False positive (drug testing)

A false positive in drug testing is a screening result that reads non-negative but is not confirmed when the sample is tested by a more specific method.

Screening immunoassays trade specificity for speed and cost, so they occasionally respond to compounds that resemble the target, to certain medications, or to laboratory error. The confirmatory step, using mass spectrometry, is designed to catch these cases by identifying the actual compound. In regulated programs a result is not reported as positive until confirmation is complete.

For cannabinoids, a non-negative screen might reflect THC-COOH from actual THC exposure, cross-reactivity from another compound, or a technical issue, and only the confirmatory test can tell which. A confirmed result is by definition not a false positive, even when the source of the THC was a legal hemp product. The term is often used loosely to mean any unexpected result; in laboratory usage it refers strictly to a screen that confirmation did not support.