Low Saliva Volume Changed the Dilution of Cannabis Test Samples
A saliva specimen can contain THC and still have a collection problem that affects its reported concentration. In a controlled cannabis-smoking study, many early oral-fluid samples contained less saliva than the collection device intended. THC was nevertheless identified in every low-volume specimen collected after smoking. Detection did not make the volume issue disappear.
What the human experiment collected
Newmeyer and colleagues studied 14 frequent and ten occasional cannabis smokers after a single standardized 6.8% THC cigarette. Participants smoked as they chose for up to ten minutes. Oral fluid was collected repeatedly with the Oral-Eze device, starting thirty minutes after smoking and continuing through thirty hours.
The device used a cotton collection pad and a volume indicator designed to change color after collecting one milliliter of oral fluid. The pad then went into two milliliters of stabilizing buffer. At the intended volume, saliva constituted one-third of the combined liquid.
Low-volume collections clustered early after smoking
At the half-hour collection, ten frequent smokers’ specimens and nine occasional smokers’ specimens were recorded as low volume. At one hour, the corresponding counts were seven and eight. Additional low-volume specimens occurred at other times.
These are counts of specimens at specified collections, not separate groups of additional participants. The study obtained 404 specimens from 24 people. Repeated collections increase the information about each person’s time course without increasing the number of independent people studied.
A fixed buffer volume changes the dilution question
The investigators analyzed low-volume specimens as collected, without applying weight corrections. Less saliva entering a fixed amount of buffer means more dilution of the original fluid. The authors expressly recognized that the resulting concentrations were artificially reduced.
This is a collection-volume issue, separate from whether the instrument can identify THC. The paper reported THC in all low-volume post-smoking specimens, but that observation does not establish that all such specimens would exceed every possible decision cutoff. Identification, quantitative concentration and classification against a chosen cutoff are different questions.
The study did not validate a universal correction
The report did not supply a single multiplier that can repair any underfilled saliva collection. An indicator recording inadequate volume is not itself an exact measurement of how much saliva was collected. Applying a guessed volume correction would add an unsupported assumption.
Collection-pad recovery adds another consideration. The laboratory’s validation found that different cannabinoids were recovered from the pad at different rates. Those validation findings do not justify simply adding a fixed percentage to every authentic THC result. The specimen, buffer, calibration and validated analytical procedure must be considered together.
The low-volume observation was descriptive; the paper did not establish an experimentally isolated effect of specimen volume on driving impairment. It also reported no significant post-smoking difference between the groups’ median THC concentrations at the sampled times. That null result does not establish identical physiology or remove collection-related uncertainty.
What a laboratory-record review should establish
Review the collection device, adequacy indicator, recorded specimen volume, buffer amount and the laboratory’s handling of an underfilled collection. Determine whether the reported concentration incorporates the intended dilution and whether the procedure was validated for an inadequate sample. These records help evaluate the measurement without assuming that an underfilled specimen is automatically unusable.
This small study concerned oral fluid after smoked cannabis. It did not establish the same collection behavior for edibles, other devices, blood or urine. Nor did identifying THC in an underfilled specimen establish a person’s functional condition at an earlier incident. For a separate issue affecting the meaning of a positive result, see how AND versus OR analyte rules change cannabis detection windows.
Primary research
Newmeyer MN, Desrosiers NA, Lee D, et al. Cannabinoid disposition in oral fluid after controlled cannabis smoking in frequent and occasional smokers. Drug Testing and Analysis. 2014;6:1002–1010. doi:10.1002/dta.1632.
