How Drug Tests Work: Screens, Confirmations, Panels and Validity Checks

Updated 15 Oct 2026 · By the DrugTestAnswers editorial team

Here's the whole process, start to finish, the way we'd explain it to a friend in the parking lot before his appointment.

Step 1: the screen

Most tests start with an immunoassay: antibodies that react to a drug or its metabolite. It's fast and cheap, but it can cross-react with other substances, which is where false positives come from. Results are compared with a cutoff.

Step 2: confirmation

A non-negative screen in a lab goes to gas or liquid chromatography with mass spectrometry (GC-MS or LC-MS), which identifies the exact molecule. That's very hard to fool and rarely wrong.

Step 3: the Medical Review Officer

In regulated programs, a physician called the MRO reviews confirmed positives and contacts you about legitimate prescriptions before anything reaches your employer.

Panels

A "5-panel" tests five drug classes (typically THC, cocaine, amphetamines, opiates and PCP). 10 and 12-panel tests add things like benzodiazepines, barbiturates, methadone, oxycodone or MDMA.

Specimen validity

Labs check temperature (32-38°C within four minutes of collection), creatinine, specific gravity, pH and oxidants. Out-of-range samples are reported as dilute, substituted, adulterated or invalid, and many employers treat those as a fail or a refusal.

Specimen types

• Urine: most common, days of history.
• Saliva: observed, short window (about 1-2 days).
• Hair: about 90 days, misses the most recent week.
• Blood: hours to a day or two, mostly used after accidents.

Keep reading

See home and lab tests, the glossary and the detection table.