Whether it's a workplace test or a forensic case, drug testing almost always works in two stages: a quick, sensitive screen to flag what might be present, then a definitive test to confirm exactly what it is. Understanding the two explains both the power and the pitfalls of a "positive" result.
Stage 1: the screen
The first test is usually an immunoassay — it uses antibodies that react to a class of drug. It's fast, cheap and sensitive, ideal for processing many samples. But it's presumptive: antibodies can cross-react with similar compounds, so a screen can flag a positive that isn't what it seems (some cold and prescription medicines are classic culprits). A screen alone should never be treated as proof.
Stage 2: confirmation
Anything of significance is confirmed by gas or liquid chromatography–mass spectrometry (GC-MS / LC-MS). Chromatography separates the components of the sample, and mass spectrometry identifies each by its molecular fingerprint. This is definitive — it names and measures the specific substance, resolving the false positives a screen can throw up.
What's tested, and the window
Different samples suit different questions:
- urine — common, with a detection window of days,
- blood — best related to current effect or impairment,
- oral fluid — recent use,
- hair — a history of use over months.
Tests use defined cut-off levels so that trace or passive exposure doesn't register as a positive.
The takeaway
A trustworthy drug result is a confirmed result, interpreted in context — not a lone screen. The two-stage design exists precisely so that speed doesn't come at the cost of accuracy.