A serum follicle-stimulating hormone (FSH) should be measured in any man with small and/or soft testes. Elevated FSH suggests impaired sperm production and a poor prognosis.

Anti-sperm antibodies can be detected in the blood of most men who have undergone vasectomy. An anti-sperm antibody is the body's immune response that has been activated and targeted against the man's own sperm. This has occurred because during the vasectomy and following sperm have been allowed to exist in places in the body where they weren't normally found before the vasectomy. Although pre-operative antibody levels in the blood or seminal fluid do not project the ultimate outcome of the vasectomy reversal, the presence of sperm bound antibodies post operatively, i.e., antibodies found on sperm in a man's ejaculation after a successful vasectomy reversal does appear to predict a lower pregnancy rate.