Microencapsulated antibodies in radioimmunoassay. III. Determination of cortisol

We describe a method for directly measuring total plasma cortisol by use of a microencapsulated antibody. The antibody microcapsules were incubated with plasma and 125I-labeled cortisol in a competitive reaction at 37 degrees C for 15 min, then separated by centrifugation; the radioactivity of the pellet was counted for 1 min. Analytical recovery of cortisol from three plasma pools was 97.6% (SD 8.5%) and was unaffected by triglyceridemia, hemolysis, or icterus. The standard curve was linear to 500 microgram/L and the sensitivity was 10 microgram/l. Recovery of cortisol added as the hemisuccinate was 102% (SD 10.6%), whereas that of the conjugate cortisol hemisuccinate/bovine serum albumin was 4.3% (SD 3.5%). This confirmed that the microcapsule excludes large molecules. The within-day CV for two control pools was 9.8 and 12.8%, the day-to-day variation 13.4 and 13.8%.