Cross-Checking
Definition
The practice of running two different presumptive tests on adjacent cuttings from the same stain. Concordant results (both positive or both negative) increase confidence; discordant results suggest substrate interference or a non-blood peroxidase source.
- Method type
- Dual presumptive test on adjacent cuttings
- Concordant result
- Both tests positive or both negative
- Discordant cause
- Substrate interference or non-blood peroxidase source
- Applies to
- Leucomalachite green and similar presumptive blood tests
Common questions
Why run two tests instead of trusting one positive result?+
Presumptive blood tests react to peroxidase activity, which several plant and household chemicals also trigger. A second test on an adjacent cutting from the same stain checks whether the reaction is consistent, and a false positive from a single reagent rarely repeats the same way on a second chemistry.
What does a discordant result mean for the case?+
It flags the stain as unreliable for a presumptive call and pushes the examiner toward confirmatory testing or a note that the substrate itself may be interfering, rather than reporting the sample as blood on the strength of one reaction alone.
Related terms
- Leucomalachite Green (LMG)
- The reduced, colourless form of malachite green. It is oxidised to the coloured malachite green form by active oxygen released when haemoglobin...
- Malachite Green
- A synthetic triphenylmethane dye with an intense blue-green colour in its oxidised form. It is a common industrial dye and the coloured...
- Presumptive Colour Test
- Any rapid field or laboratory test that produces a colour change in the presence of a target substance. Positive results are indicative,...
- Substrate Control
- A sample of the unstained surface material from an area adjacent to a body fluid stain, collected alongside the stain and processed...
- Triphenylmethane Dye
- A class of synthetic dyes built around a central carbon bonded to three phenyl groups. Members include malachite green, crystal violet, and...