A new metabolomics-based strategy for identification of endogenous markers of urine adulteration attempts exemplified for potassium nitrite.

Urine adulteration to circumvent positive drug testing represents a problem for toxicological laboratories. While creatinine is a suitable marker for dilution, detection of chemicals is often performed by dipstick tests associated with high rates of false positives. Several methods would be necessary to check for all possible adulterants. Untargeted mass spectrometry (MS) methods used in metabolomics should theoretically allow detecting concentration changes of any endogenous urinary metabolite or presence of new biomarkers produced by chemical adulteration. As a proof of concept study, urine samples from 10 volunteers were treated with KNO2 and analyzed by high-resolution MS. For statistical data evaluation, XCMSplus and MetaboAnalyst were used. Compound identification was performed by database searches using an in-house database, Chemspider, METLIN, HMDB, and NIST. Principle component analysis revealed clear separation between treated and untreated urine samples. In detail, 307 features showed significant concentration changes with fold changes greater than 2 (79 decreased; 228 increased). Mainly amino acids (e.g., histidine, methylhistidine, di- and trimethyllysine) and purines (uric acid) were detected in lower amounts. 5-HO-isourate was found to be formed as a new compound from uric acid and, e.g., imidazole lactate concentrations increased due to the breakdown of histidine. This metabolomics-based strategy allowed for a broad identification range of markers of urinary adulteration. More studies will be needed to investigate routine applicability of identified potential markers exploring urinary conditions of their formation and stability. Selected markers might then be integrated into routine MS screening procedures allowing for detection of adulteration within routine MS analysis. Graphical Abstract ᅟ.