Lateral-Flow and Immunochromatographic Assays in Forensic Testing

A lateral-flow strip consists of four overlapping zones laminated onto a plastic backing card. From the sample end to the far end: the sample application pad, the conjugate pad, the nitrocellulose membrane carrying the test and control lines, and the absorbent wick. The wick maintains a pressure gradient that drives fluid forward continuously until the sample is exhausted or the absorbent is saturated.

The capillary flow rate is determined primarily by the nitrocellulose pore size. Faster-flow membranes reduce background colour but also reduce the dwell time that conjugate has to bind antigen, lowering sensitivity. Manufacturers optimise pore size for the target application: forensic body-fluid strips typically use slower flow rates than point-of-care clinical strips to maximise sensitivity with very low-concentration crime scene samples.

The test line is a narrow stripe of capture antibody that binds the antigen-conjugate complex as it flows past. The control line is coated with a secondary antibody, typically anti-species antibody, that captures the conjugated detection antibody directly, whether or not antigen is present. The control line exists to confirm that fluid has flowed to that point and that conjugate has not degraded. A strip with no control line visible is invalid: the flow may have been blocked or the conjugate inactivated, and neither a positive nor a negative test-line result can be trusted.

The reporter particle is the component that converts antigen binding into a visible signal. Two systems dominate forensic lateral-flow strips: colloidal gold and coloured latex microspheres.

Colloidal gold particles are gold nanoparticles, typically 20 to 40 nanometres in diameter, suspended in an aqueous buffer. At that size, gold exhibits strong surface plasmon resonance: the particles absorb and scatter green light and appear red to the eye. Antibodies adsorb onto the gold surface by electrostatic and hydrophobic interactions without covalent conjugation, which simplifies manufacture. Colloidal gold signal intensity is proportional to the amount of antigen within the sensitive range of the assay. The red line produced is the characteristic appearance of most forensic lateral-flow results. The colour is stable for extended periods after the strip dries, which matters for documentation when the strip is retained as evidence.

Coloured latex microspheres are polymer beads, typically 0.1 to 0.4 micrometres in diameter, loaded with coloured dye and coated with antibody. They produce a brighter, more intense signal than colloidal gold at the same antigen concentration, which is advantageous for detecting very low-abundance targets. However, they can produce higher background colour on the membrane, making faint positive lines harder to distinguish from background. Some manufacturers use latex in the control line and gold in the test line, or vice versa, to produce two-colour strips that are easier to read.

ABAcard HemaTrace, produced by Abacus Diagnostics and now distributed by Hemalex, is the most widely validated lateral-flow strip for forensic blood identification. Its detection antibody is a monoclonal antibody specific to the alpha-chain of human haemoglobin. Because the haemoglobin tetramer carries two alpha-chains and two beta-chains, and because the strip uses a sandwich architecture, a single haemoglobin molecule provides two alpha-chain epitopes, increasing sensitivity. The reported detection limit is approximately 1 in 1,000,000 dilution of blood, allowing detection from stains that have been aged, washed, or severely diluted.

The strip is used as part of a two-step screening workflow. A presumptive chemical test, such as the Kastle-Meyer phenolphthalein test or luminol, is applied first to indicate the possible presence of blood. A positive presumptive result is followed by ABAcard HemaTrace to distinguish human blood from other sources of peroxidase activity, such as plant material or rust. The HemaTrace result is still a screening result, not a species confirmation, but it narrows the interpretation significantly.

Degradation affects sensitivity but not specificity. Aged blood stains, stains exposed to bleach, or heavily contaminated stains may produce weak or absent test lines even when blood is present. A negative HemaTrace result from a heavily degraded sample does not exclude blood. Forensic examiners document all sample conditions and treat negative results from poor-quality samples with appropriate caution.

The Rapid Stain Identification (RSID) series, produced by Independent Forensics, provides separate lateral-flow strips for five body fluids. Each strip targets a protein or enzymatic marker chosen because it is specific to or highly enriched in that fluid. The series was designed to assist evidence triage: identifying which stains contain which body fluids before DNA profiling, so that limited sample material is spent on informative extractions. Studies from laboratories in the United Kingdom, United States, and the Netherlands have validated the RSID strips for forensic use, and several national accreditation frameworks formally accept them as screening methods.

The forensic value of the RSID series goes beyond simple identification. Body-fluid context shapes the interpretation of DNA profiles and informs the reconstruction of events. A semen stain on an item can be significant differently depending on whether a saliva stain is also present on the same surface. Running RSID screens in parallel across multiple stain areas, before committing any stain to DNA extraction, is standard practice in sexual-assault case examination. The RSID strips consume between 10 and 50 microlitres of extract, small enough to allow a parallel DNA extraction from the same primary extraction.

Drug-screening lateral-flow strips use a competitive immunoassay rather than the sandwich architecture described above. In a competitive assay, the test line is coated with drug-protein conjugate, not capture antibody. The conjugated antibody in the conjugate pad binds either free drug from the sample or the immobilised drug conjugate at the test line. When drug is absent from the sample, all conjugate binds the test line and the line is strongly coloured. When drug is present, free drug competes with the immobilised conjugate for the antibody, reducing test-line signal. A positive drug result therefore appears as a faint or absent test line, opposite to the sandwich-assay convention.

Multi-panel drug-screening strips carry multiple competitive test lines on a single strip, each targeting a drug class: opiates, benzodiazepines, amphetamines, cannabinoids, cocaine metabolites, and others. These are widely used in roadside impairment testing, custody suites, and workplace screening programs. In England and Wales, roadside drug testing is governed by the Road Traffic Act 1988 as amended; in the United States, federal workplace drug testing is governed by Substance Abuse and Mental Health Services Administration (SAMHSA) guidelines; in India, roadside testing authority derives from Section 185 of the Motor Vehicles Act 1988 as amended, and in the European Union, individual member states govern roadside testing with frameworks that broadly follow the DRUID project recommendations.

Cross-reactivity in competitive drug strips is a material concern. Antibodies raised against a drug or its metabolite also bind structurally related compounds. Ibuprofen can produce a false positive for cannabinoids in some strip formats. Pseudoephedrine can produce a positive for amphetamines. Poppy seed consumption raises urinary morphine above the screening threshold used in many strips. All positive drug-screen results must be confirmed by a laboratory method that identifies the specific compound, typically gas chromatography-mass spectrometry or liquid chromatography-tandem mass spectrometry, before any disciplinary, criminal, or regulatory action can be taken.

Lateral-flow strips are validated as screening tests, not as confirmatory tests. The distinction is not procedural convention; it reflects the fundamental technical limitations of antibody specificity. Any lateral-flow antibody will bind some structurally similar molecules in addition to its target, producing false-positive results at some non-zero frequency. The rate is low for well-validated forensic strips in clean samples, but it is not zero, and it rises with sample complexity, degradation, and contamination. A screening test must therefore be followed by a confirmatory test that uses a different analytical principle to verify the identity of the detected substance.

For body fluids, confirmatory methods include enzyme-linked immunosorbent assay (ELISA), which provides a quantitative result with a defined sensitivity and specificity; immunohistochemistry or immunocytology on stain material; and, ultimately, DNA profiling, which establishes the identity of the biological source rather than the fluid type. For drugs, confirmatory methods are invariably chromatographic: GC-MS (gas chromatography-mass spectrometry) or LC-MS/MS (liquid chromatography-tandem mass spectrometry) provide definitive molecular identification based on retention time and mass spectral fragmentation pattern.

The legal status of unconfirmed lateral-flow results varies by jurisdiction but the direction is consistent. Under the Bharatiya Sakshya Adhiniyam 2023, expert opinion must meet standards of reliability that unconfirmed screening results do not independently satisfy. United States federal courts applying Daubert criteria routinely exclude drug-screen results that lack GC-MS confirmation. United Kingdom Crown Prosecution Service guidance requires confirmatory testing before any body-fluid identification is included in a case file as fact. European Court of Human Rights jurisprudence on fair trial rights has addressed reliance on unconfirmed forensic screening results as a fair-trial concern. The practical consequence is that a forensic laboratory that reports a lateral-flow result as a conclusion, rather than as a preliminary observation requiring confirmation, exposes its findings to successful challenge.

The appropriate position of lateral-flow testing in a forensic workflow is early-stage triage: identify which stains are worth further analysis, direct limited sample material to the most informative extractions, and provide the investigative team with rapid preliminary information. Lateral-flow results recorded in case notes as observations support the narrative of why particular stains were selected for confirmatory testing. They do not, standing alone, constitute conclusions about the presence of blood, semen, or drugs.