Error calculation for the PSA quotient

Prostate specific antigen (PSA) assays have significant measurement errors, but the error associated with the PSA quotient (free to total PSA) often remains unknown. We used both Gaussian error calculation and measurement of imprecision to investigate the level of error associated with the PSA quotient. Surprisingly, we found that the error of the PSA quotient at low levels is markedly smaller than that of the total PSA value. The PSA quotient should be calculated and considered as a clinically relevant value.