Repeatability of oral fluid collection methods for THC measurement

Study objectives: To determine the influence of sample collection for two different collection methods on THC concentrations and to compare THC concentrations collected by both methods.

Methods: A total of 136 pairs of oral fluid samples from subjects who had recently smoked Cannabis were obtained by the non-acidic Statsure oral fluid collection device and by ordinary spit tubes. Oral fluid was analyzed for THC by LC-MS/MS. Bland-Altman plots were used for the quantitative analysis of repeatability, whereas Cohen's kappa was used for qualitative analysis to determine the consistency of the results with regard to the Belgian legal limit.

Results: Repeatability of both sampling methods was very low. The Statsure device had a better rate of agreement when compared with the Belgian legal limit than the spitting method. THC concentrations of samples collected by spit tubes were on average a factor 5.9 higher than the corresponding concentrations in samples collected by the Statsure device.

Conclusions: The repeatability of both the Statsure collection method and the ordinary spit tubes was low when applied to subjects who had consumed Cannabis very recently. Furthermore, THC concentrations were higher in samples obtained by spitting than samples collected with Statsure. These results may have implications for confirmation analysis in oral fluid, when applied for legal purposes.