USDTL resource guide
Hair drug testing
Often called a “hair follicle” test
A longer view of drug use or exposure, using a small sample of hair cut close to the scalp.
The approximate testing window
A visual reference for this specimen.
Approximate window, not guaranteed detection or clearance. The bar shows a general span, not continuous coverage or a probability. Shaded ends represent the stated range. Review the limitations and sources below. View sources.
What this test tells you
The laboratory analyzes the hair shaft. A 1.5-inch scalp-hair sample represents approximately three months of history. The drugs tested depend on the panel ordered.
What collection involves
Hair is cut close to the scalp. USDTL describes about 200 strands for a standard sample; larger panels may require more. The collector determines the appropriate sample.
Keep in mind
This is a broad historical window, not a date-by-date record. A result does not establish the exact time, dose, or impairment.
A common question
Does a hair drug panel include alcohol?
Confirm the order. Hair EtG is a separate alcohol biomarker; a drug panel should not be assumed to include it.
Read the original resources.
Source material behind this guide.
- USDTL · Hair drug testingScalp hair, specimen collection, and the approximate three-month window.
- USDTL · Reservoir matrices (PDF)Why specimen results cannot establish an exact dose or time of use.
Reviewed October 26, 2026. Laboratory guidance can change; the current order and report take priority.