External Contamination
Definition
The presence of a drug or substance on the outer surface of the hair shaft from environmental exposure rather than systemic ingestion. Distinguished from true incorporation by washing protocols, the presence of parent drug without metabolites, and uniform distribution along the shaft rather than a tapering gradient.
- Source
- Environmental exposure on hair shaft surface
- Distinguishing test
- Washing protocols before analysis
- Chemical signature
- Parent drug present without metabolites
- Distribution pattern
- Uniform along shaft, not a tapering gradient
Common questions
Why is the absence of metabolites a key indicator of external contamination?+
Systemic ingestion produces drug metabolites through the body's own metabolism before incorporation into growing hair, whereas surface contamination deposits only the parent compound from the environment, so parent drug with no metabolite points toward external exposure.
Do standard washing protocols reliably remove all external contamination before testing?+
Washing reduces surface contamination substantially but is not perfect, particularly for contamination that has begun to penetrate the shaft's outer layers, which is why laboratories combine washing with metabolite-ratio and distribution-pattern checks.
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