Severe chronic ITP patients show functional defects in platelet microaggregation upon PMA stimulation. An acquired Glanzmann patient, with functional GPIIbIIIa and an anti-GPIIbIIa antibody, and a primary Glanzmann patient, without functional GPIIbIIIa, served as negative controls for the assay (A-C). Eleven mild chronic ITP patients (D-F) and 7 severe chronic ITP patients (G-I) were tested in the platelet microaggregation test. Results are given as percentage of aggregation reached after 0, 5, and 10 minutes without (A,D,G), after ristocetin (B,E,H), or after PMA stimulus (C,F,I). Outcomes are depicted pairwise, the patient compared with a healthy control tested in parallel. Statistical analysis was performed by Wilcoxon signed ranks testing.