Is an Immunoassay Available for the Measurement of Bioactive LH in Serum?

Abstract

ABSTRACT: An in vitro bioassay for luteinizing hormone (LH) is in our opinion the “gold standard” bioassay. The rodent interstitial cell testosterone assay (RICT) is specific for bioactive LH and very sensitive, accurate, and reproducible. Diverse LH standards consistently display parallel dose‐response characteristics. Sera also manifest parallel dose‐response characteristics throughout reproductive life, with the exception of basal samples from prepubertal children. This indicates that all known hormones with LH bioactivity have a similar bioactive site. The in vivo bioassays for LH used for calibration of World Health Organization standards are more cumbersome and less precise and accurate than the in vitro bioassay. The ovarian ascorbic acid depletion assay corresponds better than the seminal vesicle weight assay with in vitro bioassay. Variation in the ratio of bioactive to immunoreactive LH (B/I) principally reflects variation in LH immunoassay dose‐response characteristics, rather than a change in the bioactive moiety of LH. The varying B/I ratio is due to molecular heterogeneity at multiple levels. Different LH standards contain different proportions of nonbioactive but immunoreactive material. The immunoreactive LH isoforms in serum contain different proportions of bioactive material and the isoform distribution differs with reproductive status. Furthermore, the antibodies comprising the various immunoassay systems detect heterogeneous epitopes on LH, which are not necessarily bioactive. B/I ratio disparities indicate lack of specificity of immunoassays for bioactive LH. Polyclonal antibody‐based radioimmunoassay requires the use of purified reagents, including a bioactive tracer, in order to achieve high specificity for bioactive LH. The new generation of monoclonal antibody‐based immunometric assays yields results that are lower than, but correlate with, LH measured by the in vitro bioassay. The purest of standards, even a recombinant standard, yields results that differ up to 50% or more from one immunoassay to another. Serum LH levels also differ up to two‐fold among assays. The immunometric assays have the advantage of being more sensitive and more specific for low levels of LH in serum than radio‐immunoassays, but B/I ratio discrepancies remain great. An immunoassay specific for the bioactive “docking site” of human LH isoforms is still needed.