Cotinine is the principal breakdown product of nicotine, making it the most reliable biomarker for determining recent tobacco or nicotine exposure. When nicotine enters the body from sources like cigarettes, vaping devices, or secondhand smoke, the liver processes it into this chemical substance. Unlike nicotine, which is rapidly cleared from the bloodstream, cotinine has a much longer half-life, meaning it stays detectable in the body for a significantly greater period. Measuring cotinine levels is widely used in clinical settings, insurance underwriting, and employment screenings to objectively verify a person’s exposure status.
Cotinine: The Primary Nicotine Metabolite
Nicotine is primarily metabolized within the liver by a specific enzyme called Cytochrome P450 2A6 (CYP2A6). This enzyme converts the active nicotine molecule into cotinine, which is the major proximal metabolite. Cotinine is then further broken down into other compounds before being eliminated from the body, primarily through urine.
Nicotine itself has a relatively short half-life, typically lasting only one to two hours, which means its concentration in the blood drops quickly after use. In contrast, cotinine has a mean half-life of approximately 16 to 20 hours, allowing it to be detected for several days following the last exposure. This extended stability makes cotinine a superior indicator of regular, sustained nicotine intake.
Cotinine testing can be performed using various biological samples, each offering a different window of detection and level of accuracy. Urine is often used for simple screening due to its high concentration of cotinine and ease of collection. Saliva is another common method, providing results that are closely correlated with the amount of nicotine absorbed. Blood, or serum, testing is generally considered the most accurate method for quantitative measurement, providing a precise level of the circulating metabolite.
Interpreting Cotinine Levels Across Sample Types
The interpretation of a cotinine test relies on established threshold values that vary significantly based on the biological sample tested. These thresholds are chosen to separate non-users from passive users and active users. Because cotinine concentrates in the urine, the numerical cut-off point for a positive result in a urine sample is substantially higher than the threshold used for a blood or saliva sample.
A non-exposure or baseline reading, which characterizes a true non-smoker with minimal environmental exposure, is typically defined by a serum or saliva cotinine level of less than 1 nanogram per milliliter (ng/mL). Levels below this point suggest no recent or significant nicotine intake. This low baseline may be used by insurance companies to qualify individuals for non-smoker rates, which are often significantly lower than those for smokers.
Passive exposure to secondhand smoke, or very minor intermittent use, often results in levels ranging from 1 ng/mL up to 10 ng/mL in serum or saliva. Individuals who have significant exposure to tobacco smoke in their environment may show levels at the higher end of this range, sometimes overlapping with light or irregular users.
The distinction between a non-smoker and an active user is often determined by a threshold of 10 to 20 ng/mL in serum or saliva. However, for a definitive active or heavy use classification, levels are significantly higher. Active, daily smokers typically present with serum or saliva cotinine concentrations that are well over 100 ng/mL, reflecting a high, continuous intake of nicotine. For urine testing, the cut-off to distinguish a smoker from a non-smoker is often set much higher, ranging between 50 ng/mL and 200 ng/mL, though mean levels for heavy users can exceed 1000 ng/mL.
Biological Factors Affecting Cotinine Clearance
The same nicotine intake can produce different cotinine readings in two separate individuals due to biological variations in the clearance rate. Genetic variability in the CYP2A6 enzyme is a primary determinant of how quickly the body metabolizes nicotine and cotinine. People categorized as “slow metabolizers” due to specific gene variants will clear cotinine at a reduced rate, leading to higher measured cotinine levels for the same amount of nicotine use.
Other physiological conditions also influence the speed of cotinine clearance from the body. Since cotinine is processed by the liver and excreted by the kidneys, any impairment in the function of these organs can slow the elimination process. Older individuals tend to exhibit a reduced metabolic rate compared to younger adults, which can prolong the cotinine half-life.
The use of certain substances can also alter the metabolic pathway of the CYP2A6 enzyme. Some medications can either inhibit the enzyme, slowing the breakdown of nicotine and cotinine, or induce it, accelerating the clearance rate. Hydration is another simple but relevant factor, particularly for urine testing, as high fluid intake can dilute the sample and artificially lower the measured concentration of cotinine.