Once- versus twice-daily aspirin treatment in patients with essential thrombocytosis.

Insufficient platelet inhibition has been reported in up to 40% of aspirin-treated patients, including patients with essential thrombocytosis. To maintain sufficient platelet inhibition, a shorter dosing interval with aspirin has been suggested. We aimed to investigate the antiplatelet effect of low-dose aspirin given twice-daily compared to standard once-daily dosing in patients with essential thrombocytosis. We included 22 patients, who were treated for 7 days with standard once-daily aspirin (75 mg once-daily) followed by 7 days treatment of twice-daily aspirin (37.5 mg twice-daily). The two regimens were separated by 14 days aspirin washout. Blood samples were obtained 1h and 24h/12h after the last pill intake in each regimen. The effect of aspirin was evaluated by: (1) platelet aggregation measured by whole blood impedance aggregometry (Multiplate® Analyser) using arachidonic acid (ASPItest 0.5 mM) as agonist and (2) serum thromboxane B levels determined using an enzyme-linked immunosorbent assay. The difference in platelet aggregation from 1h to the end of the dosing interval (24h/12h) was used to compare the two regimens. We demonstrated a significantly smaller difference in platelet aggregation in the twice-daily regimen compared to the once-daily: mean of difference = 228 AU*min (95% confidence interval (CI): 92-363, p < 0.01). In addition, a significantly smaller difference in thromboxane B was demonstrated in the twice-daily regimen compared to the once-daily regimen: mean of difference = 16.3 ng/mL (95% CI: 9.9-22.7, p < 0.01). In conclusion, twice-daily dosing with low-dose aspirin provides a more consistent platelet inhibition compared with standard once-daily dosing in patients with essential thrombocytosis.