The effect of RBCs on thrombin generation depends on the endogenous platelet concentration. Thrombin generation in reconstituted whole blood with varying hematocrit (HCT) and platelets (Plts) was analyzed by calibrated automated thrombography. Hematocrits >45% interfered with the thrombin generation assay and could not be measured reproducibly. N = 3-6 per condition; bars represent mean peak thrombin ± SD. *P < .05; **P < .005.