Pre-ejaculate – an overlooked body fluid with high amounts of DNA

Pre-ejaculate is a clear, sperm-free liquid secreted by the male bulbourethral glands. Since it is currently not a subject of routine forensic testing, pre-ejaculate is often overlooked. The aim of this study was to characterize the forensic value of pre-ejaculate: Pre-ejaculate from 58 healthy male donors (aged 20-59) was analyzed with respect to DNA yield and the reactivity of various tests specific for seminal fluid, semen and sperm, respectively. DNA was subsequently extracted, quantified, and subjected to routine forensic STR analysis. We found that the tests for the presence of seminal fluid showed varying reactivity for pre-ejaculate: While the test for semenogelin returned negative in all samples, the test for PSA showed a positive reaction in 84.5% of the samples. The acid phosphatase test returned positive in 29.3% of the samples. Furthermore, ≤ 436 ng DNA were detected, and full STR profiles were obtained in 49 of the 58 analyzed samples. Taken together, these findings show that pre-ejaculate contains high levels of DNA and, thus represents a relevant body fluid in the forensic investigation of cases of sexual assault. The varying sensitivity of tests for seminal fluid implies that positive results may originate from pre-ejaculate even in the absence of seminal fluid or semen. Conversely, negative results may lead to the exclusion of samples from further STR analysis, with the risk of missing crucial case information. Consequently, additional studies are required to confirm the presence of this body fluid and to enable a reliable differentiation between pre-ejaculate and semen.