PSA and ROC curve: Validation and cutoff for forensic sexual assault cases through sequential testing

Sex crime investigations often rely on evidence involving minimal amounts of seminal material, making it necessary to use sensitive biomarkers to detect semen. Thanks to its high concentration, prostate-specific antigen (PSA) has been extensively utilized as a forensic marker, but there remains a lack of consensus regarding its diagnostic cut-off value. The technique proposed in this study applies a tiered diagnostic algorithm that combines a highly sensitive screening assay with a highly specific assay. The objective was to validate PSA quantification as a screening tool and establish an optimal cut-off value based on the receiver operating characteristic curve. A total of 460 forensic samples from sex crime investigations were analyzed for PSA quantification using electrochemiluminescence immunoassay (ECLIA). Optical microscopy was used as the reference standard to detect spermatozoa. Receiver operating characteristic curve analysis established a cut-off value of 0.085 ng/mL, with an area under the curve (AUC) of 0.848, a sensitivity of 82.8%, and a negative predictive value of 92.8%, showing diagnostic performance in line with international standards. The established cut-off value was lower than those previously documented and made it possible to increase the detection of potential semen in samples, doubling the number of positive identifications. In child victims, PSA detection is particularly relevant, given that endogenous secretion begins around the age of 9. Presence in children, even at minimal levels, may be indicative of adult male semen. These findings confirm the role of PSA as a sensitive and reliable screening test in forensic diagnostics.