Each year, about half a million American men have a vasectomy. The operation provides permanent contraception for men by cutting off the flow of sperm. The operation is mainly considered irreversible. But about one percent of surgical sterilization procedures for men turn out to be ineffective, heightening the risk of a surprise, unintended pregnancy later on. Now a recent study indicates that not enough men are getting follow-up tests to be sure that their vasectomy indeed worked.

Researchers with the Cleveland Clinic Foundation tracked nearly four-hundred-fifty men who’d received vasectomies. Twenty-five percent failed to return a semen sample to establish that the surgery had been a success and that they were indeed sterile, although they’d been advised by their doctor to do so. Additionally, about half the men studied also failed to turn in a second sample, an extra measure of safety ordered by many urologists.

Doctors reported that about one-quarter of men who had an initial semen analysis two months after their vasectomy still had sperm in their samples. However, only sperm from a trio of men were still active. The rest weren’t mobile. Some urologists, however, believe the sample should be completely void of sperm to ensure the procedure was a success.

Physicians emphasize that vasectomy is generally effective. But they say this study shows that men who go under the knife need to remember to follow up with postoperative tests, to be sure all goes as planned.