Presumptive Colour Test
Definition
Any rapid field or laboratory test that produces a colour change in the presence of a target substance. Positive results are indicative, not conclusive, and require a subsequent confirmatory test.
- Result type
- Colour change on contact with a target substance class
- Speed
- Seconds to a few minutes
- Evidentiary weight
- Indicative only, not conclusive
- Next step on positive
- Confirmatory laboratory testing
Common questions
Why can't a colour test alone identify a substance?+
Most colour reagents respond to a chemical class rather than one specific compound, so unrelated substances can give the same colour change. A positive result narrows the possibilities and justifies further work, but does not by itself name the substance in a report.
What causes a false positive on a presumptive colour test?+
Cross-reacting compounds, contaminants on the item, or reagent degradation can all produce a colour change that mimics a true positive. Laboratories control for this with reagent blanks and known negative controls run alongside the sample.
Related terms
- Cross-Checking
- The practice of running two different presumptive tests on adjacent cuttings from the same stain. Concordant results (both positive or both negative)...
- Leucomalachite Green (LMG)
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- 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...
- 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...