Human sperm hypo-osmotic swelling test: relationship to sperm fertilizing ability

The simple hypo-osmotic swelling test (HOS) has been proposed as a replacement for the labor-intensive sperm penetration assay (SPA), but its reliability is still controversial. To investigate the clinical correlation between results of the HOS and the SPA, we studied the subpopulation of washed "motile" and capacitated spermatozoa as well as sperm in the original sample in both a heterogeneous patient population being evaluated for infertility (n = 47) and a donor population (n = 13). In our patient population, the correlation between HOS results and conventional semen parameters, i.e., motility and vitality, was poor. Although the mean HOS values of the donor and patient populations differed significantly, the overlap in individual variation was too great for practical application. Using motile sperm tested two hours after separation from the seminal plasma, the correlation between a positive HOS and a positive SPA was good, with only a 4% false-negative rate. However, the correlation in capacitated sperm (20 hours) or semen itself was far less reliable. Although HOS tests correlated well with the SPA in individuals with normal sperm, in those with negative results on SPA the predictive value was extremely poor: divergent findings ranged from 32% using semen samples to 74% in the two-hour motile population. The events in the fertilizing process measured by the sperm penetration assay appear to be different from those assessed by the hypo-osmotic test. The HOS test, therefore, could not be used to screen or predict results of the SPA.