Clinical Diagnostic Assessment of Serum Testosterone in Intact and Castrated Tomcats

Basal serum testosterone may help determine reproductive status in tomcats with an uncertain surgical history, but interpretation of a single result is limited by episodic secretion and reduced analytical precision at very low concentrations. We evaluated the analytical performance and diagnostic accuracy of an automated fluorescence enzyme immunoassay (FEIA) in 80 male cats (40 intact and 40 castrated). Within-run and between-day precision and dilutional recovery were assessed using pooled feline serum, and diagnostic performance was evaluated by receiver operating characteristic analysis. Testosterone concentrations were significantly higher in intact than castrated cats (p < 0.001), although overlap occurred at low concentrations. ROC analysis yielded an AUC of 0.773 (95% CI, 0.665–0.859). The Youden-derived threshold > 0.124 ng/mL had 60.0% sensitivity (95% CI, 43.3–75.1%) and 90.0% specificity (95% CI, 76.3–97.2%). No castrated cat had a concentration > 0.271 ng/mL; this cohort-specific rule-in threshold had 47.5% sensitivity (95% CI, 31.5–63.9%) and 100% observed specificity (95% CI, 91.2–100.0%). Therefore, clearly elevated basal testosterone can support the presence of functional testicular tissue, but concentrations ≤ 0.271 ng/mL cannot exclude intact status. Low or equivocal results require integration with clinical examination and, when indicated, GnRH stimulation and/or anti-Müllerian hormone measurement. Independent validation is required before broad clinical application.