Interpretation of Toxic & Controlled Substance Reports

The detection of a substance in a biological sample does not by itself establish impairment, recent use, or causation. Interpretation depends on sample type, collection timing, analytical method, cutoff values and individual variables.

Different biological matrices represent different time windows. Blood and oral fluid reflect recent exposure; urine typically provides a longer detection window; hair offers a historical record spanning months. Positivity is not always synonymous with impairment, and negativity does not always mean no use occurred.

One of the most frequently requested analyses in traffic cases is back-calculation from a measured BAC to the time of the incident. Such calculations carry inherent uncertainty due to inter-individual variation in elimination rate and the contribution of the absorption phase. Results must always be expressed as a range rather than a single value.

• Timing mismatch: Is the collection time consistent with the expected pharmacokinetic window?
• Legitimate use: Can documented prescription or therapeutic use explain the finding?
• Metabolite interpretation: Which metabolite was detected, and what does its presence represent?
• Analytical quality: Were screening and confirmation methods applied? Are cutoffs and units reported?
• Chain of custody: Is sample identity, storage and transfer documented?