How should urinary cotinine concentrations be adjusted for urinary creatinine concentration?

Area of Science:

• Biochemistry
• Analytical Chemistry
• Tobacco Research

Background:

• Biomarker measurement in biological fluids is essential for exposure assessment.
• Cotinine is a primary metabolite of nicotine, used to assess tobacco smoke exposure.
• Urinary dilution can affect biomarker concentrations, necessitating adjustment for accurate interpretation.

Purpose of the Study:

• To evaluate the correlation between serum and urinary cotinine concentrations in male smokers.
• To determine the optimal method for adjusting urinary cotinine levels to account for urinary dilution.
• To assess the impact of different adjustment methods on the correlation between serum and urinary cotinine.

Main Methods:

• Collected paired urine and serum specimens from 279 male smokers.
• Measured cotinine concentrations in both urine and serum.
• Adjusted urinary cotinine levels using regression analysis based on urinary creatinine concentration.
• Compared correlations using raw urinary cotinine, ratio-based adjustment, and regression-based adjustment.

Main Results:

• A high initial correlation (0.83) was observed between serum and urinary cotinine.
• Adjusting urinary cotinine for creatinine concentration via regression analysis significantly increased the correlation to 0.91.
• Expressing urinary cotinine as a simple ratio to creatinine did not improve the correlation.

Conclusions:

• Regression-based adjustment of urinary cotinine for creatinine concentration is superior to simple ratio methods for improving correlation with serum levels.
• Accurate adjustment for urinary dilution is critical for reliable cotinine measurements in smokers.
• The principle of regression-based adjustment for urinary dilution is applicable to other urinary biochemical measurements.