Interlaboratory exercise on the use of immunofluorescence microscopy on the blood smear for recognizing inherited platelet disorders: Communication from the ISTH SSC Subcommittee on Platelets in Health and Disease.

Immunofluorescence-based platelet phenotyping using peripheral blood smears has recently emerged as a promising method for characterizing a subgroup of inherited platelet disorders (IPD). A single-center study demonstrated its potential for accurate diagnosis of 9 disorders with characteristic platelet structural changes. The aim of this study was to evaluate the reproducibility of this approach through an interlaboratory validation study. Native, air-dried blood smears from healthy controls and patients with confirmed IPD were shipped to 7 participating laboratories, blinded for the sample origin. Samples were fixed and stained using a shared panel of 13 commercially available primary antibodies and 2 fluorescence-labelled secondary antibodies. Laboratories formulated diagnostic predictions based solely on immunofluorescence findings. The pre-workshop method establishment involved other samples and feedback with the coordinating laboratory to address technical issues before blinded sample validation. All 7 laboratories (Brisbane, Greifswald, Murcia, Paris, Pavia, Perugia, and Tübingen) correctly diagnosed MYH9-related disease, Bernard-Soulier syndrome, Glanzmann thrombasthenia, and GFI1B-related thrombocytopenia. Six of 7 laboratories accurately identified TUBB1-related disorder and quantitative δ-storage pool disorder, while 5 of 7 correctly diagnosed GATA1-related thrombocytopenia. Immunofluorescence-based platelet phenotyping on peripheral blood smears demonstrated high sensitivity for diagnosing MYH9-related disease, Bernard-Soulier syndrome, Glanzmann thrombasthenia, and GFI1B-related thrombocytopenia, and reasonable sensitivity for TUBB1-related disorder, quantitative δ-storage pool disorder, and GATA1-related thrombocytopenia. Immunofluorescence analysis of blood smears may be of help in the diagnostic work-up of IPD.