A Saliva-to-Plasma THCCOOH Ratio Reported in ng/µg Is Not a Dimensionless Ratio
A ratio near one can mislead when its numerator and denominator use different units. In a controlled cannabis study, oral-fluid-to-plasma THCCOOH ratios were reported in ng/µg because oral-fluid metabolite concentrations were much lower.
Paired specimens under controlled dosing
Eleven daily cannabis smokers resided on a research unit for 51 days. Oral THC maintenance sessions involved 0, 30, 60 or 120 mg/day, followed by a smoked-cannabis challenge at the end of each session.
Researchers analyzed paired oral fluid and plasma. Ratios were calculated when both specimens were positive for the analyte, selecting a subset of all collections.
One does not mean equal concentrations
The paper expressed THCCOOH ratios in ng/µg. One microgram equals 1,000 nanograms. A value of 1 ng/µg therefore equals 0.001 after converting the concentrations to matching units.
For illustration, 1 ng/L divided by 1 µg/L is 1 ng/µg, even though plasma concentration is 1,000 times higher. This example explains the units; it does not reconstruct a participant’s result.
The parent drug followed another pattern
Parent-THC ratios were strongly influenced by time since smoking and mouth contamination. THCCOOH ratios were generally more stable, but still varied between participants and conditions.
Not every active-dose comparison or metabolite-time relationship was significant. A relatively stable metabolite pattern does not establish a universal partition factor. Repeated specimens also do not represent additional independent participants.
Review the units before making an inference
Identify the analyte, matrices, units and whether the ratio retains units. Check how below-limit observations and repeated samples were handled. Each decision can affect the apparent relationship.
The small daily-user sample and positive-pair selection limit individual predictions. Unit conversion clarifies what the number means; it cannot remove biological variability or turn a concentration ratio into an impairment measure.
For the separate forensic question, see our discussion of cannabinoid concentrations and evidence of impairment.
Primary research
Oral fluid/plasma cannabinoid ratios following controlled oral THC and smoked cannabis administration. Analytical and bioanalytical chemistry (2013). DOI: 10.1007/s00216-013-7159-8.
