NEW YORK (Reuters Health) - Following vasectomy, more than half of men who initially show motile sperm go on to demonstrate azoospermia, Canadian researchers report in the May issue of Fertility and Sterility.

"Motile sperm at the time of the first semen analysis done around 3 months," lead investigator Dr. Michel Labrecque told Reuters Health, "does not automatically mean that vasectomy has failed and that the procedure has to be repeated."

Dr. Labrecque of Hopital Saint-Francois d'Assise, Quebec City and colleagues note that the presence of motile spermatozoa following the first post-vasectomy analysis is a cause for concern. It is often considered to indicate the need for a repeat procedure.

Depending on the occlusion method employed, they add, motile sperm is seen in anywhere from 0.3% to 13% of cases. The most common cause is early recanalization.

To gain further information on what proportion of men show delayed vasectomy success, the researchers examined medical records and surveyed 309 men who had had a first semen analysis showing motile sperm.

In total, 174 men (56.3%) had delayed vasectomy success. Significant independent factors associated with eventual infertility were a lower sperm count in the first analysis and performing such an analysis at 13 weeks or less after vasectomy.

Another 113 men (36.6%) had repeated vasectomies. These were mainly due to similar or increasing numbers of motile sperm found in subsequent analyses or persistence of motile sperm 6 months or more after the original vasectomy.

Given these findings, the researchers conclude that, "the decision to repeat vasectomy should not rely on a single semen analysis showing motile sperm."

Moreover, added Dr. Labrecque, recanalization can be avoided "by combining thermal cautery of the vas lumen with fascial interposition rather then using simple ligation -- either with clips or suture material -- and excision of a small vas segment."

Fertil Steril 2005;83:1435-1441.

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