Serum TXB₂ levels in polycythemia vera patients. Serum TXB₂ levels were measured in polycythemia vera patients (n = 22) as well as in healthy controls (n = 6) after an 8-week treatment with low-dose aspirin (100 mg/od). represents the upper limit of values in healthy volunteers (4.9 ng/mL, mean + 2SD).