Immunoassays in Forensic Science: Principle, Types, Techniques and Applications

Every immunoassay rests on one event: a specific antibody binding its target antigen. The Y-shaped immunoglobulin uses its two variable Fab arms to lock onto an epitope through hydrogen bonds, van der Waals forces, electrostatic and hydrophobic interactions. Binding is non-covalent and reversible, described by an equilibrium dissociation constant (Kd) in the nanomolar to picomolar range for analytically useful antibodies.

That binding event is invisible on its own. To read it, the assay couples a label to either the antigen or the antibody. The label is what is actually measured: gamma emission from 125-I in RIA, colour developed from an enzyme-substrate reaction in ELISA, the change in fluorescence polarization when a small labelled tracer is bound by a large antibody in FPIA, or the appearance of a coloured band where colloidal gold has accumulated in a lateral-flow strip.

Two consequences follow that NTA likes to test. First, specificity comes from the antibody, so cross-reactivity with structurally similar molecules (the classic poppy-seed false positive for opiates, or pseudoephedrine reading as amphetamine) is intrinsic and cannot be engineered away completely. Second, sensitivity comes from the label, which is why RIA dominated forensic toxicology in the 1970s and 1980s, why chemiluminescent ELISA replaced it once non-radioactive labels caught up, and why LC-MS/MS now sits downstream as the confirmation method.

NTA's preferred frame is the two-axis classification. Memorise both axes, because MCQs mix them ("EMIT is a ... immunoassay" can take either axis as the answer).

Axis 1, format.

• Competitive. Labelled antigen and sample antigen compete for a limited number of antibody binding sites. More analyte in the sample means less labelled antigen captured, so signal is inversely proportional to analyte concentration. Used for small molecules (drugs, hormones) where a single antibody site is all there is room for.
• Non-competitive (sandwich). A capture antibody bound to the solid phase binds the analyte, then a second labelled detection antibody binds a different epitope. Signal is directly proportional to analyte. Used for larger antigens (proteins, hCG, PSA, viruses) that present at least two non-overlapping epitopes.

Axis 2, separation.

• Heterogeneous. Requires a physical wash step to separate antibody-bound label from free label. ELISA and RIA are heterogeneous. More sensitive, more steps, slower throughput, plate-based.
• Homogeneous. No wash step; the label's signal changes only when binding occurs. EMIT (enzyme activity is modulated by antibody binding) and FPIA (polarization changes when the small tracer is bound by the large antibody) are homogeneous. Fast, automatable on clinical chemistry analysers, slightly less sensitive.

A few sub-points worth a slot in your memory.

ELISA formats, in one breath.

• Direct. Antigen on plate, enzyme-labelled primary antibody. One-step, fast, lower sensitivity.
• Indirect. Antigen on plate, unlabelled primary antibody, enzyme-labelled secondary antibody. Used to detect antibodies (serology), not antigens. Higher sensitivity through signal amplification.
• Sandwich. Capture antibody on plate, antigen, enzyme-labelled detection antibody on a different epitope. Highest specificity for proteins.
• Competitive ELISA. Sample antigen competes with labelled antigen for the antibody; signal goes down as analyte goes up. The format used for small-molecule drugs.

Why RIA fell out of favour in India. Radioactive 125-I needs an AERB licence, lead shielding, dedicated waste pathways and a short shelf life (60-day half-life). Once chemiluminescent ELISA and LC-MS/MS reached comparable sensitivity, most CFSL toxicology units retired RIA. A few endocrine labs still run it for the steroids it does best.

The single most-tested fact across this bullet is the screen-then-confirm workflow. Immunoassays are presumptive. A positive result on an immunoassay screen must be confirmed by a chromatographic technique (GC-MS or LC-MS/MS) before it goes into a Section 293 CrPC report or a Section 329 BNSS expert report. The two-tier pipeline is also the reason every screening protocol is locked down under formal method validation and proficiency testing, so a defence counsel cannot challenge the cut-off, the calibrator or the controls used on the day.

The forensic homes for immunoassays:

1. Drugs-of-abuse screening. Urine, blood and oral-fluid panels for opiates, cocaine metabolites, cannabinoids (THC-COOH), amphetamines, methamphetamine, benzodiazepines, barbiturates, methadone and PCP. Used at CFSL toxicology divisions, state FSL drug sections, hospital emergency toxicology and de-addiction centres. Positive screens go to GC-MS or LC-MS/MS for confirmation and quantitation.
2. Body-fluid identification. Sandwich ELISA against semen-specific antigens (prostate-specific antigen, semenogelin) and saliva-specific antigens (alpha-amylase) for sexual-assault casework. Replaces or supplements older acid-phosphatase and starch-iodine tests.
3. Sexual-assault kit serology. Lateral-flow rapid tests for PSA on swabs and stains at the preliminary examination stage, before DNA workup at the CFSL or state DNA unit.
4. Wildlife forensics. ELISA against species-specific serum proteins to identify suspected wildlife meat or blood seized under the Wildlife Protection Act 1972.
5. Anti-doping and post-mortem toxicology. Same screen-then-confirm pipeline, with immunoassay narrowing the GC-MS or LC-MS/MS targets.
6. Therapeutic drug monitoring. FPIA and EMIT on hospital analysers for drugs (digoxin, theophylline, immunosuppressants) that occasionally surface in post-mortem casework as a cause-of-death contributor.

Two failure modes NTA tests as MCQ traps:

• Cross-reactivity. A polyclonal opiate antibody reads codeine, morphine, heroin metabolites and (at low specificity) poppy-seed-derived opiates. Pseudoephedrine cross-reacts in some amphetamine immunoassays. The lab reports "presumptive positive for opiate class" and lets chromatography speciate.
• Hook effect. In sandwich assays, extremely high analyte concentration saturates both the capture and the detection antibody independently and can paradoxically lower signal. Mostly a clinical-chemistry issue but worth knowing for the conceptual MCQ.

Immunoassays sit at three places in the Indian forensic workflow.

First, the field. NDPS rapid test kits (lateral-flow cards or coloured-tube presumptive kits) are issued to state police narcotics units and railway police. They give a colour or a band reading at the point of seizure, enough to support a Section 50 NDPS search and the initial recovery memo. The seized material then travels to the state FSL or CFSL for instrumental confirmation.

Second, the hospital and emergency-toxicology bench. AIIMS, PGI Chandigarh, JIPMER and the larger metropolitan hospitals run EMIT or FPIA urine drug-of-abuse panels on Abbott, Roche or Siemens clinical chemistry analysers. Results from these hospital panels often enter forensic casework when an unconscious patient turns out to be a poisoning victim.

Third, the CFSL and state FSL toxicology and serology divisions. ELISA microplate readers are standard in the toxicology and biology sections of CFSL Hyderabad, CFSL Chandigarh and the larger state FSLs (Maharashtra, Tamil Nadu, Karnataka, Delhi). They run drugs-of-abuse panels on blood and urine, semen-marker ELISA on sexual-assault swabs, and species-identification ELISA on suspected wildlife samples. Every assay sits inside an ISO/IEC 17025 / NABL validation regime that specifies method validation, controls per plate, calibrator verification and proficiency testing under DFSS or the Bureau of Indian Standards. Cross-reactivity caveats (the poppy-seed opiate trap, the pseudoephedrine-amphetamine overlap) are documented during method validation and proficiency testing so the lab's reported specificity, sensitivity and cut-offs are defensible in court.

The Section 293 CrPC (now Section 329 BNSS) expert report from a Government Scientific Officer treats immunoassay results as screening-grade evidence. A confirmed GC-MS or LC-MS/MS report is what carries weight in cross-examination.