Predictive parameters for the isolation of platelets from whole blood for diagnostic purposes in patients with immune thrombocytopenia

The accelerated clearance of platelets is a central feature of immune thrombocytopenia (ITP). Autoantibodies opsonize platelets and trigger clearance by macrophage phagocytosis. The direct monoclonal antibody immobilization test for platelet antigens (DMAIPA) assay measures autoantibodies in a direct way. Complexes of autoantibodies together with their target gylcoproteins (GPs) are taken out of the patient's platelet membranes and immobilized to a carrier in a GP-specific way. A major drawback of DMAIPA is the lack of platelets in ITP patients: with few platelets, only some or, with very few platelets, none of the GPs may be assessable.

In a prospective study design, the following parameters were collected: patient age, patient gender, haematocrit, platelet count, available volume of whole blood, time interval between blood collection and the start of processing in the laboratory, and the number of GPs that could be immobilized (referred to as number of tests, range 0 to 3). Statistics were performed using SPSS Version 29 (IBM, New York, USA).

A total of 754 patients were included in the study. The number of tests that could be performed were three in 405 patients (53.7%), two in 115 patients (15.3%), and one in 127 patients (16.8%). In 107 patients (14.2%), there were not enough platelets to perform DMAIPA at all. Median value and ranges were: haematocrit, 0.416 v/v (0.300-0.581); platelet count 49.500/µL (0-99.000); volume, 12 mL (0.1-40.0); and time interval, 2 days (0-18). Univariate regression analysis was non-significant for gender and age, but significant for haematocrit (b=0.081, p=0.027), platelet count (b=0.499, p<0.001), volume (b=0.371, p<0.001), and time interval (b= -0.17, p<0.001). Using multiple regressions with a stepwise procedure, platelet count, volume and time interval were identified as most relevant predictive parameters. If the time interval is set at 0-2 days and platelet count is >20.000/µL, 74.2% of patients can be tested for at least 2 GPs if the volume is below 16 mL, and 95.5% can be tested for at least 2 GPs if the volume is 16 mL or higher.

This is the first study to show that the performance of DMAIPA essentially depends on the platelet count, the available blood volume and the time interval between blood collection and the start of the analysis. Under favourable conditions, a sufficient analysis can be performed in 95.5% of patients with ITP if their platelet count is >20,000/µL.

No conflicts of interest to declare.