Comparison of competitive protein binding analysis and radioimmunoassay for the determination of cortisol in serum and urine.

The determination of serum and urinary free cortisol by competitive protein binding and radioimmunoassay were compared on patient samples using two commercially available kits. In addition, the Beckman RIA kit reagents were examined for precision and accuracy through the performance of reproducibility, recovery, cross reactivity and parallelism studies on serum and urine. Analytical recovery experiments with cortisol added to urine and serum showed 90% and 97% recovery respectively with the RIA reagents. Good reproducibility and parallelism criteria were also obtained using Beckman kit reagents. Antiserum specificity was documented and demonstrated no cross reactivity with structurally related steroids at clinically normal serum concentrations.