Severe chronic ITP patients show functional defects in platelet reactivity upon ADP and CVX stimulation. To determine baseline platelet reactivity, MFI in arbitrary units (AU) of fluorescent-labeled anti–P-selectin antibody (A) and of fluorescent-labeled fibrinogen (B) bound to platelets was determined in the absence of agonists. Maximal MFI in AU in response to stimulation with ADP, CVX, and TRAP and concentration of these agonists needed to obtain half-maximal MFI were determined for both platelet P-selectin expression (C-D,G-H,K-L) and opening of GPIIbIIIa receptor (E-F,I-J,M-N). The dots represent individual results for the 10 severe patients, 23 mild patients, and 8 healthy controls, with a line at the median. Statistical analysis was performed by Mann-Whitney U testing.