The ratio study collected more oral-fluid specimens than plasma specimens because blood collection was limited. Its paired comparisons represent selected collection times, not every saliva observation.
Paired sampling had a defined budget
The study restricted total blood collection to 591 mL. Each participant supplied 77 plasma specimens and 169 oral-fluid specimens across the 51-day protocol. The planned paired collections covered ad-libitum smoking, oral-THC maintenance and maintenance followed by a smoking challenge.
Missed collections, refusals and extraction losses further reduced available results. Ratio analysis then required positive findings for the analyte in both matrices. These are distinct reasons why an oral-fluid specimen might not contribute to a reported ratio.
More saliva observations do not fill missing blood values
An unpaired saliva concentration cannot supply the plasma value that was never collected. A graph based on available pairs also need not describe every interval with equal precision. The design traded dense oral-fluid observation against practical limits on repeated blood sampling.
Interpret the actual paired dataset
Before using the study to support a conversion rule, inspect the relevant times, analyte and positive-pair counts. The authors observed substantial individual variability, particularly for parent THC. A sampling schedule and a group ratio do not reconstruct an unobserved blood curve or supply a validated impairment classification for a new person.
Related reading: limits of identifying cannabis sources.
Research cited
Lee and colleagues (2013). Oral fluid/plasma cannabinoid ratios following controlled oral THC and smoked cannabis administration. DOI: 10.1007/s00216-013-7159-8.
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