Class Evidence, Not Proof of Identity: Medical Jurisprudence Perspectives on Bloodstain Serology

 Why this still matters

Before DNA profiling became the principal tool for source attribution, forensic laboratories used ABO and other inherited blood-group markers to characterise dried bloodstains. The absorption–elution technique, developed into a practical method for stains in the early 1960s, was more sensitive than the older absorption–inhibition approach and could detect surviving antigens on remnants of lysed red-cell membranes.

That history remains relevant whenever an old serology report, a cold case, a degraded exhibit, or a laboratory working without a usable DNA profile places ABO evidence before a court. The central judicial question is not merely whether absorption–elution is a recognised technique, but whether a validated version of it was correctly performed, controlled and interpreted in the particular case.

How the method works

When blood dries, red cells haemolyse, so the ordinary direct-agglutination procedure used on intact red cells cannot simply be applied to the stain. Absorption–elution instead detects antibodies that bind to antigens retained on immobilised red-cell fragments.

The core sequence is as follows:

1.      Sampling and immobilisation: A stained thread, fabric cutting or material recovered on a swab is secured or processed using the laboratory’s validated procedure. Published variants include fixing a thread to a glass, plastic or cellulose-acetate support with adhesive or heat, and preparing an ammoniacal extract that is dried onto a support.

2.     Absorption: Standardized anti-A, anti-B and, where group O/H activity is to be assessed, anti-H reagent is placed in contact with separate portions of the sample. For ABO testing, binding is commonly conducted in the cold, around 4°C; some established protocols use overnight absorption, while a published rapid fibre method used approximately 15 minutes.

3.     Washing: Unbound reagent is removed with repeated ice-cold or refrigerated saline washes. This is not a housekeeping formality: residual free antibody may later agglutinate the indicator cells and create a false-positive result.

4.     Elution: The washed material is heated, commonly to approximately 55–57°C, to disrupt non-covalent antigen–antibody binding and release the specifically bound antibody into the surrounding fluid.

5.     Detection: The eluate is tested with appropriate indicator red cells. Agglutination supports the inference that the corresponding antibody had bound to its antigen in the original stain.

The temperatures and timings are therefore protocol-dependent, not immutable universal constants. “Absorption at 4°C and elution near 56°C” accurately describes a common classical arrangement, but the report should identify the laboratory’s actual validated method, reagent titres, incubation times, number and duration of washes, indicator-cell concentration, reading time and acceptance criteria.

An exact claim that every absorption–elution test requires preliminary fixation at 100°C is unsafe. Authoritative descriptions recognize several immobilization or fixation approaches—including adhesive fixation of a stained thread, heat fixation, and heat-fixing a dried ammoniacal extract—without making 100°C a universal requirement.

Controls decide reliability

A positive reaction has evidential value only if the controls behave as expected. Depending on the validated procedure, the laboratory should use suitable positive controls, antigen-negative controls and an unstained sample of the same or nearby substrate; the substrate control is important because some materials can bind reagents nonspecifically or contain adventitious blood-group-active substances.

The archived NIJ training material specifically warns that substrates such as denim and soiled footwear may yield false-positive reactions, particularly apparent group-B activity. It also notes that weak agglutination can be observer-dependent and that delayed readings may turn positive, making defined reading times and confirmation by a second qualified reader important safeguards.

A negative result also requires caution. It may indicate absence of the antigen, but it may instead reflect degradation, insufficient stain, inappropriate reagent strength, loss of bound antibody during washing, an unsuitable substrate or another limitation of the procedure. Accordingly, “no antigen detected” is generally safer than an unqualified assertion that the antigen was biologically absent unless the method’s controls, sensitivity and sample condition justify that conclusion.

The Bombay trap

Ordinary group-O red cells lack A and B antigens but retain abundant H antigen, the precursor on which A- and B-producing enzymes act. In the classical Bombay or Oh phenotype, inherited H-system deficiency prevents expression of H and consequently prevents expression of A and B on red cells, irrespective of the person’s underlying ABO alleles.

Forward grouping limited to anti-A and anti-B therefore makes Bombay red cells appear to be ordinary group O. Differentiation requires investigation of the discrepancy, typically including anti-H lectin on the red cells and reverse grouping with O cells: Bombay cells do not react with anti-H, while the person’s serum commonly contains anti-H that reacts with ordinary O cells.

The often-quoted Indian frequency of approximately 1 in 10,000 is a defensible broad estimate, not a uniform national prevalence. Published Indian data show substantial geographical and community variation, including higher frequencies in some western, southern and tribal populations; one eastern Indian series reported an overall prevalence of about 0.011%.[^12][9]

Correct forensic consequences

The Bombay phenotype creates two distinct risks that should not be conflated.

Setting	If testing is incomplete	Proper interpretation
Crime-scene stain tested only with anti-A and anti-B	A Bombay stain and an ordinary O stain may both show no A/B activity; the result loses discrimination and may create an erroneous non-exclusion or apparent “O” classification.	The result does not identify the donor. Where H deficiency is materially in issue and the sample permits, anti-H testing and appropriate controls are needed.
Reference blood typed only by forward grouping	A Bombay individual may be reported as group O.	Resolve the discrepancy by reverse grouping and anti-H testing rather than accepting the apparent O result.
Parentage inference based on simple ABO phenotypes	A Bombay parent may phenotypically appear O while carrying an A or B allele; a child expressing A or B could then be wrongly treated as incompatible under an oversimplified ABO inheritance table.	Investigate rare phenotypes and confirm any apparent exclusion with modern genetic testing rather than treating ABO phenotype alone as decisive.

 

This corrects an important overstatement sometimes made in discussions of crime-scene evidence. If both a stain and a reference sample are tested only with anti-A and anti-B, the usual Bombay-related danger is not automatically a false exclusion; it is failure to distinguish Bombay from ordinary O, producing an overbroad class and potentially a false or misleading non-exclusion. A false exclusion is more readily illustrated in parentage analysis, where an unrecognised Bombay phenotype can conceal an A or B allele.

Nor should anti-H testing be presented as a universal statutory condition for the admissibility of every ABO result. Scientifically, it is integral to demonstrating H activity and distinguishing ordinary O from H-deficient phenotypes when that distinction is relevant; legally, omission affects the completeness and weight of the opinion, with its significance depending on the claim made, the controls, the sample and the facts of the case.

Class evidence, not identity

ABO type is a class characteristic shared by many people, not an individual characteristic unique to one person. A concordant blood group therefore means only that the person cannot be excluded on that marker; it does not establish that the person deposited the stain. Conversely, a genuine incompatibility may exclude a proposed source, but only if the sample is single-source, the typing is reliable, and recognised biological and technical explanations have been addressed.

The Supreme Court’s decision in State of Rajasthan v. Hanuman Criminal Appeal No. 631 of 2017, decided on June 19, 2025, by the Supreme Court of India (Bench: Sandeep Mehta, J. and Prasanna B. Varale, J.), reported as 2025 LiveLaw (SC) 691.illustrates the limited weight of such evidence. The Court held that even an FSL finding that blood on a recovered weapon matched the deceased’s B-positive group did not, together with the other deficient circumstances, complete the chain required for conviction; it reiterated that recovery of a bloodstained weapon bearing the victim’s blood group is not by itself sufficient to prove murder.

DNA profiling operates at a vastly higher level of discrimination and has largely displaced antigen-based ABO typing as the primary means of associating biological material with a person. Yet DNA evidence must still be interpreted within its own validated methods, statistics, chain of custody and case context; the superior discriminating power of DNA does not convert every DNA result into automatically conclusive proof.

The Indian evidence lens

Under Section 39 of the Bharatiya Sakshya Adhiniyam, 2023, the opinion of a person specially skilled in science or another relevant field is a relevant fact when the court must form an opinion on such a matter. Relevance, however, is not the same as decisive weight. The court must examine the expert’s qualifications, the condition and continuity of the exhibit, the validated procedure actually used, controls and observations, possible contamination or substrate effects, limitations, and whether the conclusion exceeds what the test can scientifically support.

Parentage requires an additional legal caution. Section 116 of the Bharatiya Sakshya Adhiniyam continues the statutory rule concerning conclusive proof of legitimacy for a child born during a valid marriage, subject to proof of non-access. The Supreme Court has repeatedly cautioned that blood or DNA testing is not to be ordered as a matter of course or for a roving inquiry, and that privacy, dignity, the child’s interests and the governing statutory presumption must be considered. Thus, the scientific possibility of an ABO exclusion and the legal permissibility or effect of obtaining parentage evidence are separate questions.

A courtroom checklist

When ABO typing of a dried stain is tendered, the following questions go directly to reliability and weight:

·      Was the material first established to be human blood by a validated test?

·      What precise absorption–elution protocol and current standard operating procedure were used?

·      How was the stain sampled and immobilised, and was an unstained area of the same substrate tested?

·      Which reagents were used—anti-A, anti-B and anti-H—and were their specificity, titre, lot and expiry documented?

·      What were the absorption temperature and duration?

·      How many cold-saline washes were performed, and how was complete removal of unbound antibody verified?

·      What were the elution temperature and duration?

·      Which indicator cells, concentrations and reaction controls were used?

·      Were the reactions read within the validated time and independently checked?

·      Did every positive, negative and substrate control produce the expected result?

·      Was the stain aged, diluted, degraded, mixed, contaminated or deposited on a substrate known to cause nonspecific reactions?

·      Does the report say “consistent with” or “cannot be excluded,” or does it improperly claim personal identification?

·      If an apparent group-O result is material, was H activity examined and was the possibility of an H-deficient phenotype considered?

·      Was DNA profiling attempted, and if not, why not?

Judicial takeaway

Absorption–elution is not unreliable merely because it is old, and it is not reliable merely because it is recognised. Its probative force depends on disciplined execution, valid controls, transparent reporting and conclusions confined to the method’s true discriminatory capacity.

The Bombay phenotype supplies a useful warning against mechanical interpretation: absence of reactions with anti-A and anti-B does not invariably establish ordinary group O. In crime-scene work, incomplete testing may create a misleading non-exclusion; in historical parentage testing, it may conceal inheritance of an A or B allele and generate an apparent exclusion. The legally sound formulation is therefore modest but exact: ABO evidence may exclude or support consistency at the class level, but it cannot, standing alone, identify the source of a stain or prove biological parentage.

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