External quality assessment of multi-analyte chromatographic methods in oral fluid

A proficiency testing scheme was set up for the DRUID (Driving under the influence of Drugs, Alcohol and Medicines) research project, funded by the European Commission, in which oral fluid is analysed by eleven laboratories. A common collection and analysis methodology is used: Statsure Saliva Sampler is used for collection and LC-MS/MS or GC-MS confirmation analysis of 22 substances is performed on all samples. Despite internal validation and quality control samples, external quality assessment is still necessary to further increase comparability of results. Four rounds of proficiency testing (PT) were organized between March 2008 and September 2009.

Qualitative results were evaluated using sensitivity and specificity. Quantitative results were evaluated using z-scores and the standard deviation of Horwitz.

Specificity was above 99% in each round, sensitivity per analyte varied between 81.7 and 100%, and 20 out of 22 analytes had a sensitivity above 90%. The percentage of satisfactory z-scores increased from 79.4% to 89.2%. This trend was seen for all drug classes, except zopiclone. Results were discussed with participating laboratories and problems were addressed.

Because of these corrective actions, DRUID laboratories have a lower variation in results than previously published PT schemes in oral fluid.