1. A comparison of six major platelet function tests to determine the prevalence of aspirin resistance in patients with stable coronary artery disease
Marie Lordkipanidzé, Chantal Pharand, Erick Schampaert, Jacques Turgeon, Donald A. Palisaitis, Jean G. Diodati

2. Aspirin is one of the most widely used drugs worldwide. Aspirin:
Introduction
Aspirin is one of the most widely used drugs worldwide.
Aspirin:
inhibits platelet aggregation through irreversible acetylation of platelet cyclooxygenase (COX) enzyme
blocks the transformation of arachidonic acid (AA) into thromboxane (Tx) A2
reduces the risk of stroke, myocardial infarction, or death by approximately 25% in patients with cardiovascular disease

3. Concept of aspirin resistance
Aspirin may not benefit all patients equally
No consensual definition of aspirin resistance exists
It is generally accepted that:
incomplete suppression of platelet aggregation as assessed by platelet function assays constitutes biochemical unresponsiveness of platelets to the inhibitory action of aspirin

4. Platelet function testing – The Gold Standard
Many tests are available to assess inhibition of platelet function induced by aspirin
Light transmission aggregometry (LTA)
Current gold standard
Evaluates luminosity as aggregation occurs in platelet-rich plasma (PRP) in response to AA
Use limited to specialized laboratories because:
poorly standardized
requires manipulation by a skilled technician

5. Other platelet function assays available
Laboratory testing
Whole blood aggregometry (WBA)
Preferred agonist: AA
Serum measurement of TxB2
Urinary measurement of dTxB2
Flow cytometry
Platelet count drop (PCD)
Point-of-care assays
PFA-100®
Using the COLL-EPI cartridge
VerifyNow Aspirin®
Using AA cartridge
Thromboelastograph (TEG®)
Cone and Plate(let) Analyzer
AspirinWorks®
Standardized urinary dTxB2
PlateletWorks®
Standardized PCD

6. Objectives
To determine the prevalence of aspirin resistance in stable CAD patients
To compare results obtained with widely available platelet function tests
This has led us to design this study, which aimed to determine the prevalence of aspirin resistance in patients suffering from stable coronary disease using a platelet function assay specific to the COX pathway and thus sensitive to inhibition by aspirin.
We further wished to compare the results obtained with other platelet function assays commonly used in the literature.

7. Patient selection (n = 201)
Inclusion criteria:
Adult, stable CAD patients
Taking aspirin daily (≥ 80 mg daily)
Exclusion criteria:
ACS ≤ 6 months
Concurrent ingestion of NSAIDs, including COX-2 selective anti-inflammatory drugs, clopidogrel, ticlopidine, dipyridamole, warfarin, or acenocoumarol
Self-reported use of non-prescription NSAIDs or aspirin-containing drugs ≤ 10 days
Major surgical procedure ≤ 1 month
Platelet count outside the 100 to 450 x 109/L range
Hb < 100 g/L or Ht < 25%
Hemorrhagic diathesis
End-stage renal disease requiring dialysis
Our population consisted of patients suffering from stable coronary artery disease and taking aspirin daily in prevention of acute ischemic events. We have excluded patients with pathologies or medications known to modulate platelet function.

8. Definitions of aspirin resistance
AA-induced LTA (LTAAA)
Platelet aggregation ≥ 20%
ADP-induced LTA (LTAADP)
Platelet aggregation ≥ 70%
AA-induced WBA
Impedance ≥ 3 Ω
PFA-100®
closure time ≤ 193 sec
VerifyNow Aspirin®
Platelet aggregation ≥ 550 Aspirin Response Units
Urinary levels of dTxB2
≥ 67.9 ng/mmol of creatinine

9. Results
Prevalence of resistance varied according to the platelet function assay used
* Definition used by Gum et al

10. Variability in platelet response to aspirin
Point-of-care assays
The measure of platelet aggregation by LTAAA segregated patients into two exclusive and distinct groups ( resistant and ￮ sensitive)
Much overlap existed above and below the specific cut-off value for other platelet function tests

11. Correlation between assay results
LTA
WBA
Point-of-care assays
Urinary dTxB2
ADP 5 µM
ADP 10 µM
ADP 20 µM
AA 1.6 mM
PFA-100®
Verify
Now Aspirin®
LTA AA 1.6 mM ADP 5 µM
ADP 10 µM ADP 20 µM
* - -
* 0.714* -
0.243* 0.290* 0.281* 0.282*
0.175* -
0.061
0.119
-0.025
Point-of-care assays PFA-100® VerifyNow Aspirin®
- -
0.189* *
* p<0.05

12. Platelet function test
Agreement between test to classify subjects as aspirin resistant, when compared with LTAAA
Platelet function test
κ statistic
p-value
LTA ADP 5 μM ADP 10 μM ADP 20 μM
0.250
0.168
0.019
<
0.002
0.531
WBA, AA 1.6 mM
0.173
0.001
Point-of-care assays PFA-100® VerifyNow Aspirin®
0.247
0.392
Urinary dTxB2
-0.033
0.475

13. Platelet function tests are NOT equally effective
Conclusion
Aspirin resistance remains rare in stable CAD, when defined by a COX-specific assay such as LTAAA
Platelet function tests are NOT equally effective
In measuring aspirin’s antiplatelet effect
Correlate poorly with the current gold standard
May explain the discrepancies reported in the literature
What should you remember from this presentation? A key point is that aspirin resistance is a rare phenomenon in stable coronary artery disease patients, when a platelet function test targeted at the COX pathway is used to quantify platelet response to aspirin.
Platelet function tests are not equally effective in measuring aspirin’s antiplatelet effect. They correlate poorly with the gold standard and among themselves. This lack of agreement may explain the discrepancies in prevalence of aspirin resistance reported in the literature.

14. Results from our study suggest that
Conclusion
Results from our study suggest that
conclusions drawn may be highly dependent on the assay used and results obtained from different assays are not interchangeable.
Further research is warranted to better understand the platelet activation pathways involved in platelet response to aspirin
in order to allow specific targeting with various platelet function assays
to determine the threshold to be used to best predict clinical outcomes.

15. For full details, please refer to:
Lordkipanidzé M, Pharand C, Schampaert E, Turgeon J, Palisaitis DA, Diodati JG. A comparison of six major platelet function tests to determine the prevalence of aspirin resistance in patients with stable coronary artery disease. Eur Heart J 2007; 28:1702-8