To assess the role of serum thromboxane B₂ (TXB₂) measurements and the correlation between platelet function studies, in patients with stable cardiovascular disease on aspirin or clopidogrel.

76 patients (47 on aspirin, 16 clopidogrel, 13 both) underwent assessment of TXB₂, whole blood aggregometry (WBA) after stimulation with (i) arachidonic acid (0.5mM), (ii) ADP (5μM), (iii) collagen (1 and 5μg/ml), PFA-100®, and Cone and Plate Analyzer. Clopidogrel patients were additionally assessed by the VerifyNow® System.

TXB₂ values ranged between 0.2 and 56.2ng/ml, with significant separation between those taking aspirin, clopidogrel and controls (0.45ng/ml vs 6.85ng/ml vs 12.97ng/ml, p<0.001). There was moderate correlation between WBA-AA and TXB₂ (r=0.487, p<0.001), PFA-100® (r=0.599, p<0.001), WBA-Col1 (r=0.424, p<0.001), WBA-Col1:5 (r=0.417, p<0.001), and between TXB₂ and PFA-100® (r=0.509, p<0.001). The prevalence of aspirin non-responders for WBA-AA, TXB₂, PFA-100®, CPA and Coll1:5 was 13.1%, 8.2%, 14.8%, 9.7% and 16.4% respectively. Individual patients were not consistently classified as aspirin non-responders in all tests. Those with inadequate aspirin response on ≥3 tests had higher TXB₂ levels (mean 1.57±1.66, range 0.553–4.45 vs mean 0.45±0.18, range 0.23–1.50) (p=0.001).

Clopidogrel suppressed TXB₂ (p=0.02), WBA-AA (p<0.001), WBA-Col1 (p=0.012) and WBA-ADP (p<0.001) compared to controls. TXB₂ in patients ingesting fish oil tablets was lower compared to those without (0.4ng/ml vs 0.52ng/ml, p=0.004). Obesity was associated with higher TXB₂ values (0.61 vs 0.41, p=0.01).

Serum TXB₂ measurements are a direct measure of the pharmacological effect of aspirin, are easily performed and correlate with other measures of platelet function. Serum TXB₂ measurements could be a useful sole measure of aspirin non-response, and may be even more predictive when performed in tandem with a global measure of platelet function. Aspirin and clopidogrel both suppressed several platelet pathways.