The impact of creatinine reference value: Normalization of urinary drug concentrations

Many illicit drug users attempt to manipulate urine drug testing; dilution is one of the methods. In screening tests, false-negative results below the cut-off values can create positive results after creatinine normalization. This study aimed to evaluate the impact of a creatinine reference value on the normalization of the drug concentration in diluted urine. The study focused on 25 630 cases and the following information: gender, age, urine collection time, drug screening test results, creatinine concentration (CR), and confirmation analysis result. Mean CR value was 143.71 ± 83.68 mg/dl. There was a significant difference between CR and gender (p = 0.03). The mean CR for women was lower than that for men. The correlation between age and CR was not significant (r = -0.08, p = 0.00). However, after grouping the sample into age groups of 10 years, there was a significant difference between age groups and mean CR (p = 0.00). The mean CR was significantly lower in the 0-9 year age group (n = 34) than in the 20-29 year age group (n = 10 943). According to the urine specimen collection time, CR levels during the early hours of the day (06:00-06:59) were lower than those during the remaining hours (p = 0.00). The highest converted drug-negative to drug-positive results were obtained from 153.23 mg/dl CR reference value. Reference CR values were evaluated according to gender, age, and urine collection time. Different rates of positive results were obtained for each reference value. There is no published local creatinine value for spot urine samples in many countries, including Turkey. This will be useful to develop appropriate normalization models when reporting drug test results.