Influence du seuil de confirmation lors du dépistage urinaire de l'usage du cannabis : à propos de cinq années de dépistages réalisés au sein de la Gendarmerie nationale.

Urinary cannabinoids screening are carried out in the framework of medical fitness examination previous to be enlisted in the French gendarmerie. This paper presents the results of 986 confirmation analyses of positive screening tests, carried out by the toxicology department of the Forensic Science Institute of the French Gendarmerie (IRCGN) between 1st January 2005 and 31 December 2009. The retrospective study of these data has evaluated the impact of lowering confirmation cut-off values. If it is set at 5 μg.L-1 of 11-nor-9-carboxy-_9- tetrahydrocannabinol (THCCOOH), instead of the international guidelines confirmation cut-off values 15 μg.L-1, this increases the number of confirmed cases by 25.2%. The positive predictive value of the screening test raises from 63.9% to 80.0%. The use of a confirmation cut-off value lower than that recommended by international authorities increases significantly the sensitivity of the urinary cannabinoids screening program without altering its specificity. Moreover, the cross reaction between the screening test antibodies directed against THCCOOH and niflumic acid has been identified as a major cause of false positive results. This substance indeed has been detected in 34.9% of samples for which the confirmation analysis has proved to be negative.