Relation between blood- and urine-amphetamine concentrations in impaired drivers as influenced by urinary pH and creatinine.

Amphetamine undergoes extensive renal excretion and significant amounts are present in urine as the unchanged parent drug. This prompted us to investigate whether a quantitative relationship existed between blood and urine concentrations of amphetamine in the body fluids of drug-impaired drivers apprehended in Sweden, where this stimulant is the major drug of abuse. The relationship between blood and urine concentrations of amphetamine was determined by multivariate analysis with urinary pH and creatinine as predictor variables. Amphetamine was determined in blood and urine by gas chromatography-mass spectrometry with deuterium-labelled internal standards. The concentration of amphetamine in urine was about 200 times greater than the concentration in blood; the mean and median urine/blood ratios were 214 and 160, respectively, with large individual variations. The Pearson correlation coefficient between urine (y) and blood (x) amphetamine was r = 0.53, n = 48, which was statistically highly significant (P < 0.001), although the residual standard deviation (SD) was large (+/- 181 mg/L). The correlation coefficient increased (r = 0.60) when the concentration of amphetamine in urine was normalized for dilution by dividing with the creatinine content. When urinary pH and creatinine were both included as predictor variables, the correlation coefficient was even higher (r = 0.69), now explaining 48% (r2 = 0.48) of the variation in urine-amphetamine concentration. However, the partial regression coefficient for creatinine (53 +/- 28.7) was not statistically significant (t = 1.85, P > 0.05), whereas the corresponding regression coefficient for pH was highly significant and had a negative sign (-102 +/- 32.6, t= -3.12, P < 0.005). Other factors could impact on the urine-blood amphetamine relationship, such as route of administration, pattern of voiding and time elapsed after use of the drug.