To compare the accuracy of testicular volume measurements with the Prader orchidometer versus ultrasonography.

The volume of 938 testes in 469 men with infertility (mean age 35.8 years) was measured using a Prader orchidometer and by ultrasonography. The ultrasound testicular volumes were calculated using the formula length × width × height × 0.71. To compare any differences between the methods according to testicular size, the patients were divided into six groups according to the ultrasound-determined volume: less than 5, 5 to 10, 10 to 15, 15 to 20, 20 to 25, and 25 cm³ or more.

The mean volume by orchidometry and ultrasonography was 18.7 and 13.7 cm³ for the right testis and 18.0 and 12.5 cm³ for the left, respectively, and was significantly overestimated by orchidometry (P <0.0001; by 5.1 cm³ on the right and 5.5 cm³ on the left). The largest differences between methods were observed for volumes of 10 to 15 cm³ on the right and 5 to 10 cm³ on the left. The difference between the methods decreased as the ultrasound-determined volume increased (r = 0.636 on the right; r = 0.598 on the left testis). Nonetheless, the Prader orchidometric measurements showed a strong correlation with the ultrasound measurements (r = 0.707 on the right; r = 0.746 on the left).

The testicular volume estimated by Prader orchidometry correlated closely with the measurements by ultrasonography. However, the orchidometer overestimated the testicular volume, especially in small testes.