Serum inhibin-b in fertile men is strongly correlated with low but not high sperm counts: a coordinated study of 1,797 European and US men.

Objective: To describe associations between serum inhibin-b and sperm counts, adjusted for effect of time of blood sampling, in larger cohorts than have been previously reported.

Design: Cross-sectional studies of spermatogenesis markers.

Setting: Four European and four US centers.

Patient(s): Fertile men (1,797) were included and examined from October 1996-February 2005.

Intervention(s): The study was observational and therefore without any intervention.

Main Outcome Measure(s): Associations between inhibin-b and semen variables controlled for time of blood sampling and other covariates.

Result(s): Inhibin-b decreased about 2.00% per hour from 8 am-12 pm and then about 3.25% per hour from 12 pm-4 pm. There was a strong positive association between inhibin-b levels less than 150 pg/mL and both sperm concentration and total sperm count (slopes of the regression lines were β=0.011 and β=0.013 for natural logarithm-transformed sperm concentration and total sperm count, respectively). For inhibin-b levels of 150-300 pg/mL the associations were not as steep (β=0.002), but still significant. For inhibin-b levels more than 300 pg/mL there was little association to the sperm counts. Neither sperm motility nor morphology was significantly related to inhibin-b level in any group.

Conclusion(s): Serum inhibin-b levels decrease nonlinearly during the daytime, and are positively correlated with sperm counts, but the predictive power is best when inhibin-b is low.