When individuals are exposed to nicotine, their bodies metabolize it into a chemical called cotinine. Because nicotine is eliminated from the body relatively quickly, it is not an ideal marker for tracking tobacco use. Cotinine, however, has a longer half-life, remaining in the body for a day or more. This makes it a more reliable and practical biomarker for measuring exposure to nicotine from any source.
Sources of Cotinine Exposure
The most direct source of cotinine is the active use of tobacco and nicotine-containing products. This includes smoking cigarettes, cigars, or pipes, as well as using smokeless tobacco products like chewing tobacco and snuff. Electronic cigarettes (vaping) and nicotine replacement therapies, such as patches, gums, and lozenges, also introduce nicotine into the body.
Exposure is not limited to those who directly use these products. Individuals can have detectable cotinine levels through passive exposure to tobacco smoke. Secondhand smoke is the combination of smoke exhaled by a smoker and the smoke from the burning end of a tobacco product. Inhaling this smoke leads to nicotine absorption.
A less obvious source is thirdhand smoke, the chemical residue left on surfaces like furniture, carpets, and walls long after a cigarette has been extinguished. People, especially children, can be exposed by touching these contaminated surfaces and then touching their mouths, or by inhaling particles stirred up from these surfaces. Even in environments where no one is actively smoking, thirdhand smoke can contribute to a person’s cotinine levels.
Cotinine Testing and Detection
Several types of laboratory tests are used to detect cotinine, each with a different sample type and detection window. The most common method is a urine test, which is non-invasive and can detect cotinine for a longer period than some other tests. For a moderate or light user, cotinine is detectable in urine for about three to four days after the last nicotine exposure. In heavy, chronic users, this window can extend longer.
Blood and saliva tests are also frequently used. A blood test is considered highly accurate and is often used in clinical settings. Saliva testing is less invasive and provides a convenient option for various screening purposes. Both blood and saliva tests have a shorter detection window, identifying cotinine for one to four days after last use.
For assessing long-term exposure to nicotine, a hair test can be utilized. Cotinine can be incorporated into the hair shaft, where it can remain for an extended period. This method allows for a detection window of up to 90 days, offering a historical record of an individual’s nicotine exposure. This type of test is less common but valuable in specific forensic or research contexts.
Interpreting Test Results
Cotinine test results are reported in nanograms per milliliter (ng/mL). The specific numbers are used to differentiate between various levels of nicotine exposure, from active use to passive exposure. While exact cutoff levels can vary between laboratories, general guidelines help in the interpretation of these values. A result is considered negative if cotinine levels are below a threshold that indicates no significant recent exposure.
For urine tests, levels below 10 ng/mL are associated with a non-smoker who has not had significant exposure to environmental tobacco smoke. Concentrations in the range of 11 ng/mL to 30 ng/mL suggest light smoking or considerable exposure to secondhand smoke. Daily, active smokers will show much higher concentrations, exceeding 100 ng/mL and sometimes reaching levels above 500 ng/mL in heavy users.
Saliva and blood tests use lower cutoff points because cotinine concentrations are not as high in these fluids as in urine. For saliva, a cutoff of around 10 ng/mL is often used to distinguish active smokers from non-smokers. Levels above this point indicate active nicotine use, while levels between 1 ng/mL and 10 ng/mL could signify passive exposure. There can be an overlap between a light smoker and a person heavily exposed to secondhand smoke, which requires careful interpretation of the results.
Factors That Influence Cotinine Concentrations
The concentration of cotinine measured in a test is not solely dependent on the amount of nicotine consumed, as several individual factors can influence these levels. The frequency and intensity of nicotine use play a large role; a person who smokes heavily will maintain higher cotinine levels than someone who smokes infrequently. The type of product used also matters, as different products deliver nicotine at different rates and amounts.
An individual’s metabolism is a significant variable. The enzyme CYP2A6 is responsible for converting nicotine to cotinine, and genetic variations can lead to different rates of metabolism. Some people are “slow metabolizers,” meaning nicotine and cotinine remain in their system longer, while “normal metabolizers” process it more quickly. This genetic difference can affect cotinine levels among people with similar smoking habits.
Other biological factors also contribute to variability. Body mass and composition can influence how chemicals are distributed and stored. Factors such as ethnicity, age, and gender can affect nicotine metabolism. For instance, women may metabolize nicotine faster than men, and metabolic rates can change with age or during pregnancy. These variables help explain why two people with identical nicotine intake can have different results.