Detection and Identification of Blood Stains in Forensic Science

Blood is the single most common biological evidence at violent crime scenes, and the entire forensic-biology workflow (species testing, ABO grouping, DNA profiling) downstream of detection only fires if you correctly locate and preserve the stain. Bloodstain work also feeds reconstruction, because the bloodstain pattern analysis of spatter, drip and transfer patterns at the scene depends on the stain being found, photographed in situ, and not contaminated by reagent overspray.

NTA almost always tests the workflow as a three-step pipeline. Memorise it as a single line.

1. Search. Visual inspection under white light, then oblique light, then alternate-light sources (UV, blue 415 nm). Suspected stains are photographed with scale before any chemical is applied.
2. Presumptive test. A fast, sensitive screening reagent is applied to a sample lifted on filter paper or a swab. Positive result means "treat as blood, continue to confirmation". Negative result usually excludes blood.
3. Confirmatory test. A specific laboratory test (Takayama or Teichmann crystal test, or an immunological human-blood test) establishes that the stain is in fact blood, and (for the immunological tests) that it is of human origin.

Two non-negotiable scene rules sit on top of this. First, document and photograph before you apply anything: presumptive reagents alter the stain. Second, every sample, swab and aliquot is logged in the chain of custody register from the moment of collection to the moment the lab report is sealed. A break in custody is the easiest line for the defence to attack.

All four classical presumptive tests exploit the same chemistry: the haem iron in haemoglobin acts like a peroxidase, splitting hydrogen peroxide and oxidising a colourless or weakly coloured indicator into a strongly coloured or luminescent product. They differ in indicator, sensitivity, stability and false-positive profile.

Kastle-Meyer (phenolphthalein) test. Phenolphthalein is first reduced (with zinc in alkali) to colourless phenolphthalin. At the scene, a drop of phenolphthalin reagent followed by a drop of hydrogen peroxide is added to the lifted sample. A bright pink colour within 10 to 15 seconds is a positive result. Sensitivity is roughly 1 in 10,000 to 1 in 100,000 dilutions of blood. A colour change before the peroxide is added is read as a false positive and the sample is rejected.

Leucomalachite Green (LMG) test. Leucomalachite green plus hydrogen peroxide gives a blue-green colour in the presence of haem peroxidase. Slightly less sensitive than Kastle-Meyer but the reagent is more stable in tropical field conditions, which is why several Indian state SFSLs prefer it in summer kits.

Benzidine test. Historically the most sensitive colour test, giving a blue colour with hydrogen peroxide. Now banned for routine use because benzidine is a Group 1 IARC human carcinogen (bladder cancer). It still appears in MCQs as a distractor; the correct answer is that it is no longer used and has been replaced by Kastle-Meyer and LMG.

Luminol test. Used when the suspect has tried to wash the scene. Luminol in alkaline hydrogen peroxide is oxidised by haem iron to 3-aminophthalate in an excited state, which emits blue light at about 425 nm visible in darkness. Sensitivity reaches roughly 1 in 1,000,000 dilution, which is why luminol picks up bloodstains weeks after a clean-up. The trade-off is that the luminescence lasts only 30 seconds or so and must be photographed quickly in long exposure. Luminol also dilutes the stain, so the sprayed area is sampled for DNA only after careful planning.

Fluorescein. A close cousin of luminol, fluorescein with hydrogen peroxide and zinc gives a yellow-green fluorescence under blue (455 nm) light. Useful on porous or coloured substrates where luminol's blue glow is hard to see.

The pattern to remember for MCQs: presumptive tests are sensitive but not specific, which is why a positive result never proves a stain is blood and never proves it is human.

Once a stain has triggered a positive presumptive, the lab confirms it as blood. NTA tests two classical crystal tests and (increasingly) one immunological test.

Takayama test (haemochromogen crystals). A small fragment of the stain on a microscope slide is treated with Takayama reagent (pyridine, glucose, sodium hydroxide, water). On warming, haem combines with pyridine and reduced glucose to give pink to salmon-coloured, feathery or needle-shaped pyridine ferroprotoporphyrin (haemochromogen) crystals visible under the microscope. Takayama works on aged and partially decomposed stains and is the preferred confirmatory test in most Indian SFSLs.

Teichmann test (haemin crystals). The stain on a slide is treated with glacial acetic acid and a trace of sodium chloride and warmed gently. Haem combines with chloride to give brown, rhombic (rhomboid plate) haemin or ferriprotoporphyrin chloride crystals. Older test, more temperamental on aged stains, but still in the syllabus and still tested.

Modern immunological alternative: ABAcard HemaTrace and equivalents. A lateral-flow immunochromatographic strip using a monoclonal antibody against human haemoglobin. A positive line confirms (a) the stain is blood, and (b) it is human (or higher primate). Sensitivity to about 50 nanograms of haemoglobin per millilitre. Increasingly used in Indian forensic labs because it collapses the species-test step (anti-human precipitin or Ouchterlony double diffusion) into the confirmatory step.

After confirmation, the lab moves to species identification (anti-human precipitin if not already covered), ABO grouping where the sample size allows, and DNA profiling at the CFSL Hyderabad central DNA division or a state SFSL DNA unit. Every transfer is logged in the chain-of-custody register and every report is signed by an analyst who can defend it under BSA Section 39 expert opinion in court.

The Indian crime-scene workflow for a suspected bloodstain follows a familiar script. The Investigating Officer cordons the scene under BNSS Section 176(3), which (for offences punishable with seven years or more) makes a forensic-team visit mandatory. The SOCO (Scene of Crime Officer) team conducts a search under white and oblique light, then alternate-light sources, photographs every suspected stain with a scale and ABFO ruler, and only then applies a presumptive reagent on a lifted sub-sample.

Samples are packed in breathable paper bags (never plastic, which traps moisture and encourages mould), labelled with case number, date, location and collector name, sealed, and dispatched to the relevant state SFSL or central CFSL. CFSL Hyderabad houses the central DNA division and handles the harder cases; routine bloodstain work goes to the nearest state SFSL serology division. The forensic photographer's record from the scene becomes a forensic photography exhibit alongside the analyst's report.

Two practical Indian rules that show up in MCQs and viva. First, never spray luminol over a stain before swabbing for DNA, because luminol dilutes the sample and complicates downstream profiling; the SOP at most SFSLs is to swab first, then luminol the residual area. Second, presumptive reagents are made fresh weekly at field labs, because phenolphthalin and luminol degrade with light and air.

Defence counsel attacks bloodstain evidence on three predictable lines, and NTA likes to test all three.

False positives from non-blood peroxidases. Horseradish, potato, some other root vegetables, and oxidising metal salts (copper, nickel) all turn Kastle-Meyer and LMG positive without any blood being present. Cleaning products with hypochlorite bleach trigger luminol. Cyanide and certain household oxidisers can give an early colour change. The SOP defence is the pre-peroxide read: if the indicator turns colour before the hydrogen peroxide is added, the result is non-specific and the stain is re-sampled. Confirmatory tests exist precisely to defeat this attack.

Species ambiguity. A positive presumptive plus a positive crystal test confirms blood but not human blood. Without an anti-human precipitin or HemaTrace step, the defence will say the stain could be from chicken or buffalo blood from the kitchen. Always read the question for whether species testing has been done.

Age estimation limits. Bloodstains darken and degrade over time (oxyhaemoglobin to methaemoglobin to haematin), but age estimation from colour is unreliable beyond the broadest range (fresh, hours, days, weeks). Several Indian textbooks list spectroscopic methods (microspectrophotometry, reflectance) for age estimation; for NET, recall that no method currently gives age to better than a few days, and any over-precise claim should be flagged.

The cross-examination playbook in Indian courts since the BSA 2023 came into force is that the analyst must be able to defend each step (search, presumptive, confirmatory, species, ABO, DNA), each reagent batch, and each chain-of-custody entry. The detection step is where most cases are won or lost.