Short Communication Communication brève
2009;73:73–76 The Canadian Journal of Veterinary Research 73
In cats, cardiac disease, diabetes mellitus, neoplasia, nephrotic
syndrome, and some drugs may lead to a thrombophilic state
predisposing the animal to thromboembolic complications (1).
Cardiomyopathy is the most frequent cause of arterial thromboembo-
lism (ATE) in cats (2–4); ATE has been reported in 48% of cats suffer-
ing from cardiomyopathy (5). A number of factors have been brought
forth to explain the development of thrombi in cats with myocardial
disease. Left atrial dilation causes stretching of the endocardium
leading to an altered endothelium on which platelets can adhere (5).
Mitral regurgitation along with an enlarged left atrium results in tur-
bulent blood flow and stasis (2). Studies of platelet function in cats
with cardiomyopathies have yielded conflicting results. In one study,
cats with HCM have been shown to have platelets that are hyperag-
gregable, but other studies did not support these observations (6,7).
The most common site for developing ATE is the aortic trifurcation
(2,3). A study of 127 cats with ATE documented a survival rate of
37% following the ATE episode (4). Cats surviving the first episode
have a recurrence rate that varies between 25% to 45% (2,4). Many
drugs, such as aspirin and warfarin, have been used to prevent ATE
in the long-term management of these patients; however, none of
these treatments appear to affect the recurrence rate (8).
Thienopyridines irreversibly inhibit the binding of adenine
diphosphate (ADP) to specific platelet ADP receptors (P2Y12). This
ADP receptor blockade impairs platelet release reaction and ADP-
mediated activation of GPIIb/IIIa, thereby reducing the aggrega-
tion response. Clopidogrel (Plavix; Sanofi-Aventis, Laval, Quebec),
a thienopyridine, is readily available and has been evaluated in
healthy cats. Clopidogrel, at dosages of 18.75 to 75 mg PO q24h, was
well tolerated and resulted in significant anti-platelet effects (9).
Several tests are available to evaluate platelet function in patients
receiving anti-platelet medication. Optical aggregometry is con-
sidered the gold standard for platelet function studies; however,
several sample manipulations require a well-trained technician
and specialized equipment. Plateletworks is a screening assay used
to monitor platelet aggregation in human patients receiving anti-
platelet therapy. It is quick, simple, inexpensive, reproducible, and
only requires standard hematology equipment. This assay evalu-
ates platelet aggregation by comparing impedance platelet counts
between a sample anticoagulated with ethylenediamine tetra-acetic
acid (EDTA) (baseline count) and a sample in a tube containing
a platelet agonist (ADP or collagen). The agonist will stimulate
platelet aggregation resulting in platelet clumping (10). This will
cause a drop in platelet count as platelet aggregates are excluded in
impedance platelet enumeration. The ADP tubes could be used to
monitor clopidogrel therapy since platelet aggregation is inhibited
by blocking the ADP receptor.
Plateletworks: A screening assay for clopidogrel therapy monitoring
in healthy cats
Avril Hamel-Jolette, Marilyn Dunn, Christian Bédard
Plateletworks is a screening assay used in human medicine to monitor platelet-inhibiting drugs. As arterial thromboembolism is
a common complication in cats suffering from cardiomyopathy, they are often treated with anti-platelet medication. Clopidogrel
(Plavix), an anti-platelet aggregation drug, has recently been evaluated in healthy cats. The purpose of this study was to determine
if the Plateletworks method can detect a decrease in platelet aggregation in cats receiving clopidogrel. Nine healthy adult cats
were used for this study. Platelet aggregation was measured before and after a 3-day clopidogrel treatment (18.75 mg SID).
Platelet aggregation after the clopidogrel treatment was significantly lower (P , 0.01). The Plateletworks method appears to be
a promising test to monitor clopidogrel therapy in cats.
En médecine humaine, la méthode des tubes Plateletworks est utilisée afin de surveiller l’efficacité des médicaments inhibant l’agrégation
plaquettaire. Chez le chat, les thromboembolismes artériels sont une complication fréquente des cardiomyopathies et ces patients sont
fréquemment traités avec des inhibiteurs plaquettaires. L’efficacité du clopidogrel (Plavix), un anti-agrégant plaquettaire, a récemment été
évaluée chez le chat en santé. L’objectif de cette étude était de démontrer que la méthode des tubes Plateletworks peut détecter une diminution de
l’agrégation plaquettaire chez les chats recevant du clopidogrel. Neuf chats en santé ont été utilisés pour cette étude. L’agrégation plaquettaire
a été mesurée avant et après 3 jours de traitement avec du clopidogrel (18,75 mg SID). L’agrégation plaquettaire était significativement
diminuée (P < 0,01) après la période de traitement. La méthode des tubes Plateletworks semble prometteuse dans le suivi des chats recevant
(Traduit par les auteurs)
Department of Clinical Sciences (Hamel-Jolette, Dunn) and Department of Pathology and Microbiology (Bédard), Faculty of Veterinary
Medicine, Université de Montréal, 3200 rue Sicotte, Saint-Hyacinthe, Québec J2S 2M2.
Address all correspondence to Dr. Christian Bédard; telephone: 450-773-8521; fax: 450-778-8107; e-mail: firstname.lastname@example.org
Received September 11, 2007. Accepted February 25, 2008.
74 The Canadian Journal of Veterinary Research 2000;64:0–00
The 1st objective of this study was to determine if Plateletworks
ADP tubes can detect a decrease in platelet aggregation in cats
receiving clopidogrel. The 2nd objective was to determine the time
period during which the results are stable. The ultimate goal of this
study was to determine if Plateletworks ADP tubes can be used as a
screening test to monitor clopidogrel therapy in cats.
Ten healthy neutered adult cats (2 males and 8 females) from the
veterinary teaching colony of the Faculté de Médecine Vétérinaire,
Université de Montréal were used in this prospective study. Their
ages ranged from 1 to 5 y. Cats were determined to be healthy based
on history and physical examination. All cats were routinely vac-
cinated and dewormed, and were enzyme-linked immunosorbent
assay- (ELISA-) negative for Feline immunodeficiency virus (FIV) and
Feline leukemia virus (FeLV). None of the cats were under treatment
prior to the testing period. Animals were cared for according to
guidelines provided by the Canadian Council on Animal Welfare.
On day 1, a 2-mL venous blood sample was collected from the jug-
ular vein using a 22-gauge needle and a 3-mL syringe. Immediately
after collection, 1 mL of blood was placed in the Plateletworks base-
line tube (EDTA), and 1 mL of blood was placed in the Plateletworks
ADP tube that had a final ADP concentration of 20 mM. Blood smears
were made and used to check for platelet clumping in EDTA tubes.
Each tube was mixed 15 to 20 times to ensure adequate mixing of
blood with the reagent. After a 5-minute delay, EDTA tubes were
analyzed using a hematology impedance cell counter (Cell Dyn 3500,
Abbott Laboratories, Mississauga, Ontario) to determine the initial
platelet count and to obtain a complete blood (cell) count (CBC).
The ADP tubes were mixed again 3 to 5 times and run immediately
in the same impedance cell counter to determine the platelet count.
The platelet aggregation (PA) was then calculated according to the
EDTA platelet count — ADP platelet count 3 100 = PA (%)
EDTA platelet count
Immediately following blood sampling, 18.75 mg of clopidogrel
was administered orally to each cat q24h for 3 d. A complete daily
physical examination was performed on each cat throughout the
study. Venipuncture and blood analysis were repeated on day 4,
in the same manner as on day 1. In 4 cats, PA was evaluated at
times 5, 10, 15, 30, and 60 min following sampling before and after
clopidogrel therapy. Statistical analysis was performed using NCSS
(version 2004, NCSS and PASS, Kaysville, Utah, USA). The effect of
clopidogrel treatment on PA was evaluated in 9 cats with a paired
t-test using data obtained at 5 min. A 2-factor analysis of variance
(ANOVA) model with repeated measures on both factors was used
to evaluate the effect of time of analysis and treatment with clopi-
dogrel on PA. Data obtained at 5, 10, 15, 30, and 60 min before and
after treatment with clopidogrel obtained for 4 cats and used in this
analysis. A post-hoc Tuckey-Kramer multiple-comparison test was
used to detect significant differences between time points. The a
level for significance was set at 0.05.
Ten cats were enrolled in this study, but only data from 9 cats
were used for statistical analysis. One cat was removed from the
study as he was thrombocytopenic at baseline (day 1). Clopidogrel
treatment was generally well tolerated; however, 3 cats developed
mild self-limiting diarrhea during the treatment period. Complete
(cell) blood count results from the remaining cats were unremarkable
before and after clopidogrel therapy. In EDTA, significant platelet
Figure 1. Platelet counts in EDTA and ADP tubes before and after clopidogrel therapy obtained in 9 healthy cats
using an impedance hematology analyzer. In EDTA tubes, decreased impedance platelet count secondary to platelet
clumping was present in cats 2, 3, 4, 5, 6, and 8 before treatment and in cats 2, 3, and 6 after treatment.
2000;64:0–00 The Canadian Journal of Veterinary Research 75
clumping resulting in artefactual thrombocytopenia was noted in
6 samples before treatment and in 3 samples after treatment with
clopidogrel (Figure 1). The PA was calculated as 70.55 6 15.59% on
day 1 [mean 6 standard deviation (s)] and 1.52 6 9.5% on day 4.
Clopidogrel therapy caused a marked decrease in PA (P , 0.01)
(Figure 2). The repeated measure ANOVA results indicate a
significant effect of time (P = 0.028), clopidogrel treatment (P = 0.006)
with a significant interaction term (P = 0.017) (Figure 3). The PA
before treatment at 5, 10, and 15 min were statistically different than
the PA at 60 min, with a marked drop between 30 and 60 min. This
effect of time was present only in PA before treatment.
This study clearly showed that Plateletworks® ADP tubes can be
used to detect decreased PA after a 3-day clopidogrel treatment in
healthy cats. Presence of pseudothrombocytopenia secondary to
in vitro platelet clumping in EDTA samples, however, is a significant
limitation of using this assay in cats. Artefactually decreased platelet
counts in EDTA samples would significantly underestimate platelet
aggregation. However, despite this limitation, significant differences
in PA before and after clopidogrel treatment were noted, even in cats
with significant platelet clumping. This study also demonstrated
that PA remains stable until 30 minutes after blood collection. This
is important, as the manufacturer recommends conducting the test
within 10 min of sampling. This time frame is one of the major
limitations of this test; having an on-site impedance cell counter
machine is mandatory.
Before treatment, decreased PA after 30 min resulted from the
combined effect of decreased platelet count in EDTA tubes, because
of in vitro platelet clumping, and increased platelet count in ADP
tubes (data not shown). Increased platelet counts in ADP tubes could
have been secondary to reversible aggregation, a phenomenon occa-
sionally observed in optical aggregometry when low concentrations
of ADP are used to induce aggregation (11). These results suggest
that tube analysis more than 30 min following blood sampling might
affect the PA results. For use in veterinary practice, determination of
the exact time after which the tubes should not be analyzed would
Figure 2. Platelet aggregation (PA) in 9 healthy cats before and after a 3-day clopidogrel treatment
of 18.75 mg q24h.
Figure 3. Platelet aggregation (PA) measured over time after blood collec-
tion in 4 cats before and after treatment with clopidogrel. Data represent
the mean and the standard deviation.
76 The Canadian Journal of Veterinary Research 2000;64:0–00 Download full-text
be useful. Further investigations with a greater number of cats must
be done to confirm these tendencies.
Mild, self-limiting diarrhea, not previously reported in cats receiv-
ing clopidogrel, was noted in 3 cats. In humans, diarrhea has been
reported in 4% of patients receiving clopidogrel (12). It was not pos-
sible for us to ascertain whether the diarrhea was a direct side effect
of the medication or if it was caused by other factors such as stress.
Diarrhea resolved in all 3 cats by the following week. No clinical
bleeding was noted during the study.
A negative PA was noted in 4 cats post clopidogrel therapy
because of lower platelet counts in the EDTA than in the ADP tubes.
This result could have been explained by in vitro EDTA-induced
platelet aggregation causing decreased impedance platelet counts in
baseline tubes (13). However, in these cases, no significant number
of platelet clumps was noted on the blood smears of EDTA samples.
Inter-assay variation of impedance platelet counts between ADP and
EDTA samples could also be a possible explanation.
There are 2 limitations in this study. First, only 9 cats were enrolled
in this study, and although significant differences between plate-
let aggregation before and after clopidogrel therapy were found
because of a strong treatment effect, more cats would have allowed
a better evaluation of the variation in clopidogrel response at the
population level. Secondly, the results of Plateletworks were not
compared with a gold standard such as platelet aggregometry or
closure time using PFA-100 methodology. However, it has already
been demonstrated that clopidogrel therapy at the dosage used in
this study (18.75 mg/d) has a significant effect on platelet aggrega-
tion measured in whole blood using ADP and collagen (9).
In conclusion, Plateletworks appears to be a promising screening
test for monitoring clopidogrel therapy in cats. Plateletworks is a
practical and affordable test, requiring a small amount of blood
(2 mL); it can be easily performed in a clinical setting if access to a
hematology impedance analyzer is possible. Further investigations
are necessary to validate this method and compare it to optic or
whole blood aggregometry. Furthermore, evaluation of different
anti-platelet drugs such as aspirin using Plateletworks with collagen
as an agonist should be explored.
The authors thank John P. Docherty at Ryan Medical for his techni-
cal support, Dr. Jocelyn Dubuc for the statistical analysis, and Maxim
Moreau for his help during the preparation of the manuscript.
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