Is flow cytometry accurate enough to screen platelet autoantibodies?

Background: The diagnosis of immune thrombocytopenic purpura (ITP) is a diagnosis of exclusion, as stated by international guidelines. Nevertheless, the assessment of platelet (PLT) antibodies has been reported as helpful for the diagnosis and the follow-up of ITP patients. PLT antibodies are detected by highly specialized assays, such as monoclonal antibody-specific immobilization of PLT antigen (MAIPA) test. Flow cytometry for PLT-associated immunoglobulin G (PAIgG) detection has been described more recently. This study was meant to evaluate the utility of flow cytometry to screen accurately patients needing further MAIPA testing.

Study Design And Methods: PAIgG, PAIgM, and PAIgA were determined in 107 consecutive patients and in 147 healthy controls in parallel. MAIPA testing was performed in all patients. The accuracy of flow cytometry was assessed with a receiver operating characteristics (ROC) curve analysis versus MAIPA.

Results: MAIPA assay found PLT-specific IgG in 27 patients (25%). The ROC curve analysis showed that no false-negative result in flow cytometry was obtained for a mean fluorescence intensity (MFI) cutoff of 0.2. With this cutoff, PAIgG were positive in 61 patients (57%). In this series, MAIPA was unnecessary in 42 percent of patients (corresponding to true-negative results). When MAIPA was positive, PAIgM values ranged from 0.1 to 1.0, and PAIgA from 0.1 to 2.

Conclusion: Flow cytometry for PAIgG assessment may be used to accurately decide whether or not MAIPA must be subsequently performed. In this series, MAIPA was unnecessary in 42 percent of patients. Moreover, PAIgM results suggested that its determination combined with PAIgG may be of interest in ITP investigation.