Vasectomy is a surgical procedure for men that is performed to seal the vasa diferentia and stop the flow of sperm from the testicles to the seminal vesicles, where sperm mix with seminal fluid to produce semen. After the vasectomy, the patient will have some sperm which were already stored in the seminal vesicles and prostate. It usually takes at least 15 to 20 ejaculations to eliminate the stored sperm. The sperm check examines the semen under the microscope to make sure that all of the sperm have been eliminated and the patient can no longer cause a pregnancy.

After vasectomy, about 70% of men will have negative sperm checks after 30 ejaculations.  However, it can take over six months for some men to completely empty the seminal vesicles of sperm.