Oral fluid drug testing is increasingly used in forensic toxicology, particularly in drug-driving investigations. However, a new study published in the Journal of Analytical Toxicology highlights an important issue: the concentration of a drug measured in an oral-fluid sample can be influenced by what happens during sample collection and preparation.
The research examined THC, cocaine and cocaine metabolites and found that these pre-analytical effects can differ considerably depending on the drug being tested.
For solicitors dealing with toxicology evidence, the findings provide a useful reminder that a reported drug concentration should always be considered alongside the method used to obtain and analyse the sample.
What did the study investigate?
Researchers examined oral-fluid samples collected using FLOQSwabs®.
They investigated concentrations of:
– delta-9-tetrahydrocannabinol (THC)
– cocaine
– benzoylecgonine (BZE)
– ecgonine methyl ester (EME)
The samples were separated into different fractions and analysed using liquid chromatography-tandem mass spectrometry (LC-MS/MS).
The researchers wanted to determine whether buccal cells — cells from the inside of the cheek — and retention of drugs within the collection swab could affect the concentrations ultimately measured.
THC and buccal cells
One of the most interesting findings concerned THC.
In 102 THC-positive specimens, both the number of buccal cells and the concentration of THC were significantly higher in the cell-rich fraction of the sample.
The median enrichment of buccal cells was approximately 4.4-fold, while the median THC enrichment was approximately 3.9-fold.
The researchers also identified a strong relationship between the amount of cellular material present and THC enrichment at concentrations above 5 ng/mL.
This suggests that buccal cells can effectively act as a reservoir for THC within an oral-fluid specimen.
Perhaps most notably, four specimens had THC concentrations below the analytical limit of detection in the homogenised sample, while THC remained detectable in the cell-rich fraction.
Cocaine behaved differently
The results for cocaine and its metabolites were different.
Concentrations of cocaine, BZE and EME were broadly comparable between the homogenised sample and the cell-rich fraction.
However, the material remaining within the swab consistently produced higher concentrations.
Across 52 specimens, the median ratios between the expressed swab and homogenised sample were:
– 1.16 for cocaine
– 1.28 for BZE
– 1.14 for EME
This suggests that, unlike THC, the main issue for cocaine-related compounds may be the incomplete release of the drug from the collection swab, rather than retention within buccal cells.
Why does this matter in forensic toxicology?
The study demonstrates that pre-analytical variability in oral fluid drug testing is analyte-dependent.
In simple terms, different drugs may behave differently within the same oral-fluid collection system.
For THC, the amount of cellular material collected may influence the measured concentration. For cocaine and its metabolites, retention within the swab itself may be more important.
This distinction matters because a laboratory result is not produced in isolation. Before analysis even begins, factors associated with sample collection, storage and preparation can potentially influence the specimen presented to the analytical instrument.
What could this mean for drug-driving cases?
Oral-fluid testing can provide valuable evidence of recent drug exposure. However, the interpretation of a measured concentration requires an understanding of both the analytical method and the limitations of the specimen being tested.
This research does not mean that oral-fluid drug testing is unreliable. Instead, it demonstrates why results should be interpreted within the context of the complete analytical process.
Where the significance of an oral-fluid result is disputed, relevant questions may include how the sample was collected, which collection device was used, how the drug was recovered from that device and how the specimen was prepared before analysis.
The study is particularly interesting because it demonstrates that these factors may not affect every drug in the same way.
The importance of expert interpretation
Forensic toxicology involves more than simply reporting a numerical result.
Understanding how a sample was obtained, processed and analysed can be important when assessing what a particular result means and what conclusions can reasonably be drawn from it.
Research such as this continues to develop our understanding of oral fluid drug testing and highlights why careful scientific interpretation remains important in cases involving toxicological evidence.
At Bericon Forensics, our toxicology experts can review analytical findings and the associated scientific evidence to assist solicitors in understanding the significance and limitations of toxicology results.
