Does size matter? The impact of the measuring method on testicular volume in cats

The determination of the testicular volume (TV) is a central component of the andrological examination and provides important information on the reproductive function, although reference values for tomcats and a comparison of the different available measuring devices are lacking. In this study, two manual measurement methods - a digital caliper (CL) and a measuring tape (MT) - as well as water displacement (WD) were compared. For volume calculation, the ellipsoid and Lambert's formula were applied, the gonadosomatic index (GSI) calculated and correlation analysis performed. The study included two cohorts: Group 1 comprised 16 normospermic tomcats before and during hormonal suppression by a deslorelin implant, group 2 consisted of 20 tomcats of different ages at routine castration. MT measurements consistently yielded larger values then CL measurements. The use of different TV formulas effectively minimized the bias and identified CL + Lambert and MT + ellipsoid as the most comparable, while the WD method proved to be unsuitable for cats. Strong (r = 0.87, group 2) and moderate (r = 0.68, group 1) correlations (p < 0.0001) were achieved between CT and MT volumes. While the TV was influenced by body weight, the GSI was size-independent and allowed a reliable differentiation between normo- and azoospermic, as well as between juvenile and adult individuals. This study recommends the combination of the CL with the Lambert formula or the MT with the ellipsoid formula as two practicable methods for TV calculation in veterinary practice. Further studies are encouraged to establish reference values and to validate the significance of the GSI in the assessment of reproductive capacity.