Reference range and cutoff value of serum inhibin B to predict successful sperm retrieval: A cross-sectional study of 30 613 Chinese men

BACKGROUND

The power of inhibin B to predict competent spermatogenesis is not fully understood. The aims of this study were to identify the reliable reference range of inhibin B among normozoospermic men in China and to evaluate the diagnostic accuracy of serum inhibin B level as a complementary predictor of successful sperm retrieval in patients with azoospermia.

METHODS

This was a cross-sectional study. The male partners of 30 613 infertile couples who visited our hospital were investigated between March 2017 and March 2019. We analysed semen parameters, serum levels of reproductive hormones (inhibin B, FSH and testosterone) and sperm retrieval results from PESA/TESE in Chinese men.

RESULTS

The normal reference range of inhibin B was 87.42-299.93 pg/mL among men with normozoospermia in China. Inhibin B levels were negatively correlated with age (r = -.111; P < .001) but positively correlated with total sperm counts in the overall population, reference group and case group (r = .311, r = .208 and r = .444, respectively; P < .001). Stepwise multiple regression analyses revealed that compared with the FSH and testosterone levels, the inhibin B level had the closest relationship with the total sperm count. The best cutoff value of inhibin B for predicting the retrieval outcome of testicular/epididymal sperm was >77.72 pg/mL (sensitivity = 59.14%, specificity = 92.00% and AUC = 0.801). The inhibin B:FSH ratio (cutoff value > 6.98, sensitivity = 56.99%, specificity = 96.00% and AUC = 0.814) performed better than either the inhibin B level or the FSH level alone.

CONCLUSION

A new reference range for serum inhibin B was established in China. However, neither serum inhibin B, FSH nor their ratio is adequate for men to decide whether to undergo PESA/TESE to determine the adequacy of spermatogenesis.

International collaborative study by in vitro bioassays and immunoassays of the First International Standard for Inhibin, Human Recombinant

The First International Standard for Inhibin, Human Recombinant, (ISI), a lyophilized preparation of rDNA-derived human 32 kDa Inhibin A in ampoules coded 91/624, was evaluated by international collaborative study for its suitability to serve as an International Standard. This study, which involved 15 laboratories in nine countries, included a variety of in vitro bioassays and immunoassays. The ISI was compared with two other lyophilized preparations of human recombinant inhibin, the International Standard for Porcine inhibin (ISP) and preparations of human follicular fluid inhibin. Predicted loss of activity based on estimates of potency of contents of ampoules which had been stored under conditions of accelerated thermal degradation indicated that the ISI has satisfactory stability. On the basis of the results of this study, the ISI was deemed suitable to serve as a standard for in vitro bioassays and immunoassays and was established by the Expert Committee on Biological Standardization of the World Health Organization as the First International Standard for inhibin, recombinant human, with an assigned unitage of 150,000 International Units per ampoule. This unitage maintains an approximate continuity of units with the ISP.

International collaborative study by in vitro bioassays of the first international standard for porcine inhibin

A lyophilized preparation of inhibin from porcine ovarian follicular fluid, ampoule code 86/690, was made internationally available as a research standard for in vitro bioassays in 1987. A study involving ten participants in eight countries assessed the stability and suitability of this research standard to serve as an international standard. Each of the participants used in vitro assays, the majority of which depended upon the inhibition of release of follicle-stimulating hormone from dispersed rat anterior pituitary cells. The research standard 86/690 was compared with coded ampoules of 86/690 stored under conditions of accelerated thermal degradation and with inhibins from different species. Intra- and interlaboratory variation for estimates of potency of a coded duplicate ampoule of the research standard provided the basis for comparisons of non-identical inhibins, but the fourfold variability of potency estimates for identical ampoules was such that no conclusions about the differences seen for non-identical inhibins could be made. Predictions of stability from consensus estimates of potency of ampoules that have undergone accelerated thermal degradation indicated that the research standard had satisfactory stability. On the basis of this study, the research standard 86/690 was deemed sufficiently stable and suitable to serve as a standard for in vitro bioassays and was established by the World Health Organization Expert Committee on Biological Standardization as the First International Standard for Porcine Inhibin. The possible presence, in biological extracts (standard or sample), of other bioactive proteins, such as activin and follistatin, complicates the quantitative interpretation of bioassay data. A standard of highly purified human inhibin is now required as a standard for immunoassays used for clinical research purposes; sufficient quantities of recombinant human inhibins have recently been donated for ampouling and evaluation by bio- and immunoassay in the subsequent phase of the standardization of inhibins.

An enzyme-linked immunosorbent assay for the detection of human seminal plasma inhibin

A simple and sensitive enzyme-linked immunosorbent assay (ELISA) is described for the measurement of inhibin in urine and seminal plasma. Standards used cover a range from 50 ng to 0.05 ng/0.1 ml with a detection limit of 0.098 ng/0.1 ml. Coefficients of variation for intra-assay precision and for inter-assay precision were obtained and compared favourably with RIA. Given the ease of application, this technique is an useful alternative to existing radioimmuno-assays for this peptide.