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doi: 10.1136/oem.2007.038448
2008
2008 65: 835-842 originally published online May 27,Occup Environ Med
R de Graaf, R C Kessler, J Fayyad, et al.
WHO World Mental Health Survey Initiative
on the performance of workers: results from the
attention-deficit/hyperactivity disorder (ADHD)
The prevalence and effects of adult
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The prevalence and effects of adult attention-deficit/
hyperactivity disorder (ADHD) on the performance of
workers: results from the WHO World Mental Health
Survey Initiative
R de Graaf,
1
R C Kessler,
2
J Fayyad,
3
M ten Have,
1
J Alonso,
4
M Angermeyer,
5
G Borges,
6
K Demyttenaere,
7
I Gasquet,
8
G de Girolamo,
9
J M Haro,
10
R Jin,
2
E G Karam,
3
J Ormel,
11
J Posada-Villa
12
For numbered affiliations see
end of article
Correspondence to:
Dr Ron de Graaf, Netherlands
Institute of Mental Health and
Addiction, Da Costakade 45,
3521 VS Utrecht,
The Netherlands;
rgraaf@trimbos.nl
Accepted 20 March 2008
ABSTRACT
Objectives: To estimate the prevalence and workplace
consequences of adult attention-deficit/hyperactivity dis-
order (ADHD).
Methods: An ADHD screen was administered to
18–44-year-old respondents in 10 national surveys in the
WHO World Mental Health (WMH) Survey Initiative
(n = 7075 in paid or self-employment; response rate
45.9–87.7% across countries). Blinded clinical reappraisal
interviews were administered in the USA to calibrate the
screen. Days out of role were measured using the WHO
Disability Assessment Schedule (WHO-DAS). Questions
were also asked about ADHD treatment.
Results: An average of 3.5% of workers in the 10
countries were estimated to meet DSM-IV criteria for
adult ADHD (inter-quartile range: 1.3–4.9%). ADHD was
more common among males than females and less
common among professionals than other workers. ADHD
was associated with a statistically significant 22.1 annual
days of excess lost role performance compared to
otherwise similar respondents without ADHD. No
difference in the magnitude of this effect was found by
occupation, education, age, gender or partner status. This
effect was most pronounced in Colombia, Italy, Lebanon
and the USA. Although only a small minority of workers
with ADHD ever received treatment for this condition,
higher proportions were treated for comorbid mental/
substance disorders.
Conclusions: ADHD is a relatively common condition
among working people in the countries studied and is
associated with high work impairment in these countries.
This impairment, in conjunction with the low treatment
rate and the availability of cost-effective therapies,
suggests that ADHD would be a good candidate for
targeted workplace screening and treatment programs.
Although it is now well known that attention-
deficit/hyperactivity disorder (ADHD) often con-
tinues into adulthood,
1–3
especially the inattention
symptoms,
45
adult ADHD has only recently
become the focus of clinical attention.
67
The same
is true of epidemiological research, which has
ignored adult ADHD in all but the most recent
studies carried out since the development of fully-
structured research diagnostic interviews in the
early 1980s. Prevalence estimates of adult ADHD
were consequently, until recently, based largely on
extrapolations from childhood prevalence esti-
mates using information about the proportion of
childhood cases that persist into adulthood
48–10
or
on direct estimation of prevalence in small
samples.
11–13
These studies produced adult ADHD
prevalence estimates in the range 1–6%, suggested
that adult ADHD is often seriously impairing,
14–16
and found that ADHD is more often seen among
the unemployed than the employed.
17 18
An attempt was made to confirm these results
with more representative data in the WHO World
Mental Health (WMH) Survey Initiative,
19
a series
of representative population surveys carried out in
26 countries using a common instrument to assess
the prevalence and correlates of mental disorders.
An earlier WMH report estimated that the
prevalence of DSM-IV adult ADHD is 3.4% (inter-
quartile range: 1.2–7.3%) in the 18–44-year-old
populations of the 10 WMH countries in which
this disorder was assessed.
20
That report also
documented high comorbidity and substantial role
impairment associated with adult ADHD in these
countries.
The current study goes beyond that earlier
report to estimate the prevalence of ADHD among
working people (either employed or self-employed)
and the effects of ADHD on role performance.
Previous research on these topics was confined to
patients in treatment for adult ADHD.
21 22
The present report, in comparison, considers
nationally representative samples of people in the
WMH countries in order to present representative
data on the burden of ADHD among working
people.
METHODS
Sample
Adult ADHD was assessed in 10 WMH countries
(table 1). Three of these 10 are classified as less
developed by the World Bank (Colombia, Lebanon,
Mexico). The others are classified as developed.
23
The
surveys were conducted face-to-face by trained lay
interviewers in multi-stage household probability
samples. The weighted average response rate across
all countries was 67.9% (range: 45.9–87.7%).
The WMH interview schedule consisted of two
parts. All respondents completed part I, which
contained core diagnostic assessments. All part I
respondents who met criteria for a core disorder
plus a probability subsample of others were
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administered part II, which assessed disorders of secondary
interest and a wide range of correlates. Adult ADHD was
assessed in part II. The part II sample was weighted to adjust for
the under-sampling of respondents who did not screen positive
for any part II disorders, making the weighted part II sample
representative of the full population.
As one requirement for a diagnosis of ADHD is symptom
onset in childhood, it was necessary to ask respondents to
provide retrospective reports about their childhood symptoms
of inattention and impulsivity. Based on concerns that the
accuracy of these reports might be especially low among
elderly respondents, the assessment of ADHD was limited to
respondents in the age range 18–44. A total of 11 422
respondents in this age range were screened across the 10
surveys.
The WMH interview schedule and all other study materials
were translated using standardised WHO translation and back-
translation protocols and are posted at www.hcp.med.harvard.
edu/wmh. Consistent interviewer training and quality control
procedures were used in all surveys. Procedures for informed
consent, which was obtained in all countries before beginning
interviews, as well as for protecting human subjects, were
approved and monitored for compliance by the Institutional
Review Boards of the organisations coordinating the surveys in
each country.
Adult ADHD
The retrospective assessment of childhood ADHD in the WMH
surveys was carried out as part of a larger assessment of diverse
mental disorders using version 3.0 of the WHO Composite
International Diagnostic Interview (CIDI 3.0).
24
The CIDI
module that assessed ADHD was based on questions originally
developed in the Diagnostic Interview Schedule for DSM-IV
(DIS).
25
Respondents classified retrospectively as having met full
ADHD criteria in childhood were then asked a single question
about whether they continued to have any current problems
with attention or hyperactivity/impulsivity. As described in
more detail elsewhere,
20
a clinical reappraisal interview of these
respondents was carried out in a probability subsample of
respondents in the WMH sample in the USA,
26
using the Adult
ADHD Clinical Diagnostic Scale (ACDS) v 1.2,
27 28
a semi-
structured interview that includes the ADHD Rating Scale
(ADHD-RS)
29
for childhood ADHD and an adaptation of the
ADHD-RS to assess current adult ADHD. The ADHD-RS has
been used in clinical trials of adult ADHD.
30 31
As detailed
elsewhere,
26
a strong association (with an area under the receiver
operating characteristic curve of 0.86) was found between the
questions about ADHD in the main survey and the clinical
diagnoses. Based on this result, a transformation rule was
developed to convert responses to the CIDI ADHD symptom-
recency questions into a predicted probability of adult ADHD
Table 1 Sample design characteristics
Country Survey name* Design overview{ Field dates
Sample size{
Response
rate1(n
1
)(n
2
)
Belgium ESEMeD Stratified multistage clustered probability sample of individuals
residing in households from the national register of Belgium
residents. NR
2001–2 (15) (486) 50.6
Colombia NSMH Stratified multistage clustered area probability sample of
household residents in all urban areas of the country
(approximately 73% of the total national population)
2003 (22) (1731) 87.7
France ESEMeD Stratified multistage clustered sample of working telephone
numbers merged with a reverse directory (for listed numbers).
Initial recruitment was by telephone, with supplemental in-
person recruitment in households with listed numbers. NR
2001–2 (39) (727) 45.9
Germany ESEMeD Stratified multistage clustered probability sample of individuals
from community resident registries. NR
2002–3 (19) (621) 57.8
Italy ESEMeD Stratified multistage clustered probability sample of individuals
from municipality resident registries. NR
2001–2 (32) (853) 71.3
Lebanon LEBANON Stratified multistage clustered area probability sampleof
household residents. NR
2002–3 (5) (595) 70.0
Mexico M-NCS Stratified multistage clustered area probability sample of
household residents in all urban areas of the country
(approximately 75% of the total national population)
2001–2 (27) (1736) 76.6
Netherlands ESEMeD Stratified multistage clustered probability sample of individuals
residing in households that are listed in municipal postal
registries. NR
2002–3 (22) (516) 56.4
Spain ESEMeD Stratified multistage clustered area probability sample of
household residents. NR
2001–2 (16) (960) 78.6
USA NCS-R Stratified multistage clustered area probability sample of
household residents. NR
2002–3 (139) (3197) 70.9
*ESEMeD, The European Study of the Epidemiology of Mental Disorders; LEBANON, Lebanese Evaluation of the Burden of Ailments and Needs of the Nation; M-NCS, The Mexico
National Comorbidity Survey; NCS-R, The US National Comorbidity Survey Replication; NR, nationally representative; NSMH, The Colombian National Study of Mental Health.
{Most WMH surveys are based on stratified multistage clustered area probability household samples in which samples of areas equivalent to counties in the UK were selected in
the first stage followed by one or more subsequent stages of geographic sampling (eg, towns within counties, blocks within towns, households within blocks) to arrive at a sample
of households, in each of which a listing of household members was created and one or two people were selected from this listing to be interviewed. No substitution was allowed
when the originally sampled household resident could not be interviewed. These household samples were selected from census area data in all countries other than France (where
telephone directories were used to select households) and the Netherlands (where postal registries were used to select households). Several WMH surveys (Belgium, Germany,
Italy) used municipal resident registries to select respondents without listing households. Eight of the 10 WMH surveys considered here are based on nationally representative (NR)
household samples, while the two others are based on nationally representative household samples in urbanised areas (Colombia, Mexico).
{ADHD was assessed only among respondents the age range 18–44 in the part II sample of each survey. Our focus is on the subsample of these respondents who were employed
at the time of interview. The respondents within this subsample who were classified as meeting criteria for DSM-IV adult ADHD are reported in the column labelled n
1
, while the total
subsample of employed part II respondents in the age range 18–44 are reported in the column labelled n
2
.
1The response rate is calculated as the ratio of the number of households in which an interview was completed to the number of households originally sampled, excluding from the
denominator households known not to be eligible either because they were vacant at the time of initial contact or because the residents were unable to speak the designated
languages of the survey.
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for each respondent in the larger samples. This predicted
probability was then transformed to a dichotomous case
classification by drawing from the binomial distribution
separately for each respondent based on their predicted
probability. This dichotomous case measure is the outcome
used in the current report. As noted below, appropriate
statistical techniques were used to adjust estimates of pre-
valence and significance for the fact that the outcome measures
were generated from predicted probability distributions rather
than measured directly.
Socio-demographics, role performance and service use
The main focus of analysis was the prevalence and correlates of
ADHD. We examined the associations of ADHD with socio-
demographic variables, several measures of role performance,
and responses to several questions about treatment. The socio-
demographic variables included gender, age (18–29, 30–44),
education (low, low-medium, high-medium, high), partner
status (married or cohabitating versus never married or
previously married, the latter including separated, divorced
and widowed) and broad occupational category (professional,
white collar technical, service, blue collar). The education
categories were different for each country and were designed
to divide the population into rough quartiles. The occupation
categories were based on the International Labour Organization
International Standard Classification of Occupations (ISCO)
(see http://www.ilo.org/public/english/bureau/stat/isco/).
Role performance was assessed using the WHO Disability
Assessment Schedule (WHO-DAS),
32
a battery that includes
questions about number of days out of role and quantity/
quality of role performance in the 30 days before interview.
Days out of role were assessed with the question: ‘‘Beginning
yesterday and going back 30 days, how many days out of the
past 30 were you totally unable to work or carry out your
normal activities?’’. Decreased quantity of work was assessed
with the question: ‘‘How many days out of the past 30 were
you able to work and carry out your normal activities, but had
to cut down on what you did or not get as much done as
usual?’’. Decreased quality of work was assessed with the
question: ‘‘How many days out of the past 30 did you cut back
on the quality of your work or how carefully you worked?’’.
Responses to the three questions were analysed separately and
together in a summary measure of overall role performance. The
summary measure was created by counting each day out of role
as 1 and each day of decreased quantity/quality of work as K
day of lost performance. The summary measure was truncated
at 30 in the uncommon case where the sum exceeded 30.
We asked about treatment of ADHD as well as more general
questions about treatment of any emotional problem.
Comparison of the two types of responses allowed us to
pinpoint people with ADHD who had received treatment for
co-occurring emotional problems but not for ADHD.
Analysis methods
As noted above, a prediction equation estimated in the clinical re-
appraisal sample was used to generate a probability of DSM-IV
adult ADHD for each respondent who was administered the
ADHD section in the CIDI. The method of multiple imputation
(MI)
33
was used to convert these predicted probabilities into
dichotomous diagnostic classifications and to adjust significance
tests for the fact that the predicted clinical diagnoses are
imperfectly related to actual clinical diagnoses. This method is
discussed in more detail elsewhere.
26
Simple subgroup compar-
isons of prevalence were used to study socio-demographic
correlates of ADHD in an MI logistic regression framework
where a dichotomous measure of estimated ADHD was used as
the dependent variable. Logits were exponentiated and are
reported here as odds ratios for ease of interpretation. MI linear
regression analysis was used to estimate associations of ADHD
with lost role performance. In this approach, the dichotomous
measure of ADHD was used as an independent variable to predict
each of the role performance measures with controls for age,
gender, education and occupation. The coefficients for the
regression of work performance on ADHD in these models can
be interpreted as the incrementally higher number of days of
impaired role performance in the past 30 days associated with
ADHD. These estimates were annualised by multiplying them by
12 (ie, the number of months in a year). These individual-level
annualised estimates were projected to the total civilian labour
force of each country by multiplying the individual-level
coefficients by the ADHD prevalence estimate and the size of
the labour force in that country.
All analyses were carried out on weighted data, so that all
estimates presented here can be interpreted as the estimates for
the general population of the different countries. Part I
respondents in each survey were weighted to adjust for
differential probabilities of selection within and between
households and to match sample distributions to population
distributions on socio-demographic and geographic variables.
The part II sample was additionally weighted for the under-
sampling of part I respondents without core disorders.
Significance tests were estimated using the Taylor series
linearisation method,
34
a design-based method implemented in
SUDAAN v 8.01 (Research Triangle Institute, Research Triangle
Park, NC) in order to adjust for this weighting as well as to
adjust for the fact that the vast majority of the WMH country-
specific sampling designs used geographic clustering. All
significance tests used two-sided Wald x
2
tests based on
design-corrected MI variance-covariance matrices.
RESULTS
Prevalence
As previously reported,
20
the MI prevalence estimate (standard
error in parentheses) of current DSM-IV adult ADHD pooled
across all 10 of the participating WMH surveys is 3.4% (0.4)
(table 2). The prevalence estimate among workers, in compar-
ison, is 3.5% (0.4) compared to 3.3% (0.5) among other
respondents. In the total sample, and in all countries except
the USA, estimated prevalence does not differ significantly
between workers and other respondents. In the USA, the
estimated prevalence of ADHD is significantly lower among
workers than other respondents (4.5% vs 7.2%, x
2
1
= 5.5,
p = 0.021).
Socio-demographic correlates
For all the countries combined, the prevalence of ADHD among
workers differs significantly by gender and occupation, but not
for age, education or partner status (table 3). ADHD is more
common among males than females, with an odds ratio (OR) of
1.7. ADHD is less common among professionals than other
workers, with the elevated ORs of the other occupational
groups relative to professionals ranging between 1.7 (service
workers) and 3.0 (white collar technical workers).
Interaction analyses found no significant between-country
differences in the associations of ADHD with either age or
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gender, but did find significant differences in the associations of
ADHD with education (p = 0.008), occupation (p = 0.009) and
partner status (p = 0.030). Inspection showed that the interac-
tion involving education is due exclusively to respondents in the
lowest two education categories having a very low estimated
prevalence of ADHD in Colombia. The interaction involving
occupation is due to white collar technical workers having an
exceptionally high estimated prevalence in the USA and blue
collar workers having a high estimated prevalence in France and
Spain. Occupation is unrelated to ADHD in the other countries.
Finally, the interaction involving partner status is due to
previously married people having a high estimated prevalence in
the Netherlands and never married people having a high
estimated prevalence in the USA.
ADHD and role performance
ADHD is significantly related to overall role performance in the
total sample, with an annualised individual-level regression
coefficient of 22.1 days out of role (reference: subjects without
ADHD) (table 4). Days out of role and reduced quantity/quality
of role performance are all statistically significant in the
combined data. Workers with ADHD have an average excess
8.4 days out of role, 21.7 days of decreased work quantity and
13.6 days of decreased work quality. Projections of individual-
level effects to the civilian labour force yields an estimate that
143.8 million lost days of productivity associated with ADHD
occur each year in these countries.
No significant interactions were found in the total sample
between ADHD and any socio-demographics in predicting total
role performance. However, statistically significant differences
were found across countries (p = 0.044), with the strongest
associations in Colombia, France, Italy, Lebanon and the USA.
It is noteworthy that no controls for comorbidity were
introduced into these analyses on days out of role, despite our
previous analyses documenting high comorbidity between
ADHD and other DSM-IV disorders.
20
The reasoning was that
ADHD is temporally primary to the vast majority of comorbid
disorders, meaning that any attenuation of the associations
between ADHD and role performance due to controlling for
comorbid disorders would indicate mediation (ie, ADHD
leading to secondary disorders that, in turn, cause decrements
in role performance) rather than spuriousness (ie, control
variables causing both ADHD and decrements in role perfor-
mance, with ADHD playing no causal role). Nevertheless, it is
instructive to investigate the extent to which such controls
attenuate the ADHD–impairment associations. The analyses
carried out to produce the results in table 4 were consequently
repeated with controls for the other DSM-IV mental/substance
disorders in the WMH surveys. The significant individual-level
association (standard error in parentheses) between ADHD and
overall role performance in the total sample remained signifi-
cant, but decreased from 22.1 (4.8) days per year to 15.8 (4.7)
days per year with the introduction of these controls, meaning
that most of the days out of role could be attributed to ADHD
and not to the disorders co-occurring with ADHD.
Role performance versus work performance
As noted in the section on measures, the WHO-DAS measures
decrements in role performance rather than work performance.
It is also possible that respondents counted some regularly
scheduled days off work as having role impairment if they had
difficulty with household chores or other normal activities
because of problems with their physical or mental health. We
investigated this issue by reanalysing the US data, where
workers were administered both the WHO-DAS and the WHO
Health and Work Performance Questionnaire (HPQ) assess-
ment of work performance. In the original analysis (table 4), the
overall annualised association between ADHD and days out of
role in the USA was 28.3 (8.4) days. In the analysis with the
HPQ substituted for the WHO-DAS as the outcome, the
annualised association between ADHD and days out of work
was 33.5 (10.1) days. The fact that the latter is higher than the
former was unexpected. To the extent that the same pattern
would hold in other WMH countries, the WHO-DAS analyses
reported above yielded estimates of the associations between
ADHD and role performance that were conservative relative to
work performance.
Treatment
Respondents who screened positive for current ADHD were
asked whether they received any professional treatment for
their problems with concentration, inattention or impulsivity at
any time in the 12 months before the interview. Very few
respondents reported receiving such treatment (table 5). Indeed,
Table 2 Multiply imputed prevalence estimates of current DSM-IV ADHD among respondents aged 18–44, by employment status
Overall Employed or self-employed All other respondents
% (SE) (n) % (SE) (n) % (SE) (n) x
2
1 p Value
Belgium 4.1 (1.5) (486) 3.7 (1.5) (347) 4.8 (2.5) (139) 0.0 0.86
Colombia 1.9{ (0.5) (1731) 1.9{ (0.6) (790) 1.9{ (0.6) (941) 0.0 0.92
France 7.3{ (1.8) (727) 6.3 (1.7) (533) 10.0 (3.6) (194) 0.9 0.35
Germany 3.1 (0.8) (621) 3.5 (1.1) (432) 2.1 (0.8) (189) 1.2 0.28
Italy 2.8 (0.6) (853) 3.4 (0.7) (569) 1.6{ (0.7) (284) 2.7 0.10
Lebanon 1.8{ (0.7) (595) 0.9{ (0.5) (305) 2.8 (1.4) (290) 2.9 0.09
Mexico 1.9{ (0.4) (1736) 2.4 (0.7) (749) 1.5{ (0.4) (987) 1.0 0.32
Netherlands 5.0 (1.6) (516) 4.9 (1.9) (389) 5.3 (2.8) (127) 0.1 0.79
Spain 1.2{ (0.6) (960) 1.3 (0.6) (574) 1.0{ (0.8) (386) 0.2 0.63
USA 5.2 (0.6) (3197) 4.5* (0.7) (2387) 7.2{ (1.2) (810) 5.5 0.021
Total 3.4 (0.4) (11 422) 3.5 (0.4) (7075) 3.3 (0.5) (4347) 0.0 0.99
*Significant difference in prevalence between employed and not employed at the 0.05 level, two-sided test.
{The upper end of the 95% confidence interval of this estimate is below the prevalence estimate for the total sample.
{The lower end of the 95% confidence interval of this estimate is above the prevalence estimate for the total sample.
1The prevalence estimate for the USA here is somewhat different from the estimate given in a previous report
17
because a somewhat different imputation equation was used in the
analyses reported here in order to be consistent with the equation used in the other WMH countries.
ADHD, attention-defecit/hyperactivity disorder; SE, standard error.
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it was only in the Netherlands, where 2.7% of estimated cases
reported receiving such treatment, and in the USA, where 12.6%
did so, that any respondents estimated to have ADHD reported
any treatment for the symptoms of ADHD. However, with the
exceptions of Lebanon and Mexico, considerably more of the
respondents with ADHD reported receiving treatment for some
other emotional problems in the same time period. The
proportion of these cases in treatment (other than in Lebanon
and Mexico, where none were receiving treatment) is in the
range between 3.5% (Belgium) and 9.6% (Colombia) in six of
the other countries and much higher in the Netherlands (21.3%)
and the USA (43.4%).
DISCUSSION
Several limitations are noteworthy. First, DSM-IV criteria for
ADHD were developed with children in mind and offer only
limited guidance regarding adult diagnosis. Clinical studies
make it clear that symptoms of ADHD are more heterogeneous
and subtle in adults,
35 36
leading some researchers to suggest that
assessment of adult ADHD might require an increase in the
variety of symptoms assessed,
37
a reduction in the severity
threshold,
38
or a reduction in the DSM-IV six-of-nine symptom
requirement.
39
To the extent that such changes would lead to a
more valid assessment, our estimates of prevalence and related
impairment would be conservative.
Table 3 Associations of socio-demographic variables with multiply imputed estimates of DSM-IV ADHD
among employed or self-employed respondents aged 18–44 (n = 7075)
Prevalence{
OR* (95% CI) x
2
df p Value% (SE)
Gender
Male 4.2 (0.6) 1.7* (1.1 to 2.0) 8.6 1 0.003
Female 2.5 (0.4) 1.0
Age
18–29 3.8 (0.5) 1.0 0.1 1 0.73
30–44 3.2 (0.5) 0.9 (0.7 to 1.3)
Education{
Low 4.7 (1.0) 1.3 (0.8 to 2.3) 4.0 3 0.26
Low-medium 4.5 (0.7) 1.3 (0.8 to 2.1)
High-medium 3.2 (0.5) 1.0 (0.6 to 1.5)
High 1.8 (0.4) 1.0
Partner status
Married/cohabitating 3.1 (0.5) 1.0 2.9 2 0.23
Separated/widowed/divorced 4.1 (0.9) 1.4 (0.9 to 2.1)
Never married 4.0 (0.5) 1.3 (0.9 to 1.8)
Occupation
Professional 1.7 (0.4) 1.0 13.9 3 0.003
White collar technical 5.8 (1.3) 3.0* (1.7 to 5.5)
Service 2.9 (0.4) 1.7* (1.0 to 3.0)
Blue collar 4.0 (0.6) 2.0* (1.1 to 2.0)
*Significantly different from the contrast category at the 0.05 level, two-sided test. {The prevalence of ADHD among respondents
in each of the socio-demographic categories. {See the text for a definition of the education categories.
ADHD, attention-defecit/hyperactivity disorder; SE, standard error.
Table 4 Annualised associations between multiply imputed DSM-IV ADHD and lost role performance among employed or self-employed respondents
aged 18–44{
Individual level National level (in million of days)
(n)
Absenteeism Quantity Quality Total Absenteeism Quantity Quality Total
Days (SE) Days (SE) Days (SE) Days (SE) Days (SE) Days (SE) Days (SE) Days (SE)
Belgium 10.8 (15.1) 18.3 (28.9) 9.4 (9.4) 16.5 (23.8) 1.1 (1.6) 1.9 (3.0) 1.0 (1.0) 1.7 (2.5) (347)
Colombia 21.9* (9.4) 14.3 (9.9) 13.0 (9.6) 29.4* (11.9) 6.2* (2.7) 4.1 (2.8) 3.7 (2.7) 8.3* (3.4) (790)
France 21.0 (3.9) 24.9 (27.7) 20.1 (12.9) 20.1 (17.5) 21.0 (4.0) 25.5 (28.3) 20.6 (13.2) 20.6 (17.9) (533)
Germany 13.3 (14.4) 23.6 (4.9) 3.9 (4.6) 13.6 (14.6) 10.6 (11.5) 22.8 (3.9) 3.1 (3.7) 10.8 (11.6) (432)
Italy 7.7 (9.5) 25.6* (12.6) 6.4 (6.8) 22.2 (13.9) 4.0 (5.0) 13.4* (6.6) 3.3 (3.6) 11.6 (7.2) (569)
Lebanon 5.8 (9.8) 30.7 (26.8) 24.1 (4.0) 19.4 (13.3) 0.1 (0.1) 0.4 (0.4) 20.1 (0.1) 0.3 (0.2) (305)
Mexico 5.0 (4.8) 5.5 (6.9) 20.9 (1.0) 6.1 (6.7) 2.4 (2.3) 2.6 (3.3) 20.4 (0.5) 2.9 (3.2) (749)
Netherlands 28.8 (14.2) 237.3 (16.8) 28.2 (4.7) 228.4 (15.8) 22.2 (3.5) 29.3* (4.2) 22.0 (1.2) 27.0 (3.9) (389)
Spain 22.8 (2.5) 3.5 (8.8) 7.5 (7.8) 1.1 (8.1) 20.4 (0.3) 0.5 (1.2) 1.0 (1.0) 0.1 (1.1) (574)
USA 10.0* (4.6) 29.1* (9.0) 20.6* (5.9) 28.3* (8.4) 37.0* (16.9) 107.5* (33.4) 76.1* (21.9) 104.7* (31.2) (2387)
All countries 8.4* (2.7) 21.7* (5.6) 13.6* (3.2) 22.1* (4.8) 54.8* (17.7) 141.3* (36.7) 88.6* (20.9) 143.8* (31.5) (7075)
*Significant at the 0.05 level, two-sided test.
{Estimates are based on linear regression equations in which the role performance outcomes are regressed on a dummy predictor variable that distinguishes workers estimated to
have ADHD from all other workers, controlling for age, gender, education and occupation.
ADHD, attention-defecit/hyperactivity disorder; SE, standard error.
Original article
Occup Environ Med 2008;65:835–842. doi:10.1136/oem.2007.038448 839
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Second, adult ADHD was diagnosed based entirely on adult
respondent self-report. Childhood ADHD is diagnosed largely
from parent and teacher reports because children with ADHD
have notoriously little insight into their symptoms.
40
However,
use of informants is much more difficult for adults, making it
necessary to rely on self-report.
36
Methodological studies
comparing adult self-reports versus informant reports of adult
ADHD symptoms document the same general pattern of under-
estimation as in child self-reports,
41 42
suggesting that prevalence
is probably under-estimated here.
Third, the MI imputation model used to estimate ADHD in
this study was based on a clinical assessment carried out only in
the USA. We have no way to confirm whether the calibration is
as accurate in other countries. This is especially problematic
given that little research on adult ADHD has been conducted
outside the USA, making it unclear if the same markers apply in
other countries. Given the centrality of this issue, it is important
for structured assessment of adult ADHD to be expanded for
use in future surveys and for the validity of these assessments to
be evaluated in clinical reappraisal studies outside the USA.
Within the context of these limitations, our results show
adult ADHD to be a fairly common disorder in the labour force
associated with substantial lost role performance. Our finding
that the prevalence of ADHD is generally as high among
workers as others was unexpected based on previous clinical
research that has generally found patients with ADHD to have
a high unemployment rate.
18
However, disaggregation found
that unemployed respondents have a higher prevalence of
ADHD (5.5%) than working people (3.5%), while homemakers
(1.9%) and students (2.2%) have the lowest rates.
20
The finding that adult ADHD is significantly more prevalent
among male than female workers is consistent with much
previous general population research.
18
The finding that ADHD
is less prevalent among professionals than other workers is not
surprising given that ADHD interferes with cognitive perfor-
mance and might create a selection bias against success in
professional work. The finding that ADHD is not related to age
in the range considered here (ie, 18–29 vs 30–44) extends the
broader finding that ADHD does not spontaneously remit in
early adulthood.
17
The finding that adult ADHD appears to be somewhat more
prevalent in developed than developing countries could reflect
the fact that the notion of a ‘‘deficit’’ existing in attentiveness
has to be defined in relation to the level of environment
demands on attention. A deficit exists only when demands
exceed the person’s abilities. It might be that high environ-
mental demands for attentiveness are more common in the
workplaces of developed countries, leading to the higher
recognition of adult ADHD in those countries. However, this
possibility is only a speculation that should be confirmed with
objective cognitive tests before it is accepted as true.
The key finding of the paper is that adult ADHD is associated
with significant decrements in role performance. This finding is
broadly consistent with much clinical evidence
28
and with
evidence from neuropsychological studies.
43
However, the
magnitude of the associations found here are quite large in
relation to comparable estimates reported in the literature for
other chronic physical and mental disorders.
44–46
It is noteworthy that we found more than half the days out
of role associated with ADHD to be due to reduced quantity/
quality of role performance rather than to days out of role. This
is important from an employer perspective because many
employers consider some number of days out of work (typically
1 per month) part of the cost of doing business and have
mechanisms to reduce financial losses due to larger numbers of
absence days (eg, caps on paid sick days, disability insurance).
However, employers typically expect their workers to be
working when they are on the job. To find that most ADHD-
related lost role performance occurs on days in role, then, is
both striking and disturbing from an employer perspective.
Although we found statistically significant differences in
ADHD prevalence across occupation, no between-occupation
difference were found in the association between ADHD and
role performance. Specifications involving other demographic
variables were also generally not significant. These results
suggest that the adverse effects of ADHD are widespread rather
than concentrated among workers in jobs where high concen-
tration is critical for success. The unusual finding that the
association between ADHD and role performance is positive in
the Netherlands is consequently difficult to explain and might
be due to the low number of respondents or low estimated
prevalence of ADHD in the Netherlands.
Our results regarding treatment of ADHD show clearly that
adult ADHD is not recognised as a disorder that requires
Table 5 Twelve-month treatment for emotional problems and more specifically for the symptoms of ADHD among employed or self-employed
respondents aged 18–44 with multiply imputed DSM-IV ADHD
Treatment for any emotional problem
Treatment for
ADHD
(n){
General medical Any mental health Human services CAM*
Any treatment for any
emotional problems
% (SE) % (SE) % (SE) % (SE) % (SE) % (SE)
Belgium 2.5 (5.7) 3.1 (7.0) 0.0 0.0 3.5 (7.2) 0.0 (15)
Colombia 1.0 (1.0) 8.6 (7.3) 0.0 2.3 (2.3) 9.6 (7.3) 0.0 (22)
France 2.4 (1.6) 4.7 (2.9) 0.0 0.0 5.8 (3.2) 0.0 (39)
Germany 0.0 3.8 (6.5) 0.0 0.0 3.8 (6.5) 0.0 (19)
Italy 6.2 (3.6) 3.2 (2.3) 0.0 1.6 (1.6) 7.8 (4.0) 0.0 (32)
Lebanon 0.0 0.0 0.0 0.0 0.0 0.0 (5)
Mexico 0.0 0.0 0.0 0.0 0.0 0.0 (27)
Netherlands 20.2 (11.8) 16.9 (11.8) 0.0 14.0 (11.5) 21.3 (11.9) 2.7 (2.4) (22)
Spain 4.3 (3.5) 4.3 (3.5) 0.0 0.0 4.3 (3.5) 0.0 (16)
USA 20.0 (3.5) 23.7 (5.1) 9.1 (1.8) 10.6 (2.9) 43.4 (5.1) 12.6 (3.9) (139)
*CAM: complementary and alternative medical treatment (eg, self-help group, chiropractor, faith healer, etc).
{The numbers of respondents with ADHD reported here, when divided by the total numbers of employed respondents in the age range 18–44, which were reported in table 1, do not
reproduce the prevalence estimates reported in table 1. This is because the sample sizes reported here are unweighted (ie, they represent the actual numbers of respondents in the
various samples), while the prevalence estimates are based on weighted data.
ADHD, attention-defecit/hyperactivity disorder; SE, standard error.
Original article
840 Occup Environ Med 2008;65:835–842. doi:10.1136/oem.2007.038448
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treatment in most of the countries studied. A much higher
proportion of cases in the USA and the Netherlands could be
detected if professionals treating patients with other emotional
problems screened for comorbid ADHD, as sizable minorities of
ADHD cases in both countries receive treatment for other
emotional problems. However, in the other countries only small
proportions of ADHD cases receive treatment for any emotional
problem.
The above results raise the question whether adult ADHD is a
candidate for targeted workplace screening and treatment
programs. Short screening scales that are both sensitive and
specific for adult ADHD exist.
47 48
It might be cost-effective
from the employer perspective to implement workplace screen-
ing programs with such a scale to detect and provide treatment
for workers with ADHD. The thinking here is that ADHD
among workers has non-trivial prevalence, high impairment and
a low rate of treatment, whereas cost-effective therapies exist
that are related to improvements in some objective aspects of
role performance.
49–51
The obvious next step from a public health
perspective, given these findings, is to evaluate the extent to
which best-practices outreach and treatment would result in
improvement in functioning that might have a positive return-
on-investment for employers.
Author affiliations:
1
Netherlands Institute of Mental Health and Addiction,
Utrecht, The Netherlands;
2
Harvard Medical School, Department of Health Care
Policy, Boston, MA, USA;
3
Institute for Development, Research, Advocacy and
Applied Care (IDRAAC), Department of Psychiatry and Clinical Psychology,St.
George Hospital University Medical Center and Faculty of Medicine, Balamand
University, Beirut, Lebanon;
4
Health Services Research Unit, Institut Municipal
d’Investigacio´Me`dica (IMIM), Barcelona, CIBER en Epidemiologia y Salud Publica
(CIBERESP), Spain;
5
Department of Psychiatry, University of Leipzig, Germany;
6
Instituto Nacional de Psiquiatria and Universidad Auto´noma Metropolitana-
Xochimilco, Calzada, Mexico City, Mexico;
7
Department of Neurosciences and
Psychiatry, University Hospitals Gasthuisberg, University of Leuven, Leuven,
Belgium;
8
Hoˆpitaux de Paris, Paris, France;
9
Health Care Research Agency,
Emilia-Romanga Region, Bologna, Italy;
10
Sant Joan de De´u-SSM, Fundacio´ Sant
Joan de De´u, Sant Boi de Llobregat, Barcelona, Spain;
11
Department of Psychiatry
and Department of Epidemiology and Bioinformatics, University Medical Center
Groningen, Graduate School of Behavioural and Cognitive Neurosciences and
Graduate School for Experimental Psychopathology, University of Groningen, The
Netherlands;
12
Medico Psiquiatra, U. Javerina, Centro Medico de la Sabana,
Bogota, Colombia
Acknowledgements: We thank the WMH staff for assistance with instrumentation,
fieldwork and data analysis.
Funding: The surveys discussed in this article were carried out in conjunction with the
World Health Organization World Mental Health (WMH) Survey Initiative. These
activities were supported by the United States National Institute of Mental Health
(R01MH070884), the John D. and Catherine T. MacArthur Foundation, the Pfizer
Foundation, the US Public Health Service (R13-MH066849, R01-MH069864, R01
DA016558), the Fogarty International Center (FIRCA R01-TW006481), the Pan
American Health Organization, Eli Lilly and Company, Ortho-McNeil Pharmaceutical,
Inc., GlaxoSmithKline and Bristol-Myers Squibb Company. The Colombian National
Study of Mental Health (NSMH) is supported by the Ministry of Social Protection. The
ESEMeD project is funded by the European Commission (contracts QLG5-1999-01042,
SANCO 2004123), the Piedmont Region (Italy), Fondo de Investigacio´n Sanitaria,
Instituto de Salud Carlos III, Spain (FIS 00/0028), Ministerio de Ciencia y Tecnologı´a ,
Spain (SAF 2000-158-CE), Departament de Salut, Generalitat de Catalunya,
Spain, Instituto de Salud Carlos III (CIBER CB06/02/0046, RETICS RD06/0011 REM-
TAP) and other local agencies and by an unrestricted educational grant from
GlaxoSmithKline. The Lebanese National Mental Health Survey (LEBANON) is
supported by the Lebanese Ministry of Public Health, the WHO (Lebanon), anonymous
private donations to IDRAAC, Lebanon, and unrestricted grants from Janssen Cilag, Eli
Lilly, GlaxoSmithKline, Roche and Novartis. The Mexican National Comorbidity Survey
(MNCS) is supported by The National Institute of Psychiatry Ramon de la Fuente
(INPRFMDIES 4280) and by the National Council on Science and Technology
(CONACyT-G30544- H), with supplemental support from the Pan American Health
Organization (PAHO). The US National Comorbidity Survey Replication (NCS-R) is
supported by the National Institute of Mental Health (NIMH; U01-MH60220) with
supplemental support from the National Institute of Drug Abuse (NIDA), the Substance
Abuse and Mental Health Services Administration (SAMHSA), the Robert Wood
Johnson Foundation (RWJF; grant 044708) and the John W. Alden Trust.
Competing interests: Preparation of this report was supported, in part, by an
unrestricted educational grant from Eli Lilly and Company in addition to the core WMH
funders. Eli Lilly staff were sent an information copy of the paper at the time of
submission but were not involved either in designing the study, carrying out analyses,
interpreting results, preparing the report, or deciding on whether or not the paper
should be published.
A complete list of WMH publications can be found at http://www.hcp.med.harvard.
edu/wmh/.
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