Importance of creatinine analyses of urine when screening for abused drugs

Abstract

We report here a simple method involving urine creatinine measurements for testing authenticity and reducing false-negative results in urine testing for drugs of abuse. Urinary creatinine in consecutive patient samples (n = 176) ranged between 0.1 and 31.9 mmol/L (mean 9.8 ± SD 6.2) and the osmolality in these urines ranged between 49 and 1183 mOsm/kg (mean 595 ± SD 276). With other consecutive samples in which creatinine was (arbitrarily chosen) <4.3 mmol/L (n = 85), the correlation with osmolality was lower. In 10 randomly selected urine samples from different patients, all "clean" for all drugs of abuse in initial immunological drug testing with approved methodology (in which creatinine was <4.3 mmol/L and osmolality was <200 mOsm/kg), five patients turned out to be drug positive after a simple concentration by volume. In a formerly heavy smoker of cannabis, the excretion of cannabinoids and creatinine was monitored for 93 days. The substances showed very good correlation throughout this period (r = 0.93, P <0.001), whereas simple measurements of cannabinoid concentrations would have falsely indicated several relapses of cannabis abuse. Urine samples used in drug-abuse testing should be tested for creatinine; if creatinine is <4.0 mmol/L, negative results for drugs may not be valid.