EXPRESS: Flow cytometry-based direct antiglobulin test for suspected feline immune-mediated thrombocytopenia: Assay development and demographic factors associated with DAT positivity.

Flow cytometry DAT (>10% IgG-positive platelets) aids IMT diagnosis, with younger cats at higher risk.

This study describes the development and clinical evaluation of an in-house flow cytometry-based direct antiglobulin test (DAT) for detecting IgG-bound platelets in cats with suspected immune-mediated thrombocytopenia (IMT). Platelets were identified via CD41/61 expression, and a diagnostic cutoff of >10% IgG-positive platelets was established from 100 healthy cats. The assay was subsequently applied to 461 thrombocytopenic cats (platelet count <200,000/µL). Of these, 105 cats (22.8%) were classified as DAT positive. DAT-positive cats demonstrated significantly lower platelet counts compared to DAT-negative cats (median 4,000/µL vs. 34,000/µL; P <0.05), and an inverse correlation was identified between the percentage of IgG-bound platelets and absolute platelet count (r = -0.38; P = 0.048), suggesting more severe platelet destruction in strongly positive cases. No breed predisposition was identified. Cats aged 5 years or younger were more likely to be DAT positive, indicating potential age-related susceptibility. Conversely, neutered male cats were less likely to test DAT positive, suggesting a possible protective association or hormonal influence on immune-mediated platelet destruction. The authors conclude that flow cytometry-based DAT is a valuable adjunctive diagnostic tool for evaluating suspected IMT in cats. Importantly, positive results should be interpreted alongside clinical findings and thorough investigation for underlying causes of thrombocytopenia, as secondary IMT from infectious, neoplastic, or other etiologies must be excluded before a primary IMT diagnosis is established.

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