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The growing numbers of individual and social problems associated with drug abuse necessitate new approaches in drug-testing systems. Equally, drug abusers may attempt to invalidate drug testing using different methods such as adulteration, dilution and substitution. This study aims to investigate tampering methods commonly used by Turkish substance-using probationers and evaluate their effects on toxicological drug-testing results. Initially, probationer urinary screening test results and laboratory substitution documents were evaluated to investigate the dilution and substitution attempt. Additionally, an experimental study was carried out by using readily available household products (bleach, vinegar, drain opener, eye drops) for adulteration. The effect of these agents was investigated for 11‐nor‐9‐carboxy-Δ9‐tetrahydrocannabinol (THC–COOH), amphetamine and 3,4-methylenedioxymethamphetamine (MDMA). It was determined that probationers preferred unbranded products (syringes, nylon bottles, etc.) for urine substitution. To detect dilution, screening test results were evaluated along with creatinine values. The variability of mean creatinine values can change the rate of the before-negative and after-positive ratio. For adulteration method, the high amounts of bleach provided false-negative results for THC–COOH and amphetamine, but spiking in any concentration of bleach affected MDMA results, causing a slight increase. Vinegar did not affect the THC–COOH and amphetamine results. However, false-negative results were observed for MDMA, with high amounts of vinegar-spiked urine samples. Drain opener was added in large quantities, and false-negative results were observed for all analytes. Visine eye drops did not have any effect on THC–COOH or amphetamine, but a high quantity of eye drops had a slight decreasing effect for MDMA.

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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