False Positive (Presumptive Test)
Definition
A positive colour response from a compound that is not the targeted controlled substance. Common sources include diphenhydramine (Marquis orange, Scott blue), lidocaine and procaine (Scott blue), and DXM (Marquis reddish-brown). False positives are the primary reason colour tests cannot be used as standalone drug identification methods.
- Field
- Presumptive colour testing for controlled substances
- Common interferent
- Diphenhydramine, lidocaine, procaine, DXM
- Consequence
- Colour tests cannot confirm identity alone
- Confirmation method
- GC-MS or equivalent instrumental analysis required
Common questions
Why can an over-the-counter medication trigger the same colour as an illicit drug?+
Marquis, Scott, and similar reagents react with broad chemical classes such as amines rather than one specific molecule, so common legal drugs sharing that reactive group produce the same colour shift as the targeted controlled substance.
What does a false positive mean for how the result is used in a case?+
A presumptive colour result is treated only as grounds for further testing, never as a stated identification in a report or in court; the analyst must run a confirmatory instrumental method before the substance is named.
Related terms
- Cobalt Thiocyanate
- Co(SCN)2 in aqueous solution; the active reagent in the Scott test for cocaine. Cocaine forms a lipophilic blue complex with cobalt thiocyanate...
- Duquenois-Levine Test
- A three-reagent presumptive test for cannabinoids: vanillin + acetaldehyde in ethanol (Reagent 1), concentrated hydrochloric acid (Reagent 2), and chloroform partition (Reagent...
- Mandelin Reagent
- Ammonium metavanadate (NH4VO3) in concentrated sulphuric acid. Useful for amphetamines (orange-red), MDMA (dark blue-black) and cocaine (yellow-orange). The Mandelin-Marquis panel is recommended...
- Marquis Reagent
- A presumptive drug test reagent consisting of formaldehyde in concentrated sulphuric acid, first described by Edmund Marquis in 1896. It produces characteristic...
- Mecke Reagent
- Selenious acid (H2SeO3) dissolved in concentrated sulphuric acid. More selective than Marquis for the opioid alkaloid skeleton; morphine and heroin give a...
- NIK Polytesting
- A field drug testing approach using the NIK Public Safety sealed ampoule system, where multiple colour tests are run sequentially on the...
- Scott Test
- A three-step cobalt thiocyanate presumptive test for cocaine and its salts. Step 1: cobalt thiocyanate reagent (blue in aqueous phase). Step 2:...
- Simon's Test
- A presumptive test that uses sodium nitroprusside and sodium carbonate to detect secondary amines (N-methyl group). Methamphetamine and MDMA (secondary amines) give...
- SWGDRUG Category C
- The lowest tier in the Scientific Working Group for the Analysis of Seized Drugs classification. Category C methods (including colour tests, microcrystal...