Analytical discrepancy in platelet counting in MYH9-related disease: a case report

MYH9-related disease (MYH9-RD) is a rare autosomal dominant disorder characterized by congenital macrothrombocytopenia, with giant platelets and variable extrahematologic manifestations. It is frequently underdiagnosed and misclassified as immune thrombocytopenia.

A 31-year-old pregnant woman undergoing routine first-trimester testing was shown to have severe thrombocytopenia (platelet count = 5.000 × 10³/µL) by impedance counting.

Platelet histograms and scattergrams generated on a Sysmex XN-10 hematology analyzer suggested giant platelets, while fluorescence counting using a Sysmex XN-20 hematology analyzer yielded a higher platelet count (19.000 × 10³/µL), considered a more plausible estimate in the presence of giant platelets. Peripheral blood smear examination excluded platelet clumping and demonstrated numerous giant platelets and basophilic neutrophilic inclusions, subsequently interpreted as Döhle-like bodies. Further medical history revealed a previous diagnosis of MYH9-RD. Longitudinal evaluation showed persistently low but stable platelet counts, no kidney involvement, and mild liver enzyme elevation.

This case highlights the diagnostic value of integrating automated hematology data with peripheral blood smear examination in inherited macrothrombocytopenias. Platelet counts in the presence of giant platelets may be underestimated by impedance-based analyzers because of size-dependent misclassification, whereas fluorescence-based counting may provide a more reliable estimate. Recognition of characteristic morphologic features, including Döhle-like bodies, is essential for identifying MYH9-RD and distinguishing it from immune thrombocytopenia, thereby avoiding inappropriate diagnostic and therapeutic interventions.