A detailed flow cytometric method for detection of low-level in vivo red blood cell-bound IgG, IgA, and IgM

Flow cytometric methods are commonly used to analyze white blood cell surface antigen expression. We developed a flow cytometric method to detect red blood cell (RBC)-bound immunoglobulin (Ig)G, IgA, and IgM. RBCs were washed; incubated with fluorescein isothiocyanate (FITC)-conjugated anti-IgG, -IgA, or -IgM; washed; and analyzed on the flow cytometer. The method was optimized by determining the dilution of FITC-conjugated anti-IgG, -IgA, and -IgM providing the greatest amount of fluorescence when tested with Ig-coated RBCs and the least amount of fluorescence when tested with naive RBCs. Tannic acid was used to prepare Ig-coated RBCs. Cross-reactivity of FITC-conjugated anti-IgG, -IgA, and -IgM with Ig-coated RBCs was evaluated, and a reference range was established. Use of this method may assist in clinical evaluation of patients who present with hemolysis and a negative direct antiglobulin test.