Figure 3. Total testosterone levels (±SD) in a proficiency sample (Y-04) distributed by the College of American Pathologists, and assayed using 14 different instruments in 1556 participating laboratories in 2018. The mean (±SD) value was 256 ± 51 ng/dL. The most frequent method (#6) was used by 353 laboratories. Method #4 is mass spectrometry, and was 7% lower than the mean of all methods. To convert total testosterone levels to nmol/L multiply by 0.0347