Techniques for Determining Blood Groups from Bloodstains
UGC-NET Paper 2 Unit III notes on grouping dried bloodstains: Lattes crust, absorption-inhibition, absorption-elution (Kind 1960) and mixed agglutination.
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Grouping a dried stain is a different problem from grouping a fresh tube of blood. By the time the stain reaches the serologist the red cells have lysed, the antigens are partly denatured, microbial overgrowth has set in, and the substrate (cotton, denim, wood, soil) contributes its own interference. Direct agglutination, trivial on liquid blood, gives weak or false results on most dried stains. Classical forensic serology therefore evolved a small family of indirect techniques that interrogate the antigen rather than the cell.
NTA gives this its own Unit III bullet because the techniques are distinct, the names are testable, and the sensitivity hierarchy (Lattes, then absorption-inhibition, then absorption-elution, then mixed agglutination for vanishing stains) is a recurring MCQ pattern. Modern Indian casework runs DNA STR profiling for individualisation, but ABO grouping on stains still triages cheaply, supports old case re-openings, and rescues matrices that defeat DNA extraction.
- Agglutination
- Visible clumping of red cells when their surface antigens cross-link with matching antibodies. The endpoint every classical grouping technique reads.
- Absorption-elution
- Two-step technique. Known antiserum is absorbed onto the dried stain, washed off, then heat-eluted at 56 °C and tested against indicator cells. The most sensitive classical method for stain grouping (Kind 1960).
- Absorption-inhibition
- A known volume of antiserum is exposed to the stain; the residual unbound antibody is then titrated against indicator cells. A drop in titre points to the antigen present.
- Mixed agglutination
- Indicator red cells coat onto the absorbed antibody bound to the stain fibres themselves; the rosette is read under low-power microscopy. Used when only a few fibres are available.
- Lattes crust method
- Classical direct agglutination. A crumb of the dried crust is teased into a drop of known antiserum on a slide. Leone Lattes, Turin, 1915.
- Secretor status
- About 80 percent of people secrete water-soluble ABH antigens into saliva, semen and other body fluids. Grouping a saliva or semen stain depends on secretor status.
- Antiserum
- Reagent containing known antibody (anti-A, anti-B, anti-H, anti-Rh) used to probe the unknown antigen on the stain.
- Sensitivity threshold
- The smallest stain on which a technique still gives a reliable read. The key axis for choosing between Lattes, inhibition, elution and mixed agglutination.
Why stains are different from liquid blood
Drying, lysis, denaturation, contamination and substrate, all working against you.
Liquid blood is grouped by adding a drop of anti-A and anti-B to two drops of cell suspension and reading the clumping in seconds. None of that works on a dried stain. Five things have changed by the time the stain reaches the lab.
- Red cells have lysed. The classical agglutinin endpoint needs intact red cells; antigen on stromal fragments does not clump in the textbook way.
- Antigens have partly denatured. Heat, sunlight and time degrade A, B and H epitopes. ABO is more robust than MN, which is more robust than Rh; Rh becomes almost ungroupable on old stains.
- Microbial overgrowth. Bacteria carry B-like antigens; a damp stain can falsely test as group B without controls.
- Substrate interference. Cotton, wool, denim and soil all absorb antibody non-specifically.
- Sample size. Scene stains are often a few millimetres across, with a fraction of the cell count direct grouping needs.
The classical response was to stop trying to read direct agglutination on the stain and instead use the stain as a passive antigen surface. Absorption-elution and absorption-inhibition both rely on this shift: do not look at the stain's cells, look at what the stain does to a known antibody. Before grouping is attempted, the standard Indian CFSL workflow first confirms human blood via presumptive and confirmatory blood-stain identification; grouping never precedes identification.
Lattes crust method (1915, the classical direct test)
The oldest stain-grouping technique and still the simplest demonstration.
Leone Lattes, working in Turin in 1915, was the first to show that the ABO group of a dried stain could be read directly. The method bears his name and remains the entry point in every Indian forensic serology lab manual.
Principle. A small crust of the dried stain is teased into a drop of known antiserum on a glass slide. Residual intact red cell stroma carries enough A or B antigen to bind and visibly agglutinate when the matching antibody is present.
Workflow.
- Scrape a thin flake of the dried crust with a clean scalpel.
- Place separate flakes on two slides, one with anti-A and one with anti-B (an anti-AB control is good practice).
- Cover, incubate at room temperature for 5 to 10 minutes.
- Read under low-power microscopy. Clumping with anti-A only indicates group A; with both, AB; with neither, group O (or a failed test).
Limitations. Needs a relatively fresh stain, a crust thick enough to flake, and gives weak or false results on degraded samples. For NET, hold one anchor: Lattes = direct agglutination of the dried crust. The underlying antigens are those covered in the prerequisite reading on the ABO, Rh and MNSs blood group systems; the same antisera are reused across all classical grouping techniques.
Absorption-inhibition technique
Measure what the stain takes out of a known antiserum.
When the stain is too small or degraded for Lattes, the next step up is absorption-inhibition. The logic flips: instead of looking at the stain's cells clumping, you measure how much antibody the stain pulled out of solution.
Principle. Add a known volume of standardised antiserum of known titre to the stain. The stain's surface antigen binds and removes a proportion of the antibody. After incubation, the supernatant is titrated against indicator cells. A drop in residual titre indicates the antigen was present.
Workflow. Cut three small threads of stain (and one of unstained substrate as control). Place each in a separate tube with a fixed volume of anti-A, anti-B and anti-H. Incubate at 4 °C for 1 to 2 hours. Serially dilute the supernatant and add indicator cells (A1 cells for the anti-A tube, B cells for anti-B, O cells for anti-H). Read the end-titre. A two-tube or greater drop from the reagent's starting titre is positive.
Reading the result. Drop in anti-A and not anti-B indicates group A; drops in both indicate AB; only an anti-H titre drop indicates group O (O cells carry abundant H antigen but no A or B).
Limitations. Quantitative but indirect. Needs careful reagent control, and the substrate control is essential because cotton itself can absorb low levels of antibody.
Absorption-elution technique (Kind 1960, the most sensitive classical method)
Bind the antibody to the stain, then release and assay it.
Stuart Kind, working at the British Home Office in 1960, formalised the absorption-elution test that remains the workhorse of classical stain grouping. It is roughly an order of magnitude more sensitive than absorption-inhibition and is the benchmark against which later refinements are measured.
Principle. Three steps. (1) Absorb known antiserum onto the dried stain so any matching antibody binds the antigen. (2) Wash thoroughly so only specifically bound antibody remains. (3) Elute the bound antibody by gentle heat (typically 56 °C for 10 to 15 minutes), then test the eluate against indicator red cells. Agglutination of the indicator cells proves which antibody was bound, and therefore which antigen the stain carried.
Workflow. Cut three small threads of stain plus one substrate control. Treat each with anti-A, anti-B or anti-H at 4 °C overnight. Wash repeatedly in cold saline until the wash is antibody-free. Transfer each thread into fresh saline with the matching indicator cells. Elute at 56 °C in a water bath; the released antibody now agglutinates the indicator cells. Read after centrifugation.
Sensitivity. Reliable on stains as small as a few millimetres, including stains months or years old. The DFSS Quality Manual and most state SFSL serology SOPs list absorption-elution as the technique of choice for dried-stain ABO grouping where DNA is unavailable. The grouping result still has to ride a documented chain of custody and survive scrutiny under Section 39 of the Bharatiya Sakshya Adhiniyam 2023.
Advantages. Most sensitive of the four classical techniques. Works on old stains. Gives a positive read (agglutination) rather than an inhibition read, which is easier to interpret in court.
Mixed agglutination test
Coat the indicator cells onto the antibody bound to the fibre itself.
When the available material is so small that even absorption-elution would not return enough antibody to assay (a few single fibres, a wood splinter, a fingernail scraping), the mixed agglutination test is the last classical option.
Principle. The same first step as absorption-elution: known antiserum is absorbed onto the stain so matching antibody binds the antigen. Instead of eluting, indicator red cells of the corresponding group are added directly to the fibre. The indicator cells coat onto the bound antibody, producing a rosette around the fibre read under low-power microscopy.
Workflow. Pick one or two fibres of the stained substrate. Soak in anti-A (or anti-B) at 4 °C for 1 to 2 hours. Wash thoroughly in cold saline. Add a drop of indicator A1 cells (or B cells). Mount on a slide and examine at 100x to 400x. A halo of indicator cells coating the fibre indicates the corresponding antigen.
When to use it. Single hairs, a few stained fibres on a button, a thread from inside a wound, fingernail clippings. The technique trades quantitative resolution for the ability to work on samples too small for any other method.
Limitations. Slow, microscope-dependent, easy to call false positives if substrate controls are sloppy. Modern Indian labs prefer to escalate such samples to low-template DNA STR analysis if the budget allows.
Indian forensic practice and the DNA transition
When serology still runs before DNA, and what BNSS 176(3) added.
Indian casework today treats classical ABO grouping of a stain as a screening or triage step rather than a definitive identification. It still appears in the workflow for three reasons: (1) triage, since a quick ABO group can exclude a suspect cheaply before a full STR profile is ordered; (2) old case re-openings, where pre-2000 files contain only ABO data and new samples must first be matched at that level; (3) substrate constraints, where heavily putrefied tissue or certain dyed textiles defeat DNA extraction but still yield a usable ABO read by absorption-elution.
The institutional frame to memorise: the DFSS (Directorate of Forensic Science Services, 2003, MHA) sets the Quality Manual; seven CFSLs plus the state SFSLs run the actual serology and DNA work; CFSL Hyderabad is the central DNA division. Section 176(3) of the Bharatiya Nagarik Suraksha Sanhita 2023 makes forensic examination mandatory at scenes of offences carrying seven years or more, increasing stain submissions across SFSLs. Section 39 of the Bharatiya Sakshya Adhiniyam 2023 makes the analyst's opinion admissible. Both statutes replaced the colonial-era CrPC and Indian Evidence Act in 2024. ABO grouping by absorption-elution still has a place; STR profiling carries the individualisation ABO cannot. Aspirants should know both, and which question each answers.
| Technique | Principle | Sensitivity | Sample size | When to use | Indian application |
|---|---|---|---|---|---|
| Lattes crust | Direct agglutination of dried crust with known antiserum | Low. Needs intact stromal antigen | Visible crust, ideally weeks old | Fresh, thick stains in routine triage | First-pass slide test in SFSL serology |
| Absorption-inhibition | Titre drop of antiserum after exposure to stain | Moderate. Quantitative but indirect | Small threads or scrapings | When Lattes is negative or stain is faded | Older case re-openings with limited material |
| Absorption-elution (Kind 1960) | Bind antibody to stain, wash, heat-elute, test eluate | Highest of the four classical methods | Few-millimetre stain, months to years old | Default classical technique for dried-stain ABO | DFSS Quality Manual default at CFSL/SFSL serology |
| Mixed agglutination | Indicator cells rosette onto antibody bound to fibre | Very high on tiny samples; low quantitative resolution | Single fibres, hair shaft, fingernail scraping |