Immunological Identification of Semen and Saliva

PSA is encoded by the KLK3 gene on chromosome 19q13.4 and belongs to the tissue kallikrein family. It is a 34 kDa serine protease expressed almost exclusively by prostatic epithelial cells. In seminal plasma, PSA functions physiologically to liquefy the seminal coagulum formed by fibronectin-rich secretions from the seminal vesicles: PSA cleaves semenogelin I and II, allowing sperm cells to become motile after ejaculation. This biological role requires high local concentration, which is why seminal plasma contains PSA at 0.5 to 2 mg/mL, orders of magnitude above serum PSA levels in healthy men (typically below 4 ng/mL in serum).

The forensic significance of PSA is threefold. First, it is prostate-specific: tissues outside the prostate produce PSA at trace concentrations that, in health, do not approach forensic detection thresholds. Female periurethral glands (the female prostate or Skene glands) can secrete PSA, but levels in female fluids are typically below 1 ng/mL, well under the 4 ng/mL forensic threshold of the ABAcard assay. Breast tissue and some breast tumours express PSA at elevated levels in pathological states, but this is not encountered in routine casework. Second, PSA is stable: it persists in dried stains for months under ambient conditions and has been detected in stains more than 30 years old stored in controlled environments. Third, PSA levels do not depend on sperm count: a vasectomised donor or an azoospermic individual produces PSA-containing seminal fluid at normal concentrations even though the ejaculate contains no sperm.

Two immunoassay formats are used in forensic PSA testing: ELISA and lateral-flow immunochromatography. Each has distinct operational characteristics that affect laboratory workflow and the weight a result carries in court.

The ABAcard p30 assay uses a sandwich lateral-flow format. The extract is applied to a sample pad; it migrates along the nitrocellulose membrane and encounters a colloidal-gold-labelled anti-PSA monoclonal antibody. If PSA is present at or above 4 ng/mL, it binds the labelled antibody, and the complex migrates to the test zone where a second anti-PSA antibody is immobilised, capturing the sandwich and forming a visible pink-red line. A procedural control line forms regardless of PSA concentration by capturing excess labelled antibody at the control zone. A valid result requires the control line to be present. The test line alone, control line absent, is an invalid result requiring repeat.

ELISA provides quantitative data and a lower detection threshold, typically 0.2 to 1 ng/mL depending on the kit and protocol. This is useful when a sample gives a borderline lateral-flow result, or when the analyst needs to distinguish between a low-level positive from a dilute sample and a possible cross-reactive matrix. Some laboratories use a two-step protocol: screen with the lateral-flow strip, confirm borderline or critical results with ELISA. Others use ELISA as the primary test for all high-priority cases.

Salivary alpha-amylase (sAA, EC 3.2.1.1) is the predominant protein in whole saliva, secreted by the three major salivary gland pairs (parotid, submandibular, sublingual) at a total secretion rate of 0.5 to 1.5 litres per day. Its physiological role is initiating starch digestion in the oral cavity. In forensic contexts, saliva deposits occur at bite marks, on envelopes, cigarette butts, drinking vessels, and during sexual assaults. Alpha-amylase is also present in urine, sweat, tears, and vaginal secretions at concentrations roughly 100 to 1000 times lower than in saliva, a difference that the RSID-Saliva assay exploits for selective detection.

RSID-Saliva is a lateral-flow strip using an anti-amylase monoclonal antibody conjugated to colloidal gold. The assay is validated to detect saliva at a dilution of 1:1000, which corresponds to approximately 1 ng of amylase protein loaded per strip. At this threshold, vaginal secretions and sweat, which contain amylase at 1/100th to 1/1000th the concentration found in saliva, do not normally produce a positive test line. The manufacturer's validation data show specificity of greater than 99% against non-salivary fluids tested under standard forensic dilution protocols.

The RSID-Saliva strip has a reading window of 10 minutes. Both the test line and the control line must be visible for a valid positive. A faint test line at any intensity is a positive result: the line darkness is not proportional to amylase concentration across the strip's detection range. A negative result is a control line present with no test line. If neither line appears, the strip is invalid. Laboratories record the result as positive, negative, or invalid, and invalid results are repeated with a fresh extract.

Azoospermia presents the clearest case for immunological over microscopic identification. In obstructive azoospermia (most commonly post-vasectomy), the seminiferous tubules produce sperm normally but the vas deferens is ligated or absent; the ejaculate contains full seminal plasma including PSA. In non-obstructive azoospermia (testicular failure), sperm production is absent or severely reduced. In both cases, prostatic contribution to the ejaculate is normal, and PSA immunoassay returns a positive result at full semen concentration. A sexual assault investigation that relies only on sperm microscopy will return a false negative for these donors. The PSA test is definitive confirmation regardless of sperm status.

Mixed body-fluid samples are common in sexual assault casework. A vaginal swab may contain semen, vaginal secretions, and traces of urine or blood in varying proportions. The immunoassay approach tests each body fluid independently: a separate extract or the same extract with different assay strips. A positive PSA result confirms semen in the mixture; a positive RSID-Saliva confirms saliva. The two results do not interfere with each other because the antibodies are highly specific to different target antigens.

The main interpretive challenge in mixed samples is estimating relative contribution. A positive PSA lateral-flow result from a highly diluted semen sample gives the same visible line as a concentrated one; the test is qualitative. Where quantification matters, ELISA is run on the same extract. Reported PSA concentration above 10 ng/mL in extract is consistent with primary semen deposit. Concentrations between 1 and 4 ng/mL in extract approach the threshold and may represent highly diluted semen or, in rare cases, female PSA. Concentrations below 1 ng/mL in extract on ELISA are reported as insufficient for confirmatory identification.

Forensic immunoassays must be validated before use in casework. For the ABAcard p30 assay, the Scientific Working Group for DNA Analysis Methods (SWGDAM) published validation guidelines that laboratories implement by testing known semen and non-semen samples across a range of dilutions, substrates, and storage conditions. Validation data establish the assay's sensitivity (minimum detection concentration), specificity (rate of false positives with non-semen fluids), and robustness (performance on aged, mixed, or environmentally compromised samples).

In the United States, the Daubert standard (Daubert v. Merrell Dow Pharmaceuticals, 1993) requires expert testimony to rest on methods that have been tested, have known error rates, are peer-reviewed, and are generally accepted. Both the PSA immunoassay and RSID-Saliva meet these criteria. In the United Kingdom, the Criminal Procedure Rules require experts to state the basis of their opinion, including method validation; accreditation to ISO/IEC 17025 by the United Kingdom Accreditation Service (UKAS) is required for forensic laboratories. In India, expert testimony on body-fluid identification under the Bharatiya Sakshya Adhiniyam 2023 (Section 39 et seq.) must disclose the method used and its scientific basis to be given full weight. The same principle of disclosed, validated methodology applies across all major jurisdictions.

Chain of custody requirements apply from collection through testing. A forensic laboratory receiving a sexual assault evidence kit documents the integrity of each swab packaging, records the extraction protocol, and retains a reference extract for potential retesting. The immunoassay strips, once read, are photographed and retained as physical exhibits where laboratory protocols permit. In some jurisdictions, the tested strip itself is admissible as an exhibit alongside the analyst's written report.

PSA immunoassay is highly specific for semen but is not entirely infallible. The documented sources of potential false positives include: female PSA from Skene gland secretions at borderline concentrations (addressable by quantitative ELISA), breast milk in individuals with elevated PSA due to hormone-related expression, and some therapeutic biologics that cross-react with anti-PSA antibodies used in older polyclonal assay formats. Modern forensic kits use monoclonal antibodies with narrower epitope specificity, which substantially reduces these cross-reactions.

Where a PSA or RSID-Saliva result is ambiguous, confirmatory approaches include microscopic identification of cellular material (spermatozoa, oral squamous epithelial cells), mRNA profiling using body-fluid-specific transcriptome markers, and DNA methylation-based tissue typing. mRNA profiling of seminal-specific transcripts (PRM1, PRM2, TGM4) and saliva-specific transcripts (HTN3, STATH) has been developed as a molecular alternative that identifies body fluids by gene expression rather than protein antigen, offering discrimination between anatomically similar fluids. These methods complement rather than replace immunoassay: the antibody test is fast, cheap, and field-validated; the molecular confirmatory test is run when the immunoassay result is disputed or when additional biological information is needed for case interpretation.