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Comparison of
Goldmann
applanation
tonometry and
dynamic contour
tonometry in
healthy and
glaucomatous eyes
P Ceruti, R Morbio, M Marraffa and G Marchini
Abstract
Purpose To compare the measurements of
intraocular pressure (IOP) with dynamic
contour tonometry (DCT) and Goldmann
applanation tonometry (GAT) in healthy and
glaucomatous eyes and to evaluate the
influence of corneal thickness (CCT), corneal
curvature (CC), axial length (AL), and age on
these tonometric alternatives.
Methods Three hundred eyes of 100 healthy
subjects, 100 patients with primary open angle
glaucoma, and 100 patients with primary
angle-closure glaucoma underwent IOP
evaluation with DCT and GAT, and
measurements of CCT, CC, and AL.
Bland–Altman plots were used to evaluate the
agreement between tonometers. Regression
analysis was used to evaluate the influence of
ocular structural factors on IOP measurements
obtained with both tonometers.
Results Bland–Altman plots indicated that
the 95% limits of agreement between
tonometers were 1.4 to 6.6 mmHg. DCT
values measured 2.6±1.9 mmHg higher than
GAT readings (Po0.001). The mean IOP
difference between DCT and GAT (DIOP) was
higher in healthy than in glaucomatous eyes
and the magnitude of difference increased
with increasing CCT. A significant reduction
of DIOP with an increase in both CCT
(Po0.001) and IOP values (Po0.001) was
found. Regression analysis showed no effect
of CC, AL, and age on both DCT and GAT
readings. In contrast to GAT (Po0.001), DCT
measurements were not influenced by CCT
(P¼0.43).
Conclusions IOP readings obtained by DCT
were higher and less affected by CCT than
those by GAT. The DIOP was higher in healthy
than in glaucomatous eyes and decreased in
relation to increased CCT and IOP values.
Eye (2009) 23, 262–269; doi:10.1038/sj.eye.6703102;
published online 25 January 2008
Keywords: tonometry; Goldmann applanation
tonometry; dynamic contour tonometry; corneal
thickness; glaucoma
Introduction
Over the past four decades, the Goldmann
applanation tonometry (GAT) has been the gold
standard, against which other tonometers have
been compared for routine measurements of
intraocular pressure (IOP). However, the
accuracy of GAT depends on many ocular
structural factors, including central corneal
thickness (CCT), corneal curvature (CC), and
axial length (AL).
1
Measurements of IOP made
with applanation tonometry are influenced
directly by the CCT. We assume and apply a
single value for CCT (520 mm) in applanation
tonometry estimates, although there is
compelling evidence that CCT varies between
individuals, with ethnicity, gender, age, and the
presence of pseudoexfoliation of the lens
capsule,
2
and has a diurnal variation.
3
Goldmann himself recognized that the
tonometer’s accuracy was questionable in
corneas that were not of average thickness. In
recent years, different studies demonstrated
that changes in corneal structure and thickness,
Received: 27 June 2007
Accepted in revised form:
30 December 2007
Published online: 25 January
2008
Department of Neurological
and Visual Sciences, Eye
Clinic, University of Verona,
Verona, Italy
Correspondence: P Ceruti,
Department of Neurological
and Visual Sciences,
Eye Clinic,
Borgo Trento Hospital,
University of Verona,
Piazzale Stefani 1,
Verona 37126, Italy
Tel: þ39 3492 586 819;
Fax: þ39 0458 122 025.
E-mail: pierre77@
libero.it
Eye (2009) 23, 262–269
&2009 Macmillan Publishers Limited All rights reserved 0950-222X/09 $32.00
www.nature.com/eye
CLINICAL STUDY
which are both altered by corneal refractive surgery,
might be associated with clinically significant
underestimates of IOP when measured with GAT.
4–11
Moreover, many cases of normal-tension glaucoma and
ocular hypertension have been erroneously diagnosed by
underestimation and overestimation of IOP by GAT in
patients whose corneas are thinner or thicker than
average, respectively.
12–15
Dynamic contour tonometry
(DCT), a novel digital nonapplanation contact tonometer,
has been proposed as an alternative method for obtaining
IOP readings significantly closer to true manometric
levels.
16
DCT was designed to be largely independent of
structural properties of the cornea and to eliminate most
of the systematic errors arising from physiological
variables of the eye, which render GAT less accurate.
17
Some recent studies compared the performance of the
new device against other tonometers
18–22
considering
either the influence of pachymetry on IOP readings
23–26
or the impact of refractive surgery as a source of errors in
applanation tonometry.
27–30
Only two studies evaluated
ocular structural factors other than pachymetry
influencing IOP measurements in healthy subjects
31
or
African American subjects.
32
In this study, we compared
the IOP readings obtained by DCT with those of GAT and
analysed the influence of ocular structural factors (CCT,
CC, AL) and age on IOP readings in a large group of
Caucasian healthy subjects, patients with primary open
angle glaucoma (POAG), and patients with primary
angle-closure glaucoma (PACG). Moreover, the effect of
CCT, CC, AL, IOP, and age on ocular pulse amplitude
(OPA) measurements was assessed.
Materials and methods
One hundred healthy subjects from the hospital staff, 100
patients with POAG, and 100 patients with PACG, all
attending their routine clinical appointment at the
Glaucoma Service at the Eye Clinic of the University of
Verona, contributed 300 eyes that were evaluated in a
cross-sectional study. Each participant underwent all the
measurements in one randomly selected eye. The
Institutional Review Board at the University of Verona
approved the study protocol and all study procedures
conformed to the Declaration of Helsinki for research
involving human subjects. After informed consent was
obtained, participants underwent a complete ophthalmic
examination including a review of medical history,
best-corrected visual acuity assessment, slit-lamp
examination, gonioscopy, and fundus biomicroscopy
with a þ78 D lens. The inclusion criteria included
best-corrected visual acuity of 0.7 or better and the
absence of ocular pathologies other than glaucoma. The
exclusion criteria included corneal astigmatism higher
than 2 D, corneal diseases, microphthalmos, cataract
greater than mild lens opacity (according to the Lens
Opacity Classification System III),
33
history of intraocular
or refractive surgery, ocular inflammation, and contact
lens wear. The patients were classified as glaucomatous if
they had IOP higher than 21 mmHg before medications,
optic disc with typical glaucomatous findings (cup/disc
asymmetry between two eyes X0.2, neuroretinal rim
thinning, notching, disk haemorrhage or nerve fibre layer
defects), repeatable and reliable abnormal visual field
results using the automated perimetry performed with
the 24-2 Swedish Interactive Threshold Algorithm (HFA;
Carl Zeiss Meditec, Dublin, CA, USA). Reliable visual
fields were required to have fixation losses, false-positive
results, and false-negative results below 20%. An
abnormal visual field was defined as a pattern SD with
Po5%, glaucoma hemifield test outside normal limits, or
both. The 100 healthy participants were enrolled after a
complete ophthalmic examination and a thorough
history taking, which confirmed no history of previous
ocular surgery and the absence of ocular pathologies. The
100 glaucomatous patients affected by POAG were
selected from those who were treated only with topical
IOP-lowering medications (b-blockers, carbonic
anhydrase inhibitors, prostaglandin analogues,
a
2
-agonists) and who did not show peripheral anterior
synechiae (PAS) after gonioscopy. The 100 patients with
PACG were selected from those who were receiving
topical miotic therapy (pilocarpine 2%), alone or in
association with other antiglaucoma topical medication,
who showed PAS, and who had never undergone
previous ocular surgery except for peripheral laser
iridotomy. Each participant underwent all the
subsequent measurements, which were obtained by the
same experienced examiner (PC) in one randomly
selected eye, in the following order: biometry, tonometry,
and pachymetry. AL and CC were both obtained by
optical biometry system (IOL Master; Carl Zeiss AG,
Feldbach, Switzerland). IOP was measured first as any
manipulation to the eye before tonometry (contact
ultrasonic pachymetry) could alter the pressure readings.
All measurements with GAT (Haag-Streit International,
Ko
¨niz, Switzerland) and DCT (Pascal dynamic contour
tonometer; Swiss Microtechnology AG, Port,
Switzerland) tonometer were taken by the same
experienced examiner (PC), who was masked to the
readings obtained. A different observer (RM) was used to
read and record the IOP readings from both to minimize
any potential form of bias. IOP was measured with GAT
and DCT tonometer in random order, with a 5-min time
interval between readings, to avoid the possibility of IOP
fluctuation due to diurnal variations. GAT was
assembled on a slit lamp (Haag-Streit International) and
calibrated according to the manufacturer’s guidelines
before each participant underwent tonometry.
Comparison of GAT and DAT in healthy and glaucomatous eyes
P Ceruti et al
263
Eye
Three consecutive IOP measurements were obtained
(paper strips impregnated with fluorescein were used to
stain the precorneal tear film) and averaged. Before each
reading, the measuring drum was reset to approximately
6 mmHg. If IOP fluctuated during the cardiac pulse cycle,
the measurement was recorded in the maximum of the
pulsation amplitude (systolic phase). DCT, which
automatically provides digital readout values (IOP; OPA,
which is the difference between the minimum and
maximum values of the pulsatile IOP wave contour
during the cardiac cycle; Q-value, the qualitative score of
measurement) on a liquid crystal display, was assembled
on the same slit lamp and three consecutive reliable IOP
readings were obtained and averaged. The
corresponding OPA and Q-values were also recorded
and the mean of the three readings were calculated for
each eye. Particular attention was paid to standardizing
the technique of DCT. The DCT tip contacts the cornea in
a manner similar to that of GAT, except that correct
positioning on the cornea is confirmed by an audible
regular flickering signal that changes in pitch with
changes in pressure. In our study, the observer (RM)
recorded three consecutive readings for each eye only
after she was sure that the examiner (PC) had obtained a
correct IOP measurement, according to the following
criteria: (1) the interrupting coupling procedure, which
allows a correct calculation of the values by the device,
was never performed before the audible regular
flickering signal emitted at least seven consecutive
pulsations; (2) the accuracy of each measurement was
judged using the qualitative score provided by the device
and all IOP readings were recorded only if the Q-value
ranged from 1 to 3 (Q¼1, optimum; Q¼2or3,
acceptable; Q¼4, questionable; Q¼5 or 6, repetition
recommended).
Central corneal thickness was measured with an
ultrasonic pachymeter (Pachette DGH 500; DGH
Technology Inc., Exton, PA, USA) and the probe was
placed on the centre of the cornea, over an undilated
pupil, after the administration of topical anaesthesia with
oxybuprocaine 0.4%. CCT was measured three times
consecutively and the mean of the three readings within
a range of ±5mm was calculated for each eye.
Statistical analysis was performed with SPSS statistical
software (version 13.0; SPSS Science Inc., Chicago, IL,
USA) and data were presented as the mean±SD (Po0.05
was considered significant). The comparison between all
the IOP measurements by DCT and GAT was performed
using the nonparametric Wilcoxon signed ranked test.
Furthermore, Bland–Altman plots were used to evaluate
the agreement between IOP measurements obtained by
DCT and GAT. The difference between measurements
obtained by DCT and GAT were plotted against their
mean. The method of Bland–Altman states that 95% range
of agreement should be determined and then judged for
its range of clinical significance.
34
To examine the possible
effects of pachymetry on the tonometer measurements,
patients were stratified according to CCT in intervals
(CCT p530 mm(n¼94); CCT between 531 and 560 mm
(n¼109); CCT 4560 mm(n¼97)) for statistical analysis
purposes. To divide the whole sample into three groups
with the same number of eyes, the thirty-third (533 mm)
and the sixty-sixth (558.7 mm) percentiles were estimated
and used as the cutoff values. Then we assessed the IOP
measurement of DCT by comparing it to GAT with respect
to multiple CCT and various groups of eyes, using the
Student’s paired t-test. Linear regression analysis was
used to evaluate the influence of ocular structural factors
(CCT, CC, AL) and age on IOP measurements obtained
with both tonometers. Furthermore, the Pearson’s
bivariate analysis was assessed by correlating the DIOP
(which was obtained by calculating the difference
(DCTGAT) of the IOP measurements for each patient
and then the average of the 300 differences along with the
SD) with the range of both CCT and IOP values in the
study population. Linear regression analysis was also
used to analyse the effect of CCT, CC, AL, IOP, and age on
OPA measurements.
Results
The study included 300 eyes of 300 Caucasian
participants from three different groups. All
measurements were obtained on 300 randomly selected
eyes. The main characteristics of each group are listed in
Table 1. The accuracy of the 900 IOP measurements
Table 1 Clinical characteristics of the 300 eyes included in the study
Sex
(M/F)
Age
(years)
CCT
(mm)
CC
(D)
AL
(mm)
OPA
(mmHg)
DIOP
(mmHg)
Healthy (n¼100) 48/52 65±13.2 546±27.3 (477–608) 43.6±1.8 (40.1–49.3) 23.8±1.6 (21.8–28.9) 3.02±1.35 þ3.1±1.4
POAG (n¼100) 46/54 64.2±12.4 543.6±35.6 (460–619) 43.4±2 (39.7–47.7) 24.4±2.1 (21.1–32.7) 2.97±1.08 þ2.6±2.3
PACG (n¼100) 38/62 67.7±10.2 553.6±34.4 (460–643) 43.7±1.6 (40.4–48.8) 22.8±1 (19.7–25.6) 3.87±1.38 þ2.1±1.9
AL ¼axial length; CC ¼corneal curvature; CCT ¼central corneal thickness; OPA ¼ocular pulse amplitude; DIOP ¼mean IOP value by DCTmean IOP
value by GAT.
The data are presented as the mean±SD (range).
Comparison of GAT and DAT in healthy and glaucomatous eyes
P Ceruti et al
264
Eye
obtained with DCT in our study was high. The mean
Q-value in the whole study sample was 1.9±0.7. The
examiner achieved a mean optimum value (Q¼1) in
63.4% of measurements and an acceptable value (Q¼2
or 3) in 36.6% of them.
The concordance between the two different techniques
was high. Mean IOP values obtained by DCT and GAT
were strongly correlated in all eyes (r¼0.93, Po0.001).
However, IOP readings were significantly higher with
DCT than with GAT. The assessment of the whole study
population revealed a DIOP of þ2.6±1.9 mmHg
(range ¼4.9 to 8.8 mmHg) (Po0.001). IOP values
provided by DCT were higher than those by GAT
measurements in 273 eyes (91%), lower in 26 eyes (8.7%),
and equal in 1 eye (0.3%). Considering each group
separately, the results showed a significantly higher
DIOP in normal subjects than in patients with PACG
(Po0.001), but not with respect to patients with POAG
(Table 1).
The Bland–Altman plot is shown in Figure 1. These
plots allow us to investigate the existence of any
systematic difference between the measurements. The
average of the IOP measurements by the two methods is
plotted on the xaxis and the difference (DCTGAT) on
the yaxis. The mean difference is the estimated bias and
the SD of the differences measures the random
fluctuations around this mean. The plot shows that the
mean difference (DCTGAT) is positive and that very
few points fall outside the boundary limits (95% range of
agreement).
The analysis of the IOP measurements obtained by
DCT and GAT in the subgroup of eyes whose CCT was
within 531 and 560 mm revealed a significant difference
only between healthy subjects and patients with PACG
(P¼0.009). The evaluation of the two tonometers in the
subgroup of eyes with thicker corneas (CCT 4560 mm)
showed a significant difference either between normal
subjects and patients with POAG (P¼0.001) or between
normal subjects and patients with PACG (P¼0.03). On
the contrary, the comparison of DCT with GAT in the
subgroup of eyes with thinner corneas (CCT p530 mm)
failed to show any significant difference between healthy
and glaucomatous eyes, either with open-angle or angle-
closure phenotypes (Table 2).
Furthermore, the Pearson’s bivariate analysis was
assessed by correlating the DIOP with the range of both
CCT and IOP values in the study population. The results
revealed a significant reduction of DIOP with increasing
CCT (Po0.001) (Figure 2). Considering the values
recorded in our limited study population, we found an
inverse association between DCT and GAT (IOP readings
lower with DCT than with GAT) for CCT values above
620 mm either in healthy or in glaucomatous subjects
(Figure 3). We also found a significant decrease of DIOP
with an increase in IOP value (Po0.001) (Figure 4). The
analysis of the data showed an analogous inversion of
the performances of both tonometers for IOP values
above 24 mmHg. According to this analysis, we
estimated that the difference between the two tonometers
decreased 0.7 mmHg per 10-mm increase in CCT and
0.3 mmHg per 1-mmHg increase in IOP value.
Linear regression analysis showed no significant
effects of CCT (P¼0.43), CC (P¼0.59), AL (P¼0.41), or
age (P¼0.57) on DCT readings. On the contrary, IOP
values obtained by GAT depended on CCT (P¼0.001)
but were not significantly influenced by CC (P¼0.66),
AL (P¼0.77), or age (P¼0.34).
The OPA values measured by DCT (range ¼1–7.9
mmHg) were significantly higher in the eyes with PACG
than in either normal eyes (Po0.001) or in eyes with
POAG (Po0.001) (Table 1). The linear regression analysis
revealed that OPA readings were not affected by CCT
0 10 20 30 40 50 60
10
8
6
4
2
0
-2
-4
-6
-8
AVERAGE of DCT and GAT (mmHg)
DCT - GAT (mmHg)
Mean
2.6
-1.96 SD
-1.4
+1.96 SD
6.6
Figure 1 Bland–Altman plot of the agreement between
dynamic contour tonometry (DCT) intraocular pressure (IOP)
measurements (mmHg) and Goldmann applanation tonometry
(GAT) IOP measurements. The difference between the measure-
ments is plotted against the average of the measurements. Solid
line: average of the within person differences of DCTGAT.
Broken lines: 95% limits of agreement.
Table 2 Distribution of the DIOP (mean±SD, mmHg) in the
three groups of subjects (healthy, POAG, PACG) considering the
CCT subgroup of value (mm, (n¼number of eyes))
CCT Healthy POAG PACG
p530 (n¼94) þ3.7±1.5 þ4.2±1.8 þ3.5±1.9
531–560 (n¼109) þ2.9±1.3 þ2.3±2.3 þ2±1.8
4560 (n¼97) þ2.3±0.9 þ0.7±1.5 þ1.3±1.6
CCT ¼central corneal thickness; DCT ¼dynamic contour tonometry;
GAT¼Goldmann applanation tonometry; DIOP ¼mean IOP value by
DCTmean IOP value by GAT.
Comparison of GAT and DAT in healthy and glaucomatous eyes
P Ceruti et al
265
Eye
(P¼0.96), CC (P¼0.19), or age (P¼0.09), while the
values were influenced by DIOP (Po0.001) and AL
(Po0.001).
Discussion
In this study on healthy and glaucomatous subjects,
300 eyes from three different groups of Caucasian
participants were accurately enrolled to create
comparable samples (Table 1). We found a high
correlation between DCT and GAT measurements and
the agreement analysis revealed wide 95% limits of
agreement between instruments (Figure 1). However,
IOP readings obtained by DCT were 2.6 mmHg higher
than those by GAT. Similar results, in spite of different
mean values, have been reported by previous
investigations.
16,19–21,23–31,35,36
The DIOP between DCT
and GAT readings was higher in healthy than in
glaucomatous eyes, either with open-angle or angle-
closure phenotypes (Table 1). The same outcome has
been found by analysing IOP measurements obtained by
DCT and GAT in different groups of subjects with the
same CCT value, except for POAG subjects with thinner
corneas. Furthermore, the significance of DIOP between
healthy and glaucomatous eyes intensified with
increasing CCT (Table 2). The explanation of this
outcome was unclear. Topical IOP-lowering medications
could bias the measurements of DCT on glaucomatous
eyes. Both histological changes of collagen (due to an
upregulation of matrix metalloproteinases) and
modification of the corneal endothelial pump function
have been reported after the administration of topical
antiglaucoma therapy.
37,38
Drug-induced changes of
corneal structural properties, such as hydration and
rigidity, could explain the different ability of DCT
compared to GAT to measure IOP in glaucomatous
compared to healthy eyes. This observation requires
further confirmation in healthy eyes subjected to
manometry.
Several studies showed a significantly lower
correlation of CCT with DCT than with GAT.
16,28–31
Correspondingly, the present study indicated that DCT
readings were statistically independent of CCT, CC, AL,
or age. On the contrary, IOP values obtained by GAT
depended on CCT but were not significantly influenced
by other ocular structural factors and age.
Furthermore, the OPA readings were not influenced by
CCT, CC, and age but were affected by DIOP and AL.
Larger values of OPA were associated with a decrease in
the difference between DCT and GAT and a decrease of
AL values. These findings explain why OPA values were
found to be higher in the glaucomatous patients with
angle-closure phenotype (lowest AL) than either normal
subjects (median AL) (Po0.001) or glaucomatous
patients with open-angle phenotype (highest AL)
(Po0.001). For eyes with low values of OPA, DCT
640620600580560540520500480460440
CCT (m)
10
8
6
4
2
0
-2
-4
DCT - GAT (mmHg)
Figure 2 Scatterplot of DIOP (mean IOP difference between dynamic contour tonometry (DCT) and Goldmann applanation
tonometry (GAT) readings) (mmHg) vs CCT (central corneal thickness) (microns) in 300 eyes (r¼0.492; Po0.001). Solid lines: 95%
confidence intervals. Broken line: linear regression line.
Comparison of GAT and DAT in healthy and glaucomatous eyes
P Ceruti et al
266
Eye
measurements tended to be higher than GAT, whereas
for eyes with high values of OPA, GAT measurements
tended to be higher than DCT. While DCT provides a
digital readout of the OPA, which represents the average
of the minimum IOP readings during the cardiac cycle
(diastolic phase), GAT readings were obtained during the
systolic phase. These findings explain why the
differences between DCT and GAT were related to OPA.
Similar results, in spite of different study populations,
have been reported by previous investigations.
39,40
Moreover, we evaluated the performance of DCT over
a wide range of CCT and IOP values recorded in our
study population. The DIOP between DCT and GAT
measurements decreased with an increase in CCT
(Figures 2 and 3) and IOP values (Figure 4). This finding
is in agreement with other studies that showed that the
difference between the tonometers declined as CCT
increased.
36,41
On the contrary, only Barleon et al
35
observed that the difference between the two
instruments was greatest in eyes with lower IOP value,
gradually lessening as IOP increased. Considering the
values recorded in our limited study population, we
found an inverse association between DCT and GAT for
CCT values above 620 mm and IOP values above
24 mmHg. There is evidence of varying bias, as indicated
by the slope of the regression line of the differences
between the instruments: for smaller CCT and IOP
values, DCT measurements tended to be higher than
GAT measurements, whereas for higher CCT (4620 mm)
and IOP values (424 mmHg), DCT measurements
tended to be lower than GAT measurements, whether in
healthy or in glaucomatous subjects. The existence of
varying bias indicates that the amount of disagreement
between the two instruments is not constant throughout
a
b
c
5
10
15
20
25
30
450 470 490 510 530 550 570 590 610 630
CCT (m)
CCT (m)
CCT (m)
IOP (mmHg)
gat
dct
dct
gat
G
5
10
15
20
25
30
35
450 470 490 510 530 550 570 590 610 630
IOP (mmHg)
gat
dct
dct
gat
5
15
25
35
45
55
450 470 490 510 530 550 570 590 610 630
IOP (mmHg)
gat
dct
dct
gat
Figure 3 Scattergram showing the correlation between mean IOP readings obtained by dynamic contour tonometry (DCT) and
Goldmann applanation tonometry (GAT) (mmHg) with increasing CCT (central corneal thickness) value (microns) in healthy subjects
(a), patients with POAG (b), and patients with PACG (c). Solid line: linear function of the data for GAT. Broken line: linear function of
the data for DCT.
Comparison of GAT and DAT in healthy and glaucomatous eyes
P Ceruti et al
267
Eye
the range of measurements, but rather varies with the
actual measurement.
In our study, the wide and varying 95% limits of
agreement between DCT and GAT indicated that,
although they are highly correlated, measurements
should not be used interchangeably. Our study reveals a
good overall correlation between DCT and GAT, but the
high correlation of 0.93 masks the important systematic
difference between these machines and the agreement
between instruments differs in high or low IOP ranges.
This difference between IOP values from DCT and GAT
could be clinically significant and might alter
management decisions in some cases.
Acknowledgements
Piero Ceruti has full access to all the data in the study
and takes responsibility of the integrity of the data and
the accuracy of the data analysis. We do not have
financial interest in any aspect of this study or sources of
support. We do not have any commercial or proprietary
interest in the product or company. We did not receive
payment as a consultant, reviewer, or evaluator.
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Figure 4 Correlation between DIOP (mean IOP difference between dynamic contour tonometry (DCT) and Goldmann applanation
tonometry (GAT) readings) (mmHg) with increasing IOP value (mmHg) in 300 eyes (r¼0.469; Po0.001). Solid lines: 95% confidence
intervals. Broken line: linear function of the data.
Comparison of GAT and DAT in healthy and glaucomatous eyes
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