Normalization of Urinary Drug Concentrations with Specific Gravity and Creatinine

TLDR

Excessive fluid intake can dilute urinary drug concentrations and cause false‑negative results, yet normalization methods used in anti‑doping, pain monitoring, and environmental programs are rarely applied in workplace, legal, or treatment settings. The study evaluated two urine drug‑concentration normalization procedures based on specific gravity and creatinine. These corrections were applied to specimens from three distinct groups—pain patients, heroin users, and marijuana/cocaine users—each with unique characteristics, study designs, and dosing conditions. The two procedures were highly correlated (r = 0.94) and produced small to substantial increases in percent positives depending on hydration status, demonstrating that normalization can effectively address dilute and concentrated urine samples and is recommended for urine testing programs.

Abstract

Excessive fluid intake can substantially dilute urinary drug concentrations and result in false-negative reports for drug users. Methods for correction ("normalization") of drug/metabolite concentrations in urine have been utilized by anti-doping laboratories, pain monitoring programs, and in environmental monitoring programs to compensate for excessive hydration, but such procedures have not been used routinely in workplace, legal, and treatment settings. We evaluated two drug normalization procedures based on specific gravity and creatinine. These corrections were applied to urine specimens collected from three distinct groups (pain patients, heroin users, and marijuana/ cocaine users). Each group was unique in characteristics, study design, and dosing conditions. The results of the two normalization procedures were highly correlated (r=0.94; range, 0.78-0.99). Increases in percent positives by specific gravity and creatinine normalization were small (0.3% and -1.0%, respectively) for heroin users (normally hydrated subjects), modest (4.2-9.8%) for pain patients (unknown hydration state), and substantial (2- to 38-fold increases) for marijuana/cocaine users (excessively hydrated subjects). Despite some limitations, these normalization procedures provide alternative means of dealing with highly dilute, dilute, and concentrated urine specimens. Drug/metabolite concentration normalization by these procedures is recommended for urine testing programs, especially as a means of coping with dilute specimens.