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 out 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.