Bloodstain Identification and Individualisation in Forensic Serology

When investigators find a reddish-brown stain at a crime scene, the first question isn’t “whose blood is this?” It’s simpler: is this even blood? Forensic serology answers that question in stages, moving from quick screening at the scene to rigorous laboratory confirmation, and finally to protein and enzyme analysis that narrows the stain down to a smaller group of possible sources. This layered approach has guided bloodstain examination for over a century, and understanding it also explains why DNA profiling eventually became the dominant individualisation tool.

Confirming a stain is blood: the role of presumptive tests 🔗

Not every red-brown mark at a crime scene is blood. Rust, fruit juice, certain paints, and plant residues can all mimic its appearance. Before anyone spends time on expensive confirmatory work, investigators run presumptive tests to get a fast, on-scene indication of whether a stain is worth pursuing.

How Adler’s benzidine test works 🔗

One of the oldest presumptive screening methods is the Adler’s Benzidine Test, developed in the early 1900s. It relies on an oxidation-reduction reaction: haemoglobin in blood behaves like a peroxidase enzyme, catalysing the breakdown of hydrogen peroxide and oxidising benzidine to produce a blue colour change. The reaction is fast and needs almost no equipment, which is exactly why it suited crime scene conditions before portable forensic kits existed.

The catch is specificity. Benzidine-based colour changes can also appear with plant peroxidases, certain metals, and household bleaches, producing false positives. Benzidine itself is also a recognised carcinogen, which is why many laboratories have phased it out in favour of safer presumptive reagents such as phenolphthalein (the Kastle-Meyer test) and leucomalachite green, while still relying on the same underlying peroxidase-driven chemistry. A positive presumptive result is treated only as a lead. It tells investigators the stain deserves a confirmatory test, not that the case is closed.

Confirmatory crystal tests: from suspicion to certainty 🔗

Because presumptive tests can be fooled, forensic serology relies on microcrystalline confirmatory tests to establish, beyond doubt, that a stain contains haemoglobin. These tests work by converting the haem portion of haemoglobin into a derivative that forms crystals with a distinctive, reproducible shape and colour, visible only under a microscope.

The Teichmann test and its brownish hemin crystals 🔗

The Teichmann Test was developed in 1853 by the Polish anatomist Ludwig Karl Teichmann, making it one of the earliest scientific tools ever used to confirm blood. The reagent, made of glacial acetic acid and a halide salt such as sodium chloride, is heated with the suspected stain. The heat and acid convert haemoglobin into haemin, which then combines with the halide to form rhomboid, brownish-yellow crystals with a faint violet sheen when viewed under polarised light. The test is valued for working even on old, dried stains, though rust contamination and extended environmental exposure can interfere with crystal formation.

The Takayama test and salmon-pink crystals 🔗

The Takayama Test, introduced by the Japanese scientist Masaeo Takayama in 1912, takes a different chemical route. Here, pyridine reacts with the reduced (ferrous) iron in haemoglobin to form a pyridine-haemochromogen complex, which crystallises into feathery, salmon-pink needles upon gentle warming. Because this reaction depends on reduction rather than oxidation, it tends to hold up slightly better on degraded or aged samples than the Teichmann method, which is why forensic protocols often suggest attempting the Takayama test if the Teichmann test fails to produce clear crystals. Both tests remain textbook confirmatory methods, even though some modern crime labs have supplemented or replaced them with immunological strip tests for greater sensitivity on very dilute stains.

Catalytic tests and the chemistry of peroxidase activity 🔗

Alongside crystal tests, forensic serology uses catalytic tests that exploit the same peroxidase-like behaviour of haemoglobin used in presumptive screening, but push it further for sensitivity. Haemoglobin catalyses the breakdown of hydrogen peroxide, and this reaction is coupled to a colour-forming or light-emitting substrate. Benzidine and its safer derivatives turn blue, phenolphthalein turns pink, and luminol produces a blue chemiluminescent glow visible in the dark. These reactions are remarkably sensitive, with some catalytic tests able to detect blood at dilutions of roughly 1 in 100,000, and luminol pushing sensitivity even further on cleaned or heavily diluted stains.

Why sensitive tests can also mislead 🔗

High sensitivity comes at a cost. Because these tests detect any peroxidase-like activity rather than haemoglobin specifically, they can react with plant peroxidases (found in horseradish, potatoes, and several other vegetables), certain bacterial enzymes, and metal ions like copper or iron that mimic the catalytic effect. A modern catalytic reagent such as tetramethylbenzidine, validated for both crime-scene and laboratory use, was specifically developed to improve reliability while preserving the stain’s structural integrity for any DNA work that follows. This is precisely why catalytic and colour tests are classified as presumptive or preliminary, never as final proof, no matter how strong the reaction looks.

Instrumental methods for dried and degraded stains 🔗

Dried bloodstains present a special challenge because the antigens and proteins they contain degrade over time. Instrumental methods such as the absorption-inhibition test and the absorption-elution test were developed to work around this. Both rely on antibody-antigen agglutination reactions: known antibodies are applied to the stain, allowed to bind any corresponding antigens present, and then either inhibited from reacting further or eluted back off the sample and tested against known red cells. The pattern of agglutination reveals which blood group antigens survived in the stain, allowing serologists to type dried, aged evidence that would otherwise be unusable for direct antigen testing.

From “is it blood” to “whose blood”: individualisation 🔗

Once a stain is confirmed as blood, the next question is how much it can narrow down the list of possible sources. Before DNA profiling became routine, forensic serologists relied on naturally occurring polymorphisms in blood proteins and enzymes, inherited variations that differ from person to person, to individualise a stain beyond simple ABO grouping.

Haptoglobin: a protein with three faces 🔗

Haptoglobin (Hp) is a serum protein whose job is to bind free haemoglobin released when red blood cells break down. It is controlled by two common alleles, Hp¹ and Hp², which combine to produce three detectable phenotypes: Hp 1-1, Hp 2-1, and Hp 2-2. According to standard forensic serology teaching material, these phenotypes are distinguished using starch gel or polyacrylamide gel electrophoresis, where the differently sized haptoglobin-haemoglobin complexes separate into characteristic band patterns. A person homozygous for Hp1 shows a single band, while Hp2 homozygotes and heterozygotes both throw off multiple bands with slightly different spacing, because Hp2 proteins form polymers of varying chain length rather than a single fixed structure.

Visualising these bands depends on the same peroxidase chemistry used in presumptive testing. Because haptoglobin binds free haemoglobin so tightly, the complex retains its peroxidase activity, so staining the gel with benzidine or dianisidine lights up exactly where the haptoglobin bands sit, without needing a separate haemoglobin stain. This dual use of peroxidase chemistry, first for detecting blood and later for visualising its protein variants, is one of the more elegant threads running through classical forensic serology.

Red cell enzymes add another layer of individuality 🔗

Serum proteins aren’t the only source of individualising information. Several enzymes found inside red blood cells also show genetically controlled variation, and forensic scientists have used them for decades to add extra discriminating power to bloodstain analysis.

The adenylate kinase system 🔗

Adenylate Kinase (AK) is controlled by two alleles, AK1 and AK2, which combine to produce three detectable phenotypes. Like haptoglobin, these variants are separated using gel electrophoresis and visualised with enzyme-specific staining reactions, letting analysts read off a person’s AK phenotype directly from a bloodstain extract.

The adenosine deaminase system 🔗

Adenosine Deaminase (ADA) works the same way, with two common alleles, ADA1 and ADA2, producing distinguishable phenotypes on electrophoresis. Allele frequencies for this system vary noticeably between populations. A study of red cell enzyme systems in an Eastern Uttar Pradesh population recorded ADA and AK allele frequencies that differed from figures reported in other regional and international population studies, which is exactly the kind of variation that makes these systems useful for population genetics and, historically, for forensic individualisation. Because allele frequencies shift from one population group to another, a rare phenotype combination can meaningfully shrink the pool of people a bloodstain could have come from.

It’s worth remembering that protein and enzyme polymorphisms degrade faster than DNA, so this approach worked best on relatively fresh stains. That limitation, more than anything else, is what pushed forensic serology toward DNA-based typing once STR analysis became widely available. Even so, understanding these classical systems remains essential, both because older cases and reference collections still contain this kind of data, and because the underlying logic, using inherited variation to individualise evidence, is the same principle DNA profiling later refined.

What do you think? If a bloodstain is too old or too degraded for DNA profiling to work, do you think reviving these classical protein and enzyme tests could still offer useful investigative leads? And given how easily presumptive tests can be fooled by common substances, how much weight do you think a single positive field test should carry in an actual investigation?

References

1. https://nij.ojp.gov/nij-hosted-online-training-courses/laboratory-orientation-and-testing-body-fluids-and-tissues/testing-body-fluids-tissues/blood/confirmatory-tests
2. https://onlinelibrary.wiley.com/doi/abs/10.1111/1556-4029.14091
3. https://www.sciencedirect.com/science/article/abs/pii/S2468170916300406
4. https://egyankosh.ac.in/bitstream/123456789/89069/1/Unit-8.pdf
5. https://nij.ojp.gov/nij-hosted-online-training-courses/laboratory-orientation-and-testing-body-fluids-and-tissues/testing-body-fluids-tissues/blood-grouping/serum-protein-polymorphisms
6. https://www.biorxiv.org/content/10.1101/2022.11.24.517832v1.full