High Variability in Human Sperm Membrane Potential over Time Can Limit Its Reliability as a Predictor in ART Outcomes

Sperm membrane potential (<i>E</i>m) hyperpolarization during capacitation is a functional hallmark of fertilizing ability and has been proposed as a predictive biomarker for conventional in vitro fertilization (IVF) success. However, it is unclear whether <i>E</i>m remains stable across ejaculates over time and can reliably guide assisted reproductive technology (ART) decisions in advance. Thus, we aimed to evaluate the temporal consistency of human sperm <i>E</i>m within individuals and assess its utility as a prognostic marker when measured days or weeks prior to IVF procedures. <i>E</i>m was assessed in capacitated and non-capacitated sperm from normospermic donors at three time points over 28 days, using a fluorometric assay. Capacitated values were compared to a -48.6 mV threshold previously associated with successful fertilization. Intra-donor <i>E</i>m variability and coefficients of variation (CV) were analyzed statistically. Our results showed that <i>E</i>m values exhibited significant intra-donor variability over time (<i>p</i> = 0.007), with approximately half of the donors crossing the -48.6 mV functional threshold across sessions. Capacitated sperm samples showed significantly greater variability than non-capacitated ones, with several donors exceeding a 30% CV cutoff. No consistent correlation was found between CV and mean <i>E</i>m values. While <i>E</i>m remains a promising functional marker when assessed on the day of IVF, its temporal variability undermines its reliability as a predictive tool for ART decisions made in advance. These findings underscore the importance of timing in functional sperm assessments and call for further studies to identify the physiological factors influencing <i>E</i>m stability.