Effect of GPI-deficient granulocyte clone size on incidence of venous thrombosis (primary prophylaxis patients excluded). When primary prophylaxis patients are excluded, the 10-year cumulative incidence rate of thrombosis in patients with PNH granulocyte clone size larger than 50% is 44%, compared with a thrombosis rate of 5.8% in those with clone size smaller than 50% (P < .01). P value calculated with use of the log-rank test.