Objective: To determine the centrifugal force required to pellet sperm. Design: Prospective, in vitro study. Setting: Tertiary referral center. Patient(s): Men undergoing semen processing or postvasectomy semen analyses. Intervention(s): In Phase I, postvasectomy semen samples were centrifuged at 600 X g, and the pellets were examined. Supernatants from samples with no visible sperm underwent repeat centrifugation at 1,000 X g; the supernatants were removed and centrifuged at 3,000 X g. Pellets from both centrifugations were examined. Phase II examined nonazoospermic semen that was divided into 3 aliquots and centrifuged at 500, 1,000, and 3,000 X g. The supernatants were examined for the presence of sperm. Main Outcome Measure(s): Sperm presence in centrifuged pellets or seminal supernatant. Result(s): Phase I: After centrifugation at 1,000 X g and 3,000 X g, sperm were noted in 12% and 0% of samples, respectively. Phase II: Sperm were noted in the supernatant in 100% of samples subjected to 500 X g and 1,000 X g, and in 92% of samples subjected to 3,000 X g centrifugation. Conclusion(s): If sperm are in the seminal plasma, they will also be present in the pellet following centrifugation at a force of 1,000 X g or greater. Semen samples that appear azoospermic upon initial wet mount microscopy should be centrifuged at a minimum of 1,000 X g for 15 minutes.
