Creatinine excretion in consecutive urine samples after controlled ingestion of water

Highly diluted urine is among the most commonly observed factors affecting the validity of urine testing for drug abuse. A minimum creatinine concentration of 20 mg/dL urine has been proposed as a marker for dilution of a urine sample. This study investigates the effect of water consumption on creatinine concentration, as well as its effect on specific gravity and osmolality. In this study, 22 subjects (17 women and 5 men) were included to determine the influence of sex and weight on the impact of excessive water consumption on these markers of urine dilution. The subjects consumed 0.5 L, 1.0 L, and 1.5 L of water, respectively, within 15 minutes. The mean minimum creatinine concentrations (Jaffé reaction) for the void 2 hours after fluid intake were 60.4 mg/dL, 15.8 mg/dL, and 10.9 mg/dL for the respective ingested volumes of water. Mean creatinine concentrations excreted by men were significantly higher than those excreted by women. Participants with a weight below 60 kg tended to excrete lower urine creatinine concentrations. 50% of the volunteers with a BMI < 20 kg/m² and 20% of the volunteers with a BMI > 20 kg/m² exhibited creatinine concentrations below the threshold value of 20 mg/dL. A similar pattern was established for gravity and osmolality. Due to its simple determination, creatinine may be preferred over specific gravity or osmolality. In order to evaluate the internal dilution of a urine sample for legally defensible drug testing, it may be necessary to account for sex and body weight.