The erythrocyte coating substance was identified in seventy-two patients with positive direct antiglobulin reactions. Based on these studies patients were classified as having coating with γG globulin alone, with γG plus complement or with complement alone. The complement group was further divided into those with normal cold agglutinins and those with elevated cold agglutinin titers. Seventy-six per cent of the patients had evidence of underlying disease, particularly lymphatic neoplasia, connective tissue disorders and primary atypical pneumonia. In the remaining 24 per cent no underlying disease was identified. With the exception of the previously established association of cold agglutinin elevation with primary-atypical pneumonia, there was no absolute correlation between the presence or absence of underlying disease and the type of erythrocyte coating observed. In general, however, patients with complement coating (with or without γG globulin) were more likely to have underlying disease than were patients with coating with γG alone. Twenty-four of the seventy-two patients (33 per cent) did not have clinically detectable hemolysis. In this group the positive antiglobulin reaction was seen exclusively in the setting of an underlying disease, and in almost two-thirds was due to erythrocyte coating with C′ alone. Hemolytic anemia was clinically evident in forty-eight patients (67 per cent). Most of these patients were treated initially with corticosteroids. Three-fourths of the patients with normal cold agglutinin titers had a satisfactory initial response to steroids regardless of the type of erythrocyte coating. The most satisfactory response occurred in patients with complement coating and normal cold agglutinin titers. There was no difference in initial response rate to steroids between patients with underlying disease and those with no underlying disease. Patients who responded well to steroids usually responded well to additional measures such as splenectomy, x-ray therapy, alkylating agents and immunosuppressive drugs. Patients not responding to steroids usually did not respond to these measures. As might be expected, the long-term results of all forms of treatment were more favorable in patients with no underlying disease.
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