Immunological Methods in Sexual Assault Investigations

The sexual assault forensic examination kit is the starting point for all immunological analysis. It is collected by a trained sexual assault nurse examiner or forensic physician, typically within 72 to 120 hours of the reported assault, though trace biological material has been recovered from clothing weeks or months later under dry storage conditions. The kit contains multiple swab pairs (vaginal, cervical, anal, oral, and external genital), a reference blood tube and buccal swab from the survivor, fingernail scrapings, and any clothing or bedding submitted with the survivor. Each item is labelled, sealed, and transferred under signed chain of custody to the forensic biology laboratory.

On receipt, the laboratory conducts a triage examination. Garments are examined under white light and long-wave ultraviolet (Wood's lamp) for stains that fluoresce. This is a presumptive step: semen, saliva, urine, and many non-biological substances can fluoresce, so the UV scan identifies areas for targeted sampling rather than confirming any specific fluid. Swabs are examined visually for visible staining or deposits. Acid phosphatase colour spray, which produces a purple reaction with prostatic acid phosphatase in semen, is another presumptive test applied to clothing in some protocols. Presumptive-positive areas are mapped on a body diagram or garment diagram, which becomes part of the case file.

The triage decisions determine which exhibits consume the limited biological material for DNA. Since DNA extraction is destructive, the wrong exhibit selection can exhaust the sample on a low-yield garment while a more probative swab is not tested. Immunological screening, by directing the analyst to confirmed semen-positive areas, protects against this. In high-volume units operating under caseload pressure, lateral-flow PSA strips have largely replaced the more time-consuming tube-based acid phosphatase protocol as the first confirmatory step.

Prostate-specific antigen is a 33 kDa serine protease secreted at concentrations of 0.5 to 5 mg/mL into seminal plasma. Blood serum contains PSA at much lower concentrations (below 4 ng/mL in healthy males), and very low concentrations can be detected in female periurethral glands, but the concentration in semen is orders of magnitude higher. This differential underpins the forensic utility: a positive PSA result on a vaginal swab or garment stain, at any concentration significantly above the serum baseline, is a strong indicator of semen.

The ABAcard p30 lateral-flow strip is among the most widely used forensic PSA tests. It uses a sandwich immunoassay format on a nitrocellulose membrane: a stain extract is applied to the sample pad, migrates past a zone of colloidal gold-labelled anti-PSA antibodies, and then reaches a line of immobilised anti-PSA capture antibodies. If PSA is present, it bridges the labelled and capture antibodies, forming a visible coloured band at the test line. A separate control line confirms the assay has run correctly. The strip is read visually at ten minutes. A positive result requires the test line to be visible regardless of intensity.

Sandwich ELISA offers higher sensitivity and is quantitative, allowing the analyst to report the concentration of PSA in the extract. Some protocols use a quantitative ELISA result to guide the volume of extract submitted for DNA in order to optimise input into the STR typing reaction. ELISA requires a microplate reader, incubation steps, and a trained analyst, so it is better suited to a reference laboratory than to triage at an examination centre.

PSA is relatively stable under dry conditions and has been detected on fabric stains aged six months to several years. Heat, humidity, and biological contamination degrade it faster. A negative PSA result on an aged, wet-stored, or heavily contaminated sample does not exclude the prior presence of semen. This limitation must be stated when reporting negative findings, as the absence of PSA is not the same as confirmed absence of semen.

ABO antigens are expressed on the surface of red blood cells in all individuals. In secretors, the same antigens are released into body fluids at detectable concentrations. Approximately 80 percent of people are secretors of their blood group substance. In a semen stain from a group A secretor, A antigen is present in the seminal plasma and can be detected by a serological technique.

The classic forensic method is inhibition agglutination. A small portion of the dried stain is extracted in saline. The extract is incubated with a known quantity of anti-A or anti-B antibody (depending on the antigen being sought). If the extract contains the A antigen, it binds and neutralises some of the anti-A antibody. When a suspension of known A-positive red blood cells is added to the mixture, fewer free antibody molecules are available to agglutinate them. The degree of agglutination is compared to control wells without stain extract: reduced agglutination in the stain well signals that A antigen was present. The process is repeated with anti-B antibody to determine the full ABO group of the stain donor.

ABO typing of stains has been used in forensic casework since Karl Landsteiner demonstrated in 1901 that blood types could be distinguished serologically. The method reached its peak forensic utility in the pre-DNA era when it was the primary tool for associating a stain with a suspect or excluding one. Today it is used for two specific purposes: exclusion of the survivor's own body fluids from the evidentiary stain (if the survivor is group A and the stain donor is group O, the stain is not from the survivor) and prioritisation of exhibits for DNA analysis where laboratory capacity is limited.

Sexual assault cases frequently involve body fluids other than semen. Saliva may be deposited through biting, kissing, or oral sexual acts. Blood may be present from injury or menstrual contamination. Vaginal secretions are present on any vaginal swab and must be distinguished from other contributors. Urine may be deposited. Each has a specific immunological marker for confirmatory identification.

Saliva is confirmed by detecting the salivary isoform of alpha-amylase. Salivary amylase is present at concentrations of approximately 100 to 700 U/mL in saliva, compared to less than 5 U/mL in most other body fluids. The Phadebas press test is an enzyme activity assay, not an immunoassay, but the RSID-Saliva lateral-flow strip uses anti-human salivary amylase antibody in an immunochromatographic format and is specific to the salivary isoform. A positive RSID-Saliva result confirms saliva, not just amylase activity, and does not cross-react with pancreatic amylase.

Blood is confirmed by haemoglobin-specific assays. The Hexagon OBTI lateral-flow strip detects human haemoglobin using anti-human haemoglobin antibodies and is species-specific, unlike the catalytic presumptive tests such as luminol and Kastle-Meyer reagent that react with haemoglobin from any species. Vaginal secretions are confirmed by detection of human vaginal mucosa alpha-amylase using isoform-specific antibody, or by microscopic identification of vaginal epithelial cells with cornified cell morphology.

The RSID series (Rapid Stain Identification) of lateral-flow strips, developed by Independent Forensics (US), covers semen, saliva, blood, menstrual blood, and urine in a common cassette format using species-specific antibodies. Their specificity for human-only antigens eliminates false positives from animal sources, which is an issue with catalytic presumptive tests in rural or agricultural environments. Multiple jurisdictions, including the FBI laboratory in the US and several National Policing Improvement Agency-accredited laboratories in England, have validated these strips for casework use.

The immunological findings in a sexual assault case are only as strong as the chain of custody that supports them. Chain of custody is the continuous, documented record of who had possession of each biological exhibit, when, and in what condition. A break in the chain does not automatically invalidate a finding, but it creates an opening for a defence challenge that the exhibit was contaminated, switched, or tampered with. In practice, courts weigh chain-of-custody gaps against the other evidence; a documented gap over a locked freezer access log, with no evidence of tampering, is treated differently from an undocumented gap during transit.

In India, the Bharatiya Sakshya Adhiniyam 2023 governs the admissibility of documentary and forensic evidence. Section 39 deals with expert opinion, and the courts have consistently required that the expert witness describe the handling of the exhibit from collection to analysis. The Bharatiya Nagarik Suraksha Sanhita 2023 governs the procedural requirements for collection and submission of samples to forensic science laboratories. Guidelines from the Ministry of Health and Family Welfare specify the format of the SAFE kit, the packaging requirements, and the temperature conditions for biological evidence storage.

In England and Wales, the Police and Criminal Evidence Act 1984 Code B and the Forensic Science Regulator's Codes of Practice set out the minimum documentation requirements. In the United States, the Violence Against Women Act mandates standardised SAFE kit collection protocols in federally funded programmes, and the FBI's Quality Assurance Standards for Forensic DNA Testing Laboratories specify chain-of-custody documentation for DNA exhibits, which extends to the immunological screening results that generated the DNA submission. In the European Union, Article 8 of Directive 2011/36/EU on human trafficking and associated guidelines from ENFSI apply to cross-border sexual assault evidence.

Immunological body-fluid identification and DNA profiling are sequential steps in the same analytical pathway, not alternatives. The immunological result answers the question of what biological material is present; DNA answers the question of whose material it is. In the absence of immunological confirmation, a DNA profile from a vaginal swab cannot be attributed to any particular body fluid. The DNA might represent a blood stain, a saliva smear, or skin cells transferred by touch, each of which carries a different evidential significance for the acts alleged.

The laboratory sequence is designed around this dependency. After triage and presumptive screening, confirmatory immunological assays are performed on the most probative aliquots from each exhibit. The portion of the extract used for immunological testing is kept small and non-destructive where possible (lateral-flow strips use microlitre volumes). The remaining extract is submitted for DNA isolation. The DNA extraction protocol is designed to separate cell types where a differential extraction is appropriate: in a semen-on-vaginal-swab scenario, differential lysis separates sperm cells (which lyse more slowly under standard conditions) from epithelial cells, yielding a sperm fraction enriched for the depositor's DNA and an epithelial fraction representing the survivor.

Where PSA is positive but spermatozoa are absent under microscopy (consistent with an azoospermic or vasectomised donor), the entire extract proceeds as a single fraction. The immunological result confirming semen ensures the analyst documents the absence of sperm cells and does not assume the PSA positive is a false result. In these cases the STR profile from the non-sperm fraction is the primary evidential result.

The final case report integrates both layers: the body-fluid identification results (with the assay, the control results, and the interpretation), the ABO typing result if performed, and the DNA profiling result with its statistical interpretation. In jurisdictions where courts require a body-fluid finding as a foundation for the DNA evidence, the immunological section of the report is not a preliminary or administrative section; it is part of the primary evidence. Forensic biologists who present only the DNA result without the immunological foundation risk having the DNA evidence excluded or reduced in weight.