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Summary An immunofluorescence test on paraformaldehyde-fixed platelets in suspension was used for the detection of antibodies on platelets and in the sera of patients with idiopathic or secondary thrombocytopenia.

In idiopathic thrombocytopenia, a positive test was found on the patients' platelets in 55 out of 80 cases. The indirect immunofluorescence test with the patient's serum was positive in 28 out of 80 cases. In thrombocytopenia accompanied by other autoimmune diseases, the direct test was positive in 35 out of 39; the indirect test in 13 out of 39 patients. These results are similar to those obtained with other techniques recently described in the literature.

By using FITC-labelled monospecific anti-immunoglobulin reagents, immunochemical characterization of the autoantibodies was possible. Of the 90 patients with a positive direct test, 80 were analysed in this way. Of these, platelet-bound IgG was detected in 76. This was accompanied by IgM in 17 and IgA in three. In four, only IgM was found. The IgG subclasses of platelet-bound IgG were studied in 48 patients. Mainly IgG1 was found (45), often together with IgG3 (22) or IgG4 (14), but rarely with IgG2 (1). Only a few patients had just IgG3 (2) or IgG4 (1) on their platelets.

Platelet-bound autoantibodies could often be eluted (41 out of 86 cases) with ether and were demonstrable in the eluate with the indirect immunofluorescence test. This test was usually positive when the direct test was strongly positive (40 out of 44) and usually negative when it was weakly positive (1 out of 24) or negative (1 out of 18).