A Substance Enters the Body
Every detection window starts counting from a single moment of use.
- 0 h: Time zero (ingestion or injection)
- 30–90 m: Peak blood conc. (absorption phase)
- Liver: Metabolite formation (primary site)
- 1–30 h: Half-life range (substance dependent)
Absorption begins immediately
The substance enters the bloodstream within minutes of use.
Three specimens start recording differently
Blood, urine, and hair each begin their own detection clock.
Metabolism sets the pace
Liver enzymes convert the parent drug into detectable metabolites.
Blood Reveals Very Recent Use
Blood testing captures the shortest, most immediate window of the three.
- 6–48 h: Blood window (short detection)
- Parent drug: What it detects (plasma level)
- ER / DUI: Typical use case (clinical context)
- Hours: Sample stability (degrades fast)
Blood mirrors current impairment
Blood levels track active intoxication, not past history.
Window closes within a day or two
Most substances clear blood plasma inside 48 hours.
Best for recency, worst for history
Blood cannot show use from last week.
Urine Extends the Window to Weeks
Urine testing is the most common workplace and clinical screening method.
- 2–42 d: Urine window (class dependent)
- Immunoassay: Test method (screening step)
- ng/mL: Result basis (cutoff threshold)
- Workplace: Most common use (routine testing)
Metabolites accumulate in urine
Kidneys filter and concentrate drug metabolites over days.
Chronic use extends the window further
Fat-soluble drugs like THC linger far longer in heavy users.
Cutoffs determine a positive result
A result below the threshold reads as negative.
Hair Preserves a Months-Long History
Hair testing turns hair shafts into a slow-growing timeline of use.
- ~90 d: Hair window (standard segment)
- 1 cm/mo: Growth rate (average scalp hair)
- 3 cm: Sample length (typical segment)
- Longer: Heavy use effect (extended segments)
Drugs bind into the hair shaft
Metabolites lock into keratin as the hair grows.
Growth rate sets the timeline
Each centimeter of hair roughly equals one month.
Recent days remain invisible
Hair misses very recent use near the scalp.
Choosing the Right Specimen Changes the Story
The same substance tells three different stories depending on the specimen.
- 3: Specimens compared (blood, urine, hair)
- Blood: Best for recency (hours window)
- Hair: Best for pattern (months window)
- Chain of custody: Legal requirement (for court use)
No single specimen tells the whole story
Investigators often combine specimens to triangulate use.
Timing drives specimen selection
A negative urine test does not rule out chronic use.
Forensic conclusions depend on context
Detection windows are estimates, not exact boundaries.