How drug tests work — the basics
Most routine workplace and clinical drug tests first use a screening immunoassay to detect likely positives quickly. If a screen is positive, laboratories generally perform a confirmatory test such as GC-MS or LC-MS/MS to specifically identify and quantify the drug or metabolite. Confirmatory testing reduces false positives and provides more reliable legal/medical evidence.
Screening vs Confirmatory (quick)
- Screening (immunoassay): Fast, cheaper, more likely to cross-react (possible false positives).
- Confirmatory (GC-MS / LC-MS/MS): Slower, more expensive, highly specific and quantitative — the gold standard for confirmation.
Common testing methods
Urine tests
Most common for workplace testing because collection is noninvasive and many metabolites are present in urine for days to weeks after use. Urine tests typically follow federal cutoffs for screening and confirmatory reporting.
Hair (follicle) tests
Hair testing can document months of past exposure (commonly ~90 days for head hair). It records a history rather than very recent use.
Blood tests
Blood detects parent drugs and is useful for detecting recent use or impairment. Detection windows are typically shorter (hours to a few days).
Saliva / oral fluid tests
Oral fluid is great for on-site collection and detecting recent use (hours to a few days). It's harder to adulterate than urine and detects parent drugs rather than metabolites.
Typical detection windows (estimates)
The table below gives common ranges used in public guidance and clinical summaries.
| Specimen | Typical detection window | Notes |
|---|---|---|
| Urine | Hours → several days (occasional use) | Most common workplace matrix; uses metabolite detection; subject to cutoff levels. |
| Hair | Up to ~90 days | Records past months of use; not ideal for detecting very recent consumption. |
| Blood | Hours → 1–2 days | Best for recent use; invasive collection. |
| Saliva | Hours → ~2–3 days | Good for recent use; simple onsite collection; detects parent drug. |
Estimates summarized from clinical & federal guidance sources.
What is a 5-panel / 10-panel test?
“Panel” refers to which drug classes or specific analytes are being screened.
- Typical 5-panel: THC, Cocaine, Opiates, Amphetamines/Methamphetamine, PCP.
- 10-panel (examples vary): May include the 5-panel plus benzodiazepines, barbiturates, methadone, propoxyphene, MDMA/MDxx, and others — lab-specific.
Common pitfalls & what to know
- Cutoff levels: Labs use cutoffs to reduce false positives.
- False positives: Screening immunoassays can cross-react; confirmatory testing reduces errors.
- Adulteration & sample integrity: Observed collections and adulteration testing are used to ensure validity.
Frequently asked questions
How long does marijuana (THC) show up in a urine test?
Detection depends on frequency and dose. Occasional users: up to ~3 days; regular/heavy users: weeks.
Can a screening test be trusted alone?
No — confirmatory testing (GC-MS or LC-MS/MS) is required for reliable results.
Which test detects long-term use?
Hair tests capture long-term patterns (~90 days), urine and saliva detect recent use.