Chromogenic Pad
Definition
The reactive zone on a Hemastix strip impregnated with a reduced chromogen (tetramethylbenzidine in some formulations) and a peroxide source. Contact with haemoglobin activates the colour reaction.
- Location
- Reactive tip of a Hemastix reagent strip
- Chromogen
- Tetramethylbenzidine (in common formulations)
- Co-reagent
- An organic peroxide as the oxidant source
- Trigger
- Peroxidase-like activity of haemoglobin
- Positive result
- Pad changes colour, typically green to blue
Common questions
What causes a false positive on the pad?+
Haemoglobin is not the only substance with peroxidase-like activity; plant peroxidases, certain metal ions and some cleaning agents such as bleach can trigger the same colour change, so a colour shift alone is only presumptive.
Does the pad confirm the blood is human?+
No, the chromogenic reaction responds to haemoglobin structure shared across many species, so a positive strip must be followed by a species-specific confirmatory test before a sample is reported as human blood.
Why can old or diluted stains still react?+
The chromogen reacts to catalytic haem groups rather than intact whole blood, so even a highly diluted or aged stain that has lost visible colour can still carry enough haemoglobin breakdown product to trigger a positive pad reaction.
Related terms
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