Verification of the Phadebas® forensic press test for the screening of human saliva

The Phadebas® Forensic Press Test (PFPT) is widely employed in forensic laboratories for presumptive saliva screening by observation of α-amylase activity. This enzyme hydrolyses starch embedded in the test paper, resulting in a blue reaction when positive. Although designed to assist in locating areas of possible saliva staining on exhibits, other α-amylase containing substances such as urine, vaginal secretions, faeces, and laundry detergents have also produced positive results, limiting the test's specificity. This study aimed to verify the test's suitability as a presumptive tool within the evidence recovery unit of a forensic laboratory. A range of human biological fluids and non-biological substances were tested across 10 different substrate types. Substances evaluated for specificity were in part informed by gaps identified in the literature to date and by operational casework relevance. The sensitivity and specificity were evaluated to assess practical reliability in forensic casework. Results demonstrated variability in the test's sensitivity across the 10 different substrate types, showing that material composition influences the visibility and clarity of reactions. Reactivity was observed with several non-saliva substances, including semen, breast milk, faeces, and laundry detergent. This supports earlier study findings on the test's low specificity. Given these outcomes, the test is not reliable for confirming the presence of saliva. However, it remains valuable in guiding targeted sampling for subsequent DNA analysis. Given the low specificity and limited sensitivity in certain substrates, scientists should be aware of the test's limitations and consider the case context, the substrate, and any alternative explanations for positive results.