Post‐Ejaculatory Prolactin Elevation Resolves Within 2 Hours: Implications for Blood Sampling in Male Infertility Evaluation

BACKGROUND: The acute post-ejaculatory prolactin (PRL) elevation is well recognized, but its magnitude and recovery kinetics remain poorly defined. OBJECTIVES: To delineate the post-ejaculatory PRL kinetics, define its recovery window, and assess the clinical impact of improper sampling timing on hyperprolactinemia misclassification. MATERIALS AND METHODS: This single-center retrospective cohort study included 22,590 infertile men (aged 20-45 years). Participants were classified into pre-ejaculatory (n = 8804) and post-ejaculatory (n = 13,786) groups, with the latter subdivided into 17 subgroups by sequential post-ejaculatory time intervals. Serum PRL, testosterone, luteinizing hormone (LH), follicle-stimulating hormone (FSH), and estradiol were measured. The proportion of participants with PRL exceeding the local upper reference limit (17 ng/mL) was calculated. Interaction effects of ejaculation, sampling time of day, and age were evaluated using three-way ANOVA. RESULTS: Ejaculation induced a rapid and pronounced elevation in serum PRL, peaking within the ≤ 0.1 h interval (median 17.9 ng/mL, 58.4% increase from baseline, p < 0.01) and returning to baseline by the ≤ 2 h interval. Other sex hormones showed only minimal changes. PRL exhibited significant diurnal variation and age-related decline, but no significant interactions were observed between ejaculation and sampling time of day (p = 0.111) or age (p = 0.330). Notably, the percentage of participants with PRL >17 ng/mL increased from 15.8% at baseline to > 50% within the first 0.2 h, and remained significantly elevated through the ≤ 1.5 h interval. At the ≤ 2 h interval, the proportion (17.4%) was no longer significantly different from baseline. CONCLUSIONS: Post-ejaculatory prolactin elevation resolves within 2 h. This effect is independent of diurnal variation and age. Blood for PRL measurement should be collected either before ejaculation or at least 2 h after ejaculation to avoid misdiagnosis.