Creatinine normalization of workplace urine drug tests: does it make a difference?

Area of Science:

• Forensic toxicology
• Clinical chemistry

Background:

• Urine drug testing is crucial for clinical and forensic toxicology.
• Specimen dilution can affect the accuracy of drug test results.
• Creatinine normalization is a common method to adjust for urine concentration.

Purpose of the Study:

• To evaluate the impact of creatinine normalization on urine drug concentrations.
• To determine how normalization affects the proportion of positive drug tests for five common substances.

Main Methods:

• Compared pre- and post-normalization urinary drug concentrations using Wilcoxon matched-pairs signed-ranks test.
• Utilized McNemar testing to analyze differences in proportions of positive drug tests before and after normalization.
• Analyzed data from 4460 urine drug tests.

Main Results:

• Creatinine normalization significantly increased mean urinary drug concentrations for all tested drugs (amphetamines, cocaine, marijuana, opiates, phencyclidine).
• Effect sizes for concentration increases ranged from small to medium, with cocaine showing the largest effect (r = 0.229).
• A statistically significant increase in the proportion of positive drug tests was observed post-normalization for all drugs (P = 0.0010).

Conclusions:

• Specimen dilution demonstrably influences the number of confirmed laboratory-positive drug test results.
• Creatinine normalization is an important consideration in interpreting urine drug test outcomes.
• These findings highlight the need for careful interpretation of urine drug test results in the context of specimen dilution.