The hair study washed specimens but discarded the wash solutions. Its final hair concentrations therefore were not accompanied by quantitative wash results that could reconstruct the amount removed from the surface.
Preparation differed by testing workflow
For GC–MS/MS, the protocol used a buffer wash followed by water, methanol and water washes before hair digestion and extraction. Those washes were discarded. The immunoassay workflow also included a methanol decontamination step, with the solvent discarded.
Washing was thus part of the preparation. It was not a separate measured outcome comparing cannabinoid amounts in every wash and the remaining hair.
A preparation step does not prove its complete effect
The fact that hair was washed does not establish that all external material was removed, nor that the remaining drug necessarily entered only through blood. Conversely, discarding washes does not demonstrate that the final result was contaminated. It limits the evidence available to resolve that question retrospectively.
What a case interpretation should retain
Identify the wash method, which fractions were tested and the analytes reported. This small study of documented cannabis users primarily addressed detection in hair, not a universally validated surface-versus-systemic classification rule. A positive hair result remains separate from an exact dose, last-use time or impairment assessment.
Related reading: interpreting cannabinoid assay validation.
Research cited
Huestis and colleagues (2006). Cannabinoid Concentrations in Hair from Documented Cannabis Users. DOI: 10.1016/j.forsciint.2006.08.005.
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