Cotinine is a substance used to measure a person’s exposure to nicotine. It is not nicotine itself, but a primary byproduct created when the body processes nicotine from any source, including cigarettes, e-cigarettes, or nicotine replacement products. Cotinine serves as a reliable biomarker because it remains detectable in the body for a much longer period than the nicotine that produced it. This makes it a preferred tool for screening purposes, such as employment testing, insurance qualification, or monitoring smoking cessation programs.
The Relationship Between Nicotine and Cotinine
Nicotine is the active compound absorbed into the bloodstream when tobacco or a nicotine product is used. Once absorbed, the body rapidly begins to process and eliminate it, primarily through the liver. The CYP2A6 enzyme in the liver is responsible for converting nicotine into its main metabolite, cotinine. This metabolic process is a form of detoxification, changing the active drug into a less active compound that the body can more easily excrete.
The key difference that makes cotinine the superior measurement tool is its half-life—the time it takes for a substance’s concentration in the body to reduce by half. Nicotine has a short half-life, typically around two hours, meaning it is quickly cleared from the system. Cotinine, however, has a significantly longer half-life, usually ranging from 15 to 20 hours in most adults.
This extended presence allows cotinine to serve as a stable record of nicotine exposure over the preceding several days. While nicotine might only confirm use within the last few hours, cotinine levels reliably reflect daily exposure patterns. Cotinine is then further metabolized, primarily by the CYP2A6 enzyme, into trans-3′-hydroxycotinine before final excretion.
Methods Used to Measure Cotinine
Cotinine testing is performed on several types of biological samples, each offering a different window of detection and convenience. The sample type chosen depends on the purpose of the test, such as immediate confirmation or long-term monitoring. Urine is the most common sample type because of its high concentration of the metabolite. It offers a relatively long detection window, often remaining positive for up to seven days following the last exposure for a regular user.
Saliva testing is a less invasive collection method that provides a measurement closely correlated with plasma concentrations. The detection window for cotinine in saliva is generally shorter than in urine, usually lasting up to two to three days after nicotine use. Blood (plasma/serum) testing is the most precise method, often used in research or medical assessments. While it requires a blood draw, it provides the most accurate quantitative measure of the cotinine concentration circulating throughout the body.
Hair testing offers the longest detection window, sometimes indicating chronic exposure over the previous three months. Cotinine becomes incorporated into the hair shaft as it grows, providing a historical record of nicotine exposure. However, hair testing is more complex and expensive than the other methods, limiting its use to situations where long-term history is required.
Interpreting Cotinine Test Results
Cotinine test results are typically reported in nanograms per milliliter (ng/mL) and are interpreted using established thresholds to categorize exposure status. For non-users, the cotinine level is usually very low, often less than 1 ng/mL in saliva or serum. A result below 10 ng/mL in serum or saliva is frequently used to indicate no or minimal exposure, or abstinence for two weeks.
The next category involves individuals with environmental or passive exposure, often referred to as secondhand smoke exposure. A person with significant secondhand smoke exposure may have serum or saliva levels ranging from approximately 1 ng/mL up to 10 ng/mL. Non-smokers living with a heavy smoker may have cotinine levels that approach the cutoffs used to distinguish light users.
Light or intermittent nicotine users generally demonstrate cotinine levels between 10 ng/mL and 100 ng/mL in serum or saliva. These levels are high enough to confirm active use but are below the concentration typically seen in regular, heavy users. Regular or heavy users routinely show serum or saliva cotinine concentrations exceeding 100 ng/mL, often falling into the 200 ng/mL to 800 ng/mL range.
These numerical guidelines are general, and specific laboratory cutoff values can vary based on the testing method, sample type, and context of the assessment. For instance, the threshold to distinguish a smoker from a non-smoker in urine is often much higher, sometimes set at 100 ng/mL, because cotinine is more concentrated in urine. Accurate interpretation requires considering the specific cutoff values provided by the testing laboratory.
Factors That Influence Cotinine Levels
Several biological and environmental factors can cause an individual’s cotinine level to fluctuate independently of the amount of nicotine consumed. Genetic variability in the CYP2A6 enzyme significantly affects how quickly nicotine is metabolized into cotinine. Some individuals possess genetic variations that result in a faster metabolism, meaning they clear cotinine more quickly and may have lower measured levels for the same exposure. Conversely, those with reduced enzyme activity (slow metabolizers) will accumulate cotinine to higher levels, potentially leading to an overestimation of their actual nicotine intake.
Physiological factors related to elimination also play a role in the final cotinine concentration. Kidney function is important because cotinine is ultimately excreted through the urine. Impaired kidney function can slow the clearance of cotinine, causing the level to be higher than expected. Hydration status also influences the concentration of cotinine in urine samples; a highly hydrated person will produce dilute urine, which can lower the measured cotinine concentration.
Cotinine levels can also be influenced by the source of the exposure, not just the amount. While first-hand use is the largest source, environmental exposure also contributes to the final measured level. Exposure to third-hand smoke (nicotine residue remaining on surfaces and dust after smoking) can result in low, persistent levels of cotinine in non-users. The presence of other tobacco-specific alkaloids, such as anabasine, may be required to definitively distinguish tobacco use from pure nicotine replacement therapy.