Blood: Is It Blood, Whose Species, Whose Group

In one line

A bloodstain is examined in three stages, and each stage can fail without the others noticing.

In the wording a student can write in an exam: the examination of a suspected bloodstain proceeds in three stages: a presumptive test, which indicates that the stain could be blood; a confirmatory test, which establishes that it is blood; and a species test, which establishes whether it is human; after which the stain may be grouped, which excludes but does not identify, or profiled, which can identify. Blood pattern analysis is a fourth line of inquiry, directed not at whose blood it is but at how the stain came to be where it is.

Stage one: the presumptive tests

What they do. They detect the peroxidase-like activity of haemoglobin, which catalyses the oxidation of a reagent and produces a colour or a light.

The common ones. The benzidine test and its safer successors, the Kastle-Meyer test using phenolphthalein, and luminol, which produces a faint blue luminescence in the dark and is used to reveal washed or invisible stains over a large area.

Why they are called presumptive. They are sensitive and not specific. They react with plant peroxidases, with some metal salts, with certain cleaning agents and with rust. A positive result therefore means the stain could be blood.

And why they are still used. Because they are quick, cheap and can be done at a scene, and because a negative result is worth a great deal: it substantially excludes blood.

Luminol deserves a sentence of its own. It reveals traces of blood after washing, which is why it appears in cases where a scene has been cleaned. But it also reacts with bleach, and bleach is what a scene is usually cleaned with, so a luminol reaction on a cleaned floor is weaker evidence than it appears.

Stage two: the confirmatory tests

What they do. They establish the presence of haemoglobin or its derivatives specifically, by producing characteristic crystals or by spectroscopic identification.

The classical ones. The Takayama test, which produces haemochromogen crystals, and the Teichmann test, which produces haemin crystals. Modern laboratories may use immunochromatographic tests directed at human haemoglobin, which combine the confirmatory and species steps.

What a positive result means. That the stain is blood.

Stage three: the species test

The precipitin test. An antiserum raised in an animal against human serum protein is brought into contact with an extract of the stain. If human protein is present, a visible band of precipitate forms at the interface, or a line forms in a gel where the reaction is run by diffusion or by electrophoresis.

Why it is indispensable. Without it a bloodstain proves nothing against an accused who says the blood is an animal's. In a country where the slaughter of animals at home is common, that answer is available in most cases.