The Association Between Platelet Reactivity and Bleeding Risk in Adult ITP

Overview

It seems reasonable to assume that patients who present significant bleeding symptoms may have different quality of platelets than those without bleeding. This question was addressed in a study that examined platelet function in adult ITP patients, which try to determine whether this correlated with bleeding risk. Previous reports have suggested that measuring platelet function may help define patients at highest risk of bleeding. In addition, Middelburg and colleagues corrected platelet function for quartile of platelet count, using \<32×10\^9/L as the lowest cohort and \>132×10\^9/L as the top quartile. They demonstrated that increased platelet reactivity (as measured by flow cytometry) was associated with decreased risk of bleeding but particularly for those patients with the lowest platelet counts. Further studies in a larger cohort are needed to confirm this correlation. Our study aimed at standardizing a prediction model to evaluate the bleeding risk of adult ITP patients with the use of platelet function tests.

The investigators are undertaking a prospective multicenter double-blind study of 400 adult patients with immune thrombocytopenia from 6 medical centers in China. We adopted three different assays that examined platelet function and reactivity. 1) Flow cytometry: Citrate anticoagulated whole blood was diluted in PBS to result in 20×10\^9/L platelets, and 20 μl was aliquoted into polystyrene test tubes. Ten microliters of anti-CD42b-PE was added and incubated at room temperature for 10 min. Agonists (TRAP-6 12.5 μMol/L, Collagen 20 μg/mL, ADP 2 μM, Epinephrin 20 μM, Arachidonic acid 0.275 mM, Ristocetin 1.5 mg/mL) or PBS were added (10 μl each) and incubated again for 10 min. Then mAb PAC-1-FITC or anti-CD62p-FITC (10 μl each) or the corresponding isotype-matched controls were added. After 15-min incubation in the dark, the reaction was stopped with 500 μl PBS. Samples were analyzed on a flow cytometer (FACScan, Becton-Dickinson) by measuring 10,000 events in the CD42b-positive fraction. 2) Filopodia quantification: Briefly, platelets in Tyrode's buffer were allowed to adhere to VWF (9×10\^6 cells/coverslip) in the presence of botrocetin (1 μg/mL) and Integrilin (40 μg/mL) at 37°C. After 15 min, non-adherent platelets were removed by washing and adherent platelets were fixed with 4% PFA, stained with TRITC-phalloidin (2 μg/mL) and viewed by epifluorescence microscopy for filopodia count. 3) Platelet aggregation: Measured on an automated platelet aggregation analyzer. Understanding bleeding risk and underlying determinants of bleeding is important in order to help recognize patients that will require pharmacologic therapy even at higher platelet counts. Previous studies have suggested that low platelet counts, increased patient age, use of concurrent medications, and male sex are associated with increased bleeding risk. The present investigation will answer whether platelet function predicts the severity of bleeding in adult ITP patients. Clinical information of recruited participants includes gender, age, platelet count and physical/laboratory examination. Blinding was set between investigators who evaluated bleeding risks and those who performed experiments.