Blood is the most common biological evidence — but it isn't always visible, particularly when someone has diluted or cleaned it. Investigators therefore use presumptive tests to reveal blood that the eye would miss.
The common tests
- Kastle-Meyer test — a swab of the stain plus phenolphthalein reagent turns bright pink in the presence of blood.
- Luminol — sprayed over a surface in the dark, it produces a blue chemiluminescent glow where blood is present, and can reveal traces even after attempts to wash them away.
- Leucomalachite green — another colour-change presumptive test.
These react with the haemoglobin in blood and are extremely sensitive.
Presumptive, not proof
The crucial word is presumptive. A positive result indicates blood may be present — it does not confirm it, and it doesn't say the blood is human. False positives can come from some metals, cleaning agents like bleach, and plant peroxidases (some vegetables). So a positive screen is followed by confirmatory testing and species determination, and ultimately DNA where a profile is needed.
Practical cautions
Luminol has to be used in darkness and documented carefully, and investigators avoid over-spraying, since excess reagent and other treatments can affect later analysis. Used properly, these tests turn an apparently clean scene into a map of where blood really is — the essential first step before recovery and DNA analysis.