Kastle-Meyer (KM) Test
Definition
Phenolphthalein-based colorimetric presumptive blood test relying on haem peroxidase activity. Positive result: pink colour within 2-5 seconds of hydrogen peroxide addition.
- Reagent
- Phenolphthalin, oxidised by haem peroxidase activity
- Oxidant
- Hydrogen peroxide
- Positive colour
- Pink, within 2 to 5 seconds
- Test class
- Presumptive, not confirmatory
Common questions
Why is the Kastle-Meyer test called presumptive rather than confirmatory?+
Peroxidase activity is not unique to blood. Plant peroxidases, some bacteria, rust, and certain vegetable matter can also turn phenolphthalin pink, so a positive result only justifies sending the stain for a confirmatory test such as an immunochromatographic or DNA method.
What causes a false negative on the Kastle-Meyer test?+
Very old, heavily degraded, or heat-damaged bloodstains can lose peroxidase activity, and some cleaning agents or oxidising bleaches destroy the haem before testing, so a negative result does not rule out blood on its own.
How is the test performed on a stain at a scene?+
A swab or filter paper is touched to the suspected stain, phenolphthalin reagent is added first, then hydrogen peroxide. A slow colour change after several seconds is treated with more caution than an immediate pink reaction.
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