Extreme THC Ratios Separated the Mean From the Median in a Dried-Blood-Spot Study
The dried-spot study’s average capillary-to-venous THC ratio was strongly affected by extreme observations. Its mean and median should not be presented as interchangeable conversion factors.
The distribution was highly uneven
The reported median ratio was 2.11, while the mean was 16.6 with a very large standard deviation. Ratios ranged from 0.75 to 722. After ten observations were classified as outliers, the median became 1.84 and the mean 2.57. The remaining range still extended from 0.75 to 8.18.
Outlier removal did not establish one true correction
The authors discussed possible fingertip contamination, but did not experimentally identify the cause of every extreme observation. Statistical exclusion changes the summarized dataset; it does not prove that every excluded result was chemically false or that every retained result was unaffected.
An individual specimen needs its own context
A mean influenced by rare extremes is particularly unsuitable as a universal multiplier. Even the post-exclusion median conceals substantial variation. Preserve the sampling site, preparation, analyte and paired observations rather than converting a dried-spot THC result into a venous result with one ratio. These human specimen comparisons did not measure driving performance or validate an impairment threshold.
Related analysis: the study’s broader findings and limitations.
