The testing of saliva or oral fluid at the roadside could be a powerful tool to detect drivers under the influence of drugs and has several advantages over urine screening. In 177 cases of individuals suspected of driving under the influence of drugs, oral fluid was collected at the roadside and analyzed in parallel to serum samples. The study was performed to investigate the variability of oral fluid analysis results in relation to blood/serum. In 45% of the cases single-drug use was found, and in 50% poly-drug use was found. Cannabis was most prevalent (78%), and 70% of these individuals were also positive for tetrahydrocannabinol in serum. Overall, 97% of oral fluid samples positive for any substance were also positive in serum. Comparing data of oral fluid and serum for amphetamine, MDMA, morphine, benzoylecgonine, and tetrahydrocannabinol, the sensitivities were 100%, 97%, 87%, 87%, and 92%, respectively. Overall specificity and accuracy were in the range of 91-98%. Discrepancies between a negative oral fluid sample and a positive serum sample could be explained by analytical insensitivity in the lower volume of oral fluid analyzed (estimated for 0.1 mL confirmation vs. 1 mL of serum) or a shorter detection window in oral fluid. The low prevalence of discrepancies with positive oral fluid and negative serum results (2-9% of the cases) may be explained by persistent oral contamination especially for orally consumed drugs, like MDMA and cannabis. It is concluded that the detection of a psychoactive substance in oral fluid taken at the roadside is highly predictive for the detection of the corresponding drug or its metabolite in serum. Oral fluid testing is therefore suitable for the efficient confirmation of drug use of drivers suspected of being under the influence of drugs.