Validation of self reported smoking.

elf reported smoking status has been widely used to assess detrimental health effects of smoking and to orient counselling and other preventive interventions. Self reporting, however, can be unreliable if the subject is under pressure because of social or medical disapproval. Furthermore, the quantity of smoke products actually inhaled and absorbed varies by the manner of smoking. Because of these difficulties , increased emphasis has been placed on measuring exposure through the use of biological markers to provide more accurate estimates of smoking status and of the dose received. In the past two decades, an increasing number of epidemiological studies have used biomarkers in assessing tobacco smoke exposure from active and passive smoking. Biomarkers can be used to classify people as exposed or unexposed, identify deceivers (people misreporting their smoking status), or estimate relative degree of exposure. Cotinine, a major metabolite of nicotine , is currently regarded as the best biomarker for exposure of active smokers and non-smokers to environmental tobacco smoke (ETS). 1 As a marker coti-nine has the advantage of being almost specific to tobacco. The few exceptions include occupational exposure to tobacco leaves and nicotine products, use of smokeless tobacco products, chewing of nicotine gum, and use of nicotine patches or other aids for smoking cessation. Low levels of nicotine have been found in diet vegetables, but their impact in cotinine levels can be regarded as insignificant. Cotinine can be measured in blood (that is, in serum), urine, saliva, and hair. The average half life of cotinine in different body fluids in adults is approximately 20 hours, making it a good indicator of the integrated exposure over the previous two to three days. 1 Studies comparing non-smokers and smokers have consistently found that measurement of cotinine in the urine, saliva, or serum can distinguish active smokers from non-smokers. Active smokers almost always have serum or saliva levels higher than 15 ng/ml and sometimes greater than 500 ng/ml. Non-smokers exposed to typical levels of ETS have cotinine levels of less than 1 ng/ml, with heavy exposure to ETS producing levels in the 1–15 ng/ml range. 2 A number of studies have used bio-markers to validate self reported smoking status. Self reported non-smokers who seem to be smokers on the basis of biochemical measurements are generally considered " deceivers " of their true smoking status. However, caution should be used in making quantitative comparisons of smoking misclassification across studies …