Forensic toxicology

Forensic Science (B.Sc) · Biology and chemistry · free study notes for exams, placements and interviews

In short

Forensic toxicology is the detection, identification and quantification of poisons, drugs, alcohol and their metabolites in biological specimens, and the interpretation of those findings for courts. It answers three questions: was a poison present, in what amount, and could it explain the death, illness or behaviour. In India it is practised in the toxicology divisions of the Central and State Forensic Science Laboratories (CFSL/FSL), Regional FSLs, police hospitals and NDPS testing units, and is examined in B.Sc/M.Sc Forensic Science papers, UGC-NET Forensic Science, and the recruitment tests for Scientific Assistant/Scientific Officer (Toxicology) in FSLs. It is governed by the Bharatiya Nyaya Sanhita, 2023 (poisoning offences), the Bharatiya Nagarik Suraksha Sanhita, 2023 (postmortem and expert evidence), the NDPS Act, 1985, the Drugs and Cosmetics Act, 1940 and the Poisons Act, 1919.

Explained

Scope, branches and basic concepts

Poison: any substance that, in a sufficient dose and by a suitable route, produces harmful or fatal effects. There is no chemical class of "poisons" — dose and route decide ("the dose makes the poison", Paracelsus).
Branches: *postmortem (medico-legal) toxicology* — cause of death; *clinical/behavioural toxicology* — living poisoning, overdose, drug-facilitated crime; *forensic drug testing* — workplace, sports, driving; *environmental/regulatory toxicology* — food, water, adulterants; *veterinary toxicology* — cattle poisoning cases, common in Indian rural FSL workload.
Toxicokinetics (ADME): absorption, distribution, metabolism, excretion. Toxicodynamics: the mechanism of harm at the receptor or enzyme.
Key quantities: LD50 (dose killing 50% of test animals, mg/kg), therapeutic/toxic/fatal ranges, half-life (t½), volume of distribution (Vd), first-order vs zero-order elimination (ethanol is zero-order).
Routes: oral (commonest in Indian suicidal poisoning), inhalation, injection, dermal, mucosal, rectal/vaginal.
Manner: accidental (pesticide handling, children), suicidal (pesticides, phosphides, sedatives), homicidal (arsenic, aconite, thallium — slow, tasteless agents preferred), stupefying (datura, benzodiazepines in robbery and sexual assault), abuse, and iatrogenic.

Classification of poisons

By organ/action: *corrosives* (strong acids — H₂SO₄, HNO₃, HCl; alkalis — NaOH, KOH; organic acids — oxalic, carbolic); *irritants* (metallic: arsenic, mercury, lead, copper; non-metallic: phosphorus; vegetable: croton, castor; animal: snake venom, cantharides); *neurotics* (cerebral: alcohol, opioids, barbiturates, datura; spinal: strychnine, nux vomica; peripheral: curare, conium); *cardiac* (oleander, aconite, digitalis); *asphyxiants* (CO, CO₂, H₂S, cyanide); *miscellaneous* (analgesics, antidepressants, pesticides).
By origin: inorganic (metals, acids, cyanide), organic synthetic (drugs, pesticides, solvents), plant (alkaloids, glycosides), animal (venoms), microbial toxins.
By volatility — the analytical classification that drives laboratory workflow:
1. Volatile poisons — ethanol, methanol, chloroform, kerosene, phenol, benzene, formalin: isolated by steam distillation or headspace.
2. Non-volatile organic poisons — alkaloids, barbiturates, pesticides, modern drugs: isolated by solvent extraction (Stas-Otto) or SPE.
3. Metallic poisons — As, Hg, Pb, Tl, Cu, Zn: isolated by wet digestion/ashing then AAS/ICP-MS.
4. Volatile inorganic/anions — cyanide, sulphide, phosphide: trapped as gas or by microdiffusion (Conway cell).
5. Miscellaneous — gases (CO as carboxyhaemoglobin, measured spectrophotometrically, not extracted).

Specimens, collection and preservation (Indian practice)

Postmortem viscera (standard set sent by the autopsy surgeon to FSL): stomach with contents, upper ~30 cm small intestine with contents, ~500 g liver (right lobe), one kidney (or half of each), ~100 mL blood, all available urine; brain in alkaloid/organochlorine cases; lung in inhalation/volatile cases; bone and hair in chronic/exhumation cases.
Preservatives: saturated common salt (NaCl) solution for viscera generally; rectified spirit for anything *except* alcohol, methanol and other volatile-poisoning cases (it would contaminate the result) and except phosphorus; sodium fluoride 1% w/v with potassium oxalate for blood alcohol (fluoride stops microbial fermentation/neoformation of ethanol and glycolysis, oxalate anticoagulates); phenyl mercuric nitrate / fluoride for urine.
Each container is separately sealed, labelled, sample seal impression sent separately, and dispatched with the requisition and inquest papers. The chain of custody record — who held it, when, seals intact — is as important as the analysis; a broken seal destroys the evidential value.
Ante-mortem (living) specimens: blood, urine (commonest for drugs of abuse — large volume, high concentration, long window), gastric lavage/vomit, hair (segmental analysis gives a timeline, ~1 cm ≈ 1 month of growth), nails, saliva, sweat, meconium for neonatal exposure, dialysate, and insect larvae (entomotoxicology) in decomposed bodies.
Vitreous humour is prized post-mortem: isolated, decay-resistant, resists postmortem redistribution; vitreous alcohol ≈ blood alcohol × ~1.2 (ratio varies, so it is used as corroboration, not substitution).

Analytical scheme: extraction, screening, confirmation

Step 1 — Case history and preliminary examination: symptoms, smell (bitter almonds → cyanide; garlic → phosphorus/organophosphate/arsenic; kerosene; phenol), colour of hypostasis (cherry-red → CO; brown → nitrites; blue → methaemoglobin-formers), tablet residue, pesticide container.
Step 2 — Isolation/extraction:
- *Steam distillation* for volatiles; the distillate is tested for ethanol/methanol/phenol.
- Stas-Otto process for non-volatile organics: tissue is minced, macerated with ethanol acidified with tartaric acid, filtered, evaporated; the residue is extracted with an organic solvent (ether/chloroform) at acidic pH → acidic and neutral drugs (barbiturates, salicylates); then made alkaline (ammonia/NaHCO₃) and re-extracted → basic drugs and alkaloids (morphine, strychnine, atropine, nicotine).
- *Modern replacements:* liquid-liquid extraction with buffer control, solid-phase extraction (SPE), QuEChERS for pesticides, protein precipitation and headspace sampling for volatiles.
- *Wet digestion* with HNO₃-H₂SO₄-HClO₄ or microwave digestion for metals.
Step 3 — Screening (sensitive, cheap, presumptive; may give false positives): colour/spot tests (Marquis — purple with morphine/heroin; Mecke; Simon's test distinguishes secondary amine methamphetamine/MDMA; Duquenois-Levine for cannabis; sodium nitroprusside; Reinsch test for As/Hg/Sb/Bi), crystal tests, microchemical tests, TLC (Rf value against standards, visualised with Dragendorff, ninhydrin, iodoplatinate), immunoassays — ELISA, EMIT, FPIA, and lateral-flow urine kits used for NDPS and hospital screening.
Step 4 — Confirmation (specific, court-defensible): GC-MS — the gold standard for volatiles, drugs of abuse and pesticides; LC-MS/MS — polar, thermolabile, low-dose analytes (benzodiazepines, LSD, new psychoactive substances); HPLC-UV/DAD; FTIR and Raman for bulk NDPS seizures; AAS/GF-AAS, ICP-MS, ICP-OES for metals; UV-Vis spectrophotometry for CO-Hb and nitrites; ISE/ion chromatography for fluoride and cyanide. The rule: one screening method plus one confirmation method of a *different* analytical principle, with mass spectral identification wherever possible.
Quality assurance: calibration curve with ≥5 levels, internal standard (often a deuterated analogue), matrix-matched calibrators, blanks, positive/negative controls, LOD and LOQ, recovery, precision, measurement uncertainty, and NABL accreditation to ISO/IEC 17025 for the laboratory (ISO 17020 for inspection bodies). Case files and raw data are retained for cross-examination.

Important poisons in Indian casework

Organophosphates and carbamates (malathion, chlorpyriphos, monocrotophos): inhibit acetylcholinesterase → muscarinic + nicotinic + CNS signs (SLUDGE: salivation, lacrimation, urination, defecation, GI cramps, emesis, pinpoint pupils, bronchorrhoea). Diagnosis supported by depressed plasma/RBC cholinesterase. Antidote: atropine + pralidoxime (oxime reactivates the enzyme before ageing). India's single largest poisoning burden.
Aluminium/zinc phosphide (grain fumigant tablets): liberates phosphine gas; garlicky breath, refractory shock, myocarditis; no specific antidote — high mortality; detection by silver nitrate paper test on gastric contents and GC-headspace for phosphine.
Pyrethroids, paraquat (progressive pulmonary fibrosis, late death), endosulfan and other organochlorines (now restricted/banned).
Alcohols: ethanol — commonest intoxicant; methanol — hooch tragedies, metabolised by alcohol dehydrogenase to formaldehyde and formic acid → metabolic acidosis and blindness; treated with ethanol or fomepizole (competitive ADH inhibition) plus bicarbonate and dialysis.
Opioids (heroin, tramadol, codeine): pinpoint pupils, respiratory depression, coma — the opioid triad; naloxone reverses. Marker metabolite 6-monoacetylmorphine (6-MAM) proves heroin rather than morphine or poppy intake.
Cannabis (ganja, charas, bhang), cocaine (metabolite benzoylecgonine), amphetamine-type stimulants, benzodiazepines (drug-facilitated sexual assault), and new psychoactive substances (mephedrone/"meow meow", synthetic cannabinoids) notified under the NDPS Act.
Metals: arsenic (homicidal, chronic — hair/nail/bone, Mees' lines; Reinsch, Gutzeit and Marsh tests historically, now AAS/ICP-MS), lead (industrial, basophilic stippling), mercury, thallium (alopecia), copper sulphate.
Plant and animal poisons: *datura* (atropine, hyoscine — stupefying/railway robbery, "mad as a hatter" anticholinergic picture), *Abrus precatorius* (ratti needle used for cattle poisoning), oleander (yellow oleander *Cerbera/Thevetia* — cardiac glycosides), aconite (Bish), nux vomica (strychnine), castor/ricin, *Cleistanthus collinus*, snake venom (neurotoxic cobra/krait, vasculotoxic viper — ELISA detection).
Gases: CO — cherry-red postmortem lividity, carboxyhaemoglobin saturation measured; cyanide — bitter almond smell, rapid death, antidotes sodium nitrite/sodium thiosulphate or hydroxocobalamin; H₂S in sewers and tanks.

Interpretation and Indian legal framework

Postmortem changes that mislead: postmortem redistribution (drugs such as tricyclics and digoxin diffuse from liver/lung into central blood, inflating the result — always prefer peripheral femoral blood); neoformation of ethanol by fermentation in decomposed bodies (a fluoride-preserved sample and a urine/vitreous cross-check separate this); drug instability (cocaine hydrolyses unless fluoride-preserved); embalming artefacts.
Tolerance means a concentration fatal to a naive person may be sub-therapeutic in a chronic user; interpretation must combine concentration, history, autopsy findings and scene.
A negative toxicology report does not exclude poisoning — the agent may be undetectable (insulin, succinylcholine), fully metabolised, or outside the screening panel.
Statutes and provisions: BNS, 2023 — administering poison/stupefying substance to cause hurt or to facilitate an offence, adulteration of food or drugs, and sale of noxious food and drink (the BNS replaced IPC sections 272-276, 284, 299-304, 324, 328 from 1 July 2024; cite the BNS section only when certain, as numbering changed). BNSS, 2023 — the provision for reports of government scientific experts being admissible without the expert's personal appearance (the former Section 293 CrPC), and postmortem/inquest procedure.
Other laws: Poisons Act, 1919 (possession and sale of listed poisons, with state rules); Drugs and Cosmetics Act, 1940 and Rules 1945 (Schedule H/H1/X drugs); NDPS Act, 1985 — small/commercial quantity thresholds notified for each drug decide punishment, and a government analyst's report is central; Insecticides Act, 1968; Food Safety and Standards Act, 2006; Motor Vehicles Act, 1988 Section 185 — driving offence at BAC above 30 mg per 100 mL of blood (30 mg/100 mL = 0.03% w/v).
Expert evidence: the toxicologist's report must state the method, the LOD/LOQ, the uncertainty, and the limits of interpretation; opinions on cause of death are formed jointly with the autopsy surgeon.

Key points

Poison - any substance causing harm at a sufficient dose by a suitable route; dose and route, not chemistry, define it.
Analytical classification - volatile, non-volatile organic, metallic, volatile-inorganic/anionic, and gases; it decides the extraction method.
Stas-Otto process - acid-alcohol maceration, then solvent extraction at acidic pH (acidic/neutral drugs) and at alkaline pH (alkaloids and basic drugs).
Steam distillation - isolation of choice for ethanol, methanol, phenol, chloroform and other volatiles.
Wet digestion then AAS/ICP-MS - the route for As, Hg, Pb, Tl and other metallic poisons.
Preservative rule - saturated NaCl for viscera; rectified spirit never in suspected alcohol/volatile poisoning; 1% sodium fluoride + potassium oxalate for blood alcohol.
Screening vs confirmation - presumptive colour tests, TLC and immunoassay screen; GC-MS or LC-MS/MS confirms; the two must use different principles.
GC-MS - the court-accepted gold standard, giving retention time plus a mass spectral fingerprint.
Organophosphate poisoning - cholinesterase inhibition, SLUDGE + miosis; atropine plus pralidoxime; India's largest poisoning load.
Aluminium phosphide - phosphine release, garlic odour, shock and myocarditis, no antidote; silver nitrate paper test.
Methanol - toxicity is from formic acid; antidote is ethanol or fomepizole plus bicarbonate and haemodialysis.
6-MAM - the metabolite that proves heroin use as distinct from morphine or codeine.
CO poisoning - cherry-red lividity, carboxyhaemoglobin measured spectrophotometrically; no extraction possible.
Postmortem redistribution - drug levels rise in central blood after death; use peripheral femoral blood.
Ethanol neoformation - microbial fermentation creates alcohol in decomposed bodies; fluoride preservation and vitreous/urine cross-checks expose it.
Negative report ≠ no poisoning - insulin, succinylcholine, fully metabolised or off-panel agents escape detection.
MV Act Section 185 - the drink-driving threshold is 30 mg ethanol per 100 mL blood.
NABL/ISO 17025 accreditation - the quality framework expected of an Indian toxicology laboratory.

Formulas and examples

Widmark equation: A = c × W × r ÷ 100 — A = amount of absorbed alcohol (g); c = blood alcohol concentration (mg/100 mL); W = body weight (kg); r = Widmark factor (volume of distribution, ≈ 0.68 for men, ≈ 0.55 for women, dimensionless); the ÷100 converts mg/100 mL into g/L, the unit c must be in (120 mg/100 mL = 1.2 g/L).

Back-extrapolation: C₀ = Ct + (β × t) — C₀ = BAC at the time of incident; Ct = measured BAC (mg/100 mL); β = elimination rate, 15 mg/100 mL/h (range 10-25); t = hours between incident and sampling.

BAC from a drink: c = (V × strength × 0.789 × 1000) ÷ (W × r) in mg/L, ÷10 for mg/100 mL — V in mL, strength as a fraction (v/v), 0.789 g/mL = density of ethanol.

Beer-Lambert law: A = ε c l — A = absorbance; ε = molar absorptivity (L mol⁻¹ cm⁻¹); c = concentration (mol/L); l = path length (cm). Basis of CO-Hb, nitrite and metal colorimetry.

Retardation factor (TLC): Rf = distance travelled by solute ÷ distance travelled by solvent front (dimensionless, 0-1).

Elimination kinetics: first order, C = C₀e^−kt, t½ = 0.693/k; ethanol is zero order, C = C₀ − βt.

Concentration from a dose: C = (D × F) ÷ (Vd × W) — D = dose (mg); F = bioavailability; Vd = volume of distribution (L/kg); W = weight (kg); C in mg/L.

Blood ↔ breath: BAC = BrAC × 2100 (the statutory partition ratio used by most breath analysers; actual ratio varies ~1800-2400, which is a standard cross-examination point).

Example 1 — Widmark, amount of alcohol consumed. A man weighing 70 kg has a BAC of 120 mg/100 mL. Find the alcohol absorbed and the equivalent quantity of 42% v/v whisky.

A = c × W × r ÷ 100 = 120 × 70 × 0.68 ÷ 100 = 5712 ÷ 100 = 57.1 g of ethanol.
Volume of pure ethanol = 57.12 ÷ 0.789 = 72.4 mL.
Whisky = 72.4 ÷ 0.42 = 172 mL ≈ 0.17 litre, i.e. about 2.3 pegs of 75 mL — a quantity consistent with heavy social drinking, and the figure itself tells the court about the pattern of drinking. (Elimination during drinking is ignored here; the classic Widmark calculation therefore gives a minimum.)

Example 2 — Back-extrapolation for Section 185 MV Act. A driver's blood drawn 2 h 30 min after an accident reads 48 mg/100 mL.

C₀ = 48 + (15 × 2.5) = 48 + 37.5 = 85.5 mg/100 mL at the time of the accident.
Using the slow and fast limits: 48 + (10 × 2.5) = 73 and 48 + (25 × 2.5) = 110.5 → report the range 73-110 mg/100 mL.
Both the measured value and the whole range exceed 30 mg/100 mL, so the offence under Section 185 is established on the chemistry; the range must be reported, not the single figure.

Example 3 — Expected BAC after a known drink. A 55 kg woman (r = 0.55) drinks 180 mL of 40% v/v rum, fully absorbed.

Ethanol mass = 180 × 0.40 × 0.789 = 56.8 g.
Body water equivalent = W × r = 55 × 0.55 = 30.25 L.
c = 56.8 ÷ 30.25 = 1.878 g/L = 187.8 mg/100 mL ≈ 188 mg/100 mL at peak.
After 4 hours: 188 − (15 × 4) = 128 mg/100 mL, still over four times the driving limit.

Example 4 — Breath analyser to blood equivalent. A breath analyser shows 0.035 mg ethanol per 100 mL of breath (0.35 mg/L).

BAC = BrAC × 2100 = 0.35 mg/L × 2100 = 735 mg/L = 73.5 mg/100 mL — above the 30 mg/100 mL limit; a confirmatory blood sample preserved in fluoride-oxalate should still be collected because the partition ratio varies between individuals.

Example 5 — Quantification of arsenic in liver by AAS. A 2.00 g liver portion is digested and made up to 25.0 mL. The AAS reading against the calibration line (A = 0.0420 × C, C in µg/mL) is 0.0294.

C = 0.0294 ÷ 0.0420 = 0.700 µg/mL.
Total arsenic = 0.700 × 25.0 = 17.5 µg.
Tissue concentration = 17.5 µg ÷ 2.00 g = 8.75 µg/g (ppm).
Normal liver arsenic is below about 0.5 ppm; chronic/fatal arsenic poisoning typically shows levels of a few ppm upward, so 8.75 ppm strongly supports arsenic poisoning and justifies hair segmental analysis for the timeline. (Reference cut-offs vary between laboratories and must be quoted from the laboratory's own validated data.)

Technique	Principle	Best for	Role
Colour/spot tests	Chemical colour reaction	Alkaloids, metals, cannabis	Presumptive, field
TLC	Differential adsorption, Rf	Multi-drug screen, cheap	Screening
Immunoassay (ELISA/EMIT)	Antigen-antibody	Drugs of abuse in urine	Screening, class-level
GC-MS	Volatility + mass fragmentation	Volatiles, pesticides, abuse drugs	Confirmation (gold standard)
LC-MS/MS	Liquid chromatography + MS/MS	Polar, thermolabile, trace NPS	Confirmation, ultra-trace
HPLC-DAD	Retention + UV spectrum	Therapeutic drugs	Screening/quantification
AAS / ICP-MS	Atomic absorption / mass of ions	As, Hg, Pb, Tl, multi-metal	Confirmation, metals
UV-Vis	Beer-Lambert absorbance	CO-Hb, nitrite, cyanide	Quantification

Poison	Clue at autopsy/scene	Confirmatory analysis	Antidote
Organophosphate	Garlicky smell, miosis, froth	GC-MS; cholinesterase assay	Atropine + pralidoxime
Aluminium phosphide	Garlic breath, shock, gastric congestion	Silver nitrate paper; headspace GC	None specific
Methanol	Blindness, acidosis, hooch history	GC (head-space), formic acid	Ethanol/fomepizole, dialysis
Cyanide	Bitter almond smell, rapid death	Microdiffusion, ISE, GC-MS	Nitrite-thiosulphate, hydroxocobalamin
Carbon monoxide	Cherry-red lividity	CO-Hb by UV-Vis/CO-oximetry	100% O₂, hyperbaric
Arsenic (chronic)	Mees' lines, GI symptoms, exhumation	AAS/ICP-MS of hair, nail, bone	Chelation (BAL/dimercaprol, DMSA)
Opioid	Pinpoint pupils, injection marks	6-MAM/morphine by LC-MS/MS	Naloxone
Oleander/aconite	Vomiting, arrhythmia, seed fragments	LC-MS/MS for glycosides/aconitine	Supportive, digoxin Fab

Where it is used

Government forensic laboratories: Toxicology and Narcotics divisions of CFSLs (DFSS, MHA — Delhi, Hyderabad, Kolkata, Chandigarh, Bhopal, Pune, Guwahati) and State/Regional FSLs — viscera analysis, NDPS seizures, blood alcohol, cattle poisoning.
Hospitals and medical colleges: clinical toxicology and poison information centres (for example the National Poisons Information Centre at AIIMS New Delhi and the centre at CMC Vellore), emergency cholinesterase and drug-level assays, therapeutic drug monitoring.
Regulatory and testing industry: NABL-accredited private labs, pharmaceutical bioanalytical/toxicokinetic CROs, FSSAI food testing labs, pesticide residue labs, CDSCO-linked drug testing, workplace drug screening for shipping, mining and aviation.
Sports and transport: NDTL New Delhi for anti-doping (WADA-linked); traffic police breath analysis and Section 185 prosecutions.
Job roles: Scientific Assistant/Scientific Officer (Toxicology), Government Analyst under the Drugs and Cosmetics Act, Narcotics Analyst, bioanalytical scientist, QC chemist (LC-MS/MS), poison information pharmacologist, and expert witness consultancy.

Interview questions

Q: How is a poison classified for laboratory purposes rather than clinically? A: By volatility and extractability — volatile, non-volatile organic, metallic, volatile-inorganic/anionic, and gases — because that decides the isolation technique used.
Q: Why is rectified spirit not used to preserve viscera in a suspected alcohol case? A: It is itself ethanol and would contaminate the sample, making the alcohol result meaningless; saturated common salt solution is used instead.
Q: Why is blood for alcohol preserved in sodium fluoride with potassium oxalate? A: Fluoride inhibits microbial fermentation and glycolysis so no ethanol is created or lost; oxalate prevents clotting so a representative aliquot can be taken.
Q: What is the Stas-Otto process and what does pH control achieve? A: Extraction of non-volatile organic poisons from tissue with acidified alcohol followed by solvent extraction — acidic pH extracts acidic and neutral drugs, alkaline pH extracts alkaloids and basic drugs.
Q: Why must a presumptive positive always be confirmed? A: Colour tests and immunoassays respond to a chemical class and cross-react, so a confirmation by a different principle — ideally mass spectrometry — is needed before the result can go to court.
Q: What is postmortem redistribution and how do you guard against it? A: After death, lipophilic drugs diffuse from liver, lung and gut into central blood, inflating concentrations; sampling peripheral femoral blood and cross-checking vitreous or urine controls it.
Q: Does a negative toxicology report rule out poisoning? A: No — insulin, succinylcholine and some plant toxins are hard or impossible to detect, the agent may be fully metabolised, or it may lie outside the screening panel.
Q: Which single metabolite proves heroin use? A: 6-monoacetylmorphine; morphine alone could come from codeine, pharmaceutical morphine or poppy seeds.
Q: What is the blood alcohol limit for driving in India and in what units? A: Section 185 of the Motor Vehicles Act, 1988 sets it at 30 mg of ethanol per 100 mL of blood, that is 0.03% w/v.
Q: Why is methanol poisoning treated with ethanol? A: Ethanol competes for alcohol dehydrogenase with a much higher affinity, so methanol is not converted to formaldehyde and formic acid and is excreted unchanged; fomepizole does the same more safely.
Q: How would you prove aluminium phosphide poisoning in the laboratory? A: Phosphine liberated from gastric contents blackens silver nitrate impregnated paper, and headspace GC/GC-MS confirms phosphine; the garlicky odour and refractory shock support it.
Q: Which provisions make a government scientific expert's report admissible without the expert testifying? A: The provision of the Bharatiya Nagarik Suraksha Sanhita, 2023 corresponding to the former Section 293 CrPC, which lists government scientific experts whose reports may be used as evidence, subject to the court summoning the expert if required.

Common mistakes

Writing "rectified spirit" as the universal viscera preservative and forgetting that it is prohibited in alcohol, volatile and phosphorus cases.
Confusing screening with confirmation — reporting a Marquis or immunoassay positive as identification of a specific drug.
Reporting a back-extrapolated BAC as a single number instead of a range, and ignoring that absorption may still have been ongoing.
Mixing up BAC units — mg/100 mL, mg%, g/L and % w/v — and dropping the ÷100 that converts mg/100 mL to g/L in the Widmark equation.
Applying first-order half-life kinetics to ethanol, which is eliminated at a nearly constant zero-order rate.
Interpreting a drug concentration without tolerance, history and autopsy findings, or without noting postmortem redistribution and ethanol neoformation.
Neglecting the chain of custody, seals, blanks and controls — a technically perfect analysis is worthless if the sample's integrity or the quality data cannot be shown.