Nicotine metabolite ratio: Comparison of the three urinary versions to the plasma version and nicotine clearance in three clinical studies.

Background: Variation in CYP2A6 activity influences tobacco smoking behaviors and smoking-related health outcomes. Plasma Nicotine Metabolite Ratio (NMR) is a robust phenotypic biomarker of CYP2A6 activity and nicotine clearance. In urine, the NMR has been calculated as a ratio of free trans-3'-hydroxycotinine to free cotinine (NMR), total trans-3'-hydroxycotinine to free cotinine (NMR), or total trans-3'-hydroxycotinine to total cotinine (NMR).