Key Points• ITP patients differ in their tendency to bleed despite similarly low platelet counts, thereby confounding treatment decisions.• Platelet function tests, independent of platelet count, are associated with bleeding severity in ITP patients.Immune thrombocytopenia (ITP) patients with similarly low platelet counts differ in their tendency to bleed. To determine if differences in platelet function in ITP patients account for this variation in bleeding tendency, we conducted a single-center, cross-sectional study of pediatric patients with ITP. Bleeding severity (assessed by standardized bleeding score) and platelet function (assessed by whole blood flow cytometry) with and without agonist stimulation was evaluated in 57 ITP patients (median age, 9.9 years). After adjustment for platelet count, higher levels of thrombin receptor activating peptide (TRAP)-stimulated percent P-selectin-and activated glycoprotein (GP)IIb-IIIa-positive platelets were significantly associated with a lower bleeding score, whereas higher levels of immature platelet fraction (IPF), TRAP-stimulated platelet surface CD42b, unstimulated platelet surface P-selectin, and platelet forward light scatter (FSC) were associated with a higher bleeding score. Thus, platelet function tests related to platelet age (IPF, FSC) and activation through the protease activated receptor 1 (PAR1) thrombin receptor (TRAPstimulated P-selectin, activated GPIIb-IIIa, and CD42b), independent of platelet count, are associated with concurrent bleeding severity in ITP. These tests may be useful markers of future bleeding risk in ITP. (Blood. 2015;126(7):873-879)

Key Points Platelet function in WAS/XLT, measured by agonist-induced surface-activated GPIIb-IIIa and P-selectin, is proportional to platelet size. Eltrombopag increased platelet counts, but did not improve platelet activation, in most WAS/XLT patients.