Determination of optimal storage time and temperature for the detection of red blood cell and platelet surface-associated immunoglobulin by flow cytometry in healthy horses

Differentiating immune-mediated causes from other causes of anemia and thrombocytopenia can be challenging. Flow cytometry can detect surface-associated immunoglobulin (sIg) on red blood cells (RBC) and platelets (PLT) in dogs and horses. Sample storage parameters for ideal assay performance has not been evaluated in horses. The study objective is to identify optimal storage time and temperature of equine whole blood for the detection of RBC-sIg and PLT-sIg via flow cytometry. Both assays were performed on samples at time 0, 4, 24, 48, and 72 h post collection. RBC-sIg samples were stored at 4 °C and PLT-sIg samples were stored at 4 °C and room temperature. RBC-sIg percentages were stable up to 72 h storage. Platelet surface-associated IgG percent positive platelets increased above baseline at all timepoints and percent positive platelets were inconsistent across timepoints for IgM and IgA. PLT-sIg testing should ideally be performed within 4 h of collection. In instances where this is not feasible, samples should be stored at 4 °C and analyzed no later than 24 h after collection. Whereas cutoff values for RBC-sIg remained similar across timepoints, results for PLT-sIg should be compared to time-specific cutoff or reference intervals established by the laboratory running the test.