Comparison of Radial Diffusion, Rapid Stain Identification Series Test for Saliva (RSID), and DNA Test Positivity in Saliva Specimens Over Time

Abstract

Saliva evidence can provide important information in the forensic investigation of sexual assaults.  Presumptive tests for saliva, sometimes used to determine when to process for DNA evidence, rely on the enzyme α-amylase found in the liquid fraction of saliva, the degradation rates of which are not well known.  It is possible, then, that DNA, found in the cellular fraction of saliva, may be present after the enzyme can no longer be detected.  This project examined two different presumptive assays for saliva, the Radial Diffusion assay and the Rapid Stain Identification Series Test for Saliva (RSID) assay in order to compare the test positivity rates against the persistence of cellular DNA.  Presumptive assays were tested at 7 different time points over the course of 68 hours (two subjects) and 93 hours (one subject) for saliva deposited on human skin, and at various time points for 40 days for saliva deposited on glass slides.  On glass, both types of presumptive assays showed positive results, and sufficient DNA (∼0.045ng for Y-filer) was recovered up to day 40.  On human skin, the test positivity of both presumptive assays and DNA recovery varied widely between subjects and with sample location on the subject.  In the majority of cases (88%), DNA was recovered for at least 35 hours.  In nearly half of cases (44%), sufficient DNA was recovered at either the same time point or after both presumptive assays no longer showed positive results.  In all cases the RSID-Saliva assay outperformed the Radial Diffusion.  The results of this study indicate that forensically-useful DNA does in fact remain on the skin after the presumptive assays are no longer positive.  This may support the practice of routine DNA testing even when presumptive assays are negative or inconclusive.