A concentration peak in hair can look like a timestamp. A review by Shima and colleagues illustrates why the number of peaks can be misleading when a laboratory is trying to distinguish one exposure from several.
The authors describe two regions of incorporation near the hair root. In the zolpidem examples they reviewed, the larger peak was associated with incorporation near the bulb and a smaller peak with incorporation higher in the root. A single exposure can therefore give rise to a two-peak distribution. The relative size and shape of these regions vary by compound. Hair pigmentation and cosmetic treatment can also affect the amount recovered.
The interpretation should identify the compound, the segment length and orientation, the method’s sensitivity, and whether the pattern is supported across strands. A pooled segment may be suitable for detecting a drug while blurring the distribution needed to evaluate timing. Where the dispute is about one episode versus two, the number of peaks alone is insufficient. Ask whether the observed spacing and distribution are consistent with the published incorporation pattern for that drug.
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