Comparative Diagnostic Performance of the Acid Phosphatase Test and Seminal PSA Rapid Test in Forensic Sexual Assault Swab Samples at Udon Thani Hospital

Abstract

        The acid phosphatase test (ACP test) is widely used as a presumptive screening method for detecting seminal stains in sexual assault investigations; however, its limited specificity necessitates confirmatory testing with the seminal PSA test. This study aimed to compare the diagnostic performance of the ACP test and the seminal PSA test and to evaluate an appropriate ACP reaction time cutoff for screening purposes. An analytical cross-sectional study was conducted using 152 forensic swab specimens from sexual assault cases submitted to Udon Thani Hospital during fiscal years 2024–2025. ACP reaction times were recorded and compared with seminal PSA test results as the reference method, and diagnostic parameters including sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), accuracy, and likelihood ratios were calculated. The seminal PSA test was positive in 46 samples (30.3%). Using the conventional ACP positivity criterion (color development ≤ 60 seconds) yielded high sensitivity (91.3%) but low specificity (50.9%), whereas applying a shorter cutoff (≤ 30 seconds) markedly improved specificity to 96.2%, with an overall accuracy of 85.5% and a positive likelihood ratio (LR+) of 16.03. Samples with ACP reaction times ≤ 30 seconds were strongly associated with PSA positivity (87.5%, p < 0.001), indicating a substantial reduction in false-positive results. These findings suggest that adopting a shorter ACP reaction time cutoff (≤ 30 seconds) may improve specificity and overall diagnostic accuracy while maintaining the ACP test as a presumptive screening tool; nevertheless, confirmatory testing with the seminal PSA test remains essential in all cases to ensure the highest reliability of forensic interpretation.