Objective: The dilution of urine is a serious problem in drugs of abuse (DOA) testing. Therefore, methods and procedures that identify such adulterations are mandatory.

Method: Creatinine concentrations from 15,978 urine samples and DOA concentrations from 9,937 urine samples were determined. 11.4% of these samples were from prisoners and 88.6% from patients undergoing substitution therapy in drug substitution clinics. In our study, patients with established diagnosis for nephropathies were excluded.

Results: The mean creatinine value of 0.83 g/L that we found in DOA samples was in the same area of healthy male individuals (0.89 g/L). From the 9,937 DOA urine samples, 6,019 (60.57%) were tested positive. Data from samples with diluted urine defined as < 0.2 g/L creatinine (SAMHSA) or < 0.4 g/L (clinical chemistry reference value) were analyzed further. A protocol for handling samples with diluted urine in the analysis of DOA is presented. The procedure takes advantage of the fact that analytical limits of quantification are lower than cutoff values of DOA analysis. The method relies on a mathematical calculation of cutoff concentrations in relation to the quotient of the urine creatinine concentration, measured in the urine specimen, and the lower reference value of creatinine in urine.

Conclusions: Using this mathematical procedure for calculation of cutoffs for diluted urine (adulteration), we could demonstrate that creatinine- dependent cutoffs are highly suitable to identify positive samples in DOA testing.