False positives and false negatives with a cocaine-specific field test and modification of test protocol to reduce false decision.

The specificity of the Scott test, which is widely used in the field to detect cocaine, was investigated. Several drugs and medicines were applied to the test, and the conditions leading to false positives or false negatives were defined. The Scott test consists of three steps, each involving the addition of a certain reagent and observation of the color that consequently develops. In the first step, blue precipitates appear. In the second, these precipitates completely disappear. In the third step, blue appears again, but in the lower layer. It became clear that proper sample size is critical for correct decision, since too much heroin or dibucaine showed exactly the same color sequence as cocaine HCl and thus gave false positives, and too much cocaine HCl showed persisting precipitates in the second step, yielding a false negative. The appropriate sample size was 1mg or smaller. Freebase (crack) cocaine gave false negatives even when the sample size was appropriate, and it could not be distinguished from a newer substance of abuse, 5-methoxy-N,N-diisopropyltryptamine (5-MeO-DIPT, foxy). The authors developed a new protocol to distinguish crack from 5-MeO-DIPT.