Measurements of human luteinizing hormone in plasma by in vitro bioassay and by conventional and improved radioimmunoassays.

Daily plasma samples obtained from normally menstruating women were measured for human luteinizing hormone (hLH) by in vitro bioassay and by conventional and improved radioimmunoassays (RIA). The improvement of hLH RIA consisted in the choice of standard and the extensive purification of tracer. The profiles of the biological and immunological activity of hLH throughout the cycle showed a qualitative similarity with the mid-cycle hLH surge. The estimated values obtained with the conventional RIA were consistently and significantly higher than those by the improved RIA (p less than 0.001). The elevation of the Y-intercept on RIA (5.22 +/- 3.22 uIU/ml) indicated the possible influences of crossreacting substances on the estimates obtained with the conventional RIA. The Y-intercept values calculated from the bioassay and the improved RIA estimates were indistinguishable from the origin (0.45 +/- 1.81 mIU/ml). The inter-individual variations of the biological/immunological activity ratios indicate the different proportions of multiple sub-populations of hLH molecules with various biological and immunological activities in each case. The improved RIA was concluded to be more preferable for detecting hLH levels in biological fluid than the conventional RIA. The need for more suitable standard preparations for hLH RIA was proposed.