Evaluation of Diagnostic Sensitivity and Specificity of a New Automatized Assay for Indirect Synthetic Urine Identification

Synthetic urine mimicking normal urine is difficult detectable by standard specimen validity testing. In this study, the VDx True Urine LD Test and True Urine SD Test, analysing 'long duration' (LD) and 'short duration' (SD) markers, were evaluated for their ability to detect synthetic urine. Two synthetic urines from the US market and 1188 real-life urine specimens were analysed for LD markers, SD markers, uric acid and temperature. Forty-seven specimens (set 1) showed uric acid and/or temperature suspicious results and were analysed by LC-MS/MS for 10 endogenous biomolecules. Diagnostic sensitivities and specificities were calculated for LD and SD markers based on uric acid and LC-MS/MS results of set 1. The false-positive rate of the SD marker was further evaluated with uric acid results of the other 1141 specimens (set 2). Additionally, direct synthetic urine markers were analysed by LC-(HR)MS. Uric acid and LC-MS/MS could identify synthetic urine. In one US synthetic urine, low concentrations of uric acid were found. Specimens of the Austrian/German region were reliably classified congruently by both methods. The LD and the SD markers detected both synthetic urines. For set 1, the LD had 100% sensitivity and 88.9% specificity, and the SD had 84.2% sensitivity and 44.4% specificity for authentic/synthetic specimens. In set 2, the SD marker specificity depended on the upper limit of the acceptance criteria. In one US synthetic urine, carbendazim was identified. Overall, several tests should be applied to confirm authentic or synthetic urine. Especially, simultaneous analysis of indirect and direct synthetic urine markers should be considered.