SERUM BINDING OF STEROID TRACERS AND ITS POSSIBLE EFFECTS ON DIRECT STEROID IMMUNOASSAY

We studied the serum protein binding of H-3-labelled progesterone, oes tradiol and testosterone, and five I-125-labelled analogues of these s teroids. All tracers investigated appeared to be bound by proteins in every serum sample tested. The addition of blocking agents caused a su bstantial reduction in serum protein binding of H-3-labelled steroids, but had relatively little effect on the binding of analogue steroid t racers. Use of analogue steroid tracers in conventional direct immunoa ssays for oestradiol and progesterone produced anomalous results for s ome patient samples when compared to extraction radioimmunoassays, but assays where tracer binding to serum constituents was prevented by ad option of two-step procedures appeared to avoid anomalous results. The results suggest that serum protein binding of steroid analogue tracer s may be a source of interference in some direct steroid immunoassays.