A radioimmunoassay for fibrinopeptide A (FPA) has been developed. This assay uses rabbit antibodies induced by injection of native FPA human serum albumin conjugates and 125I introduced into tyrosine FPA synthesized in the laboratory. Plasma FPA is separated from fibrinogen by TCA extraction. The assay is capable of detecting as little as 50 pg/ml of FPA. In 20 normal donors this assay revealed a mean concentration of 0.9 ng/ml (0.3 SD). In five patients with disseminated intravascular coagulation, FPA concentrations ranged from 13.0 to 346 ng/ml. Two groups of patients with systemic lupus erythematosus (SLE) whose disease had achieved complete remission were studied; one consisted of four patients with no history of lupus nephritis and another with a history of nephritis. Mean FPA concentrations of 1.5 ng/ml (range, 0.7-1.8 ng/ml) and 2.7 ng/ml (range, 1.1-5.6 ng/ml) were found in these two groups, respectively. Another group of nine patients with active SLE, but without evidence of lupus nephritis, had a mean FPA concentration of 4.5 ng/ml (range, 2.4-7.8 ng/ml). Finally, a group of seven patients with active SLE, including active nephritis, had a mean FPA concentration of 10.2 ng/ml (range, 5.3-17.0 ng/ml). A positive correlation was found between the concentration of plasma FPA and serum DNA binding activity and an inverse correlation was found between plasma FPA and the concentration of serum C3. No correlation existed between plasma FPA and concentration of serum creatinine. Several possibilities for the origin of plasma FPA in patients with SLE were considered; at present it seems most likely that FPA arises through the action of thrombin on fibrinogen.
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