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(Chest. 2005;128:1233-1238.)
© 2005 American College of Chest Physicians

Self-Reported Smoking Status and Exhaled Carbon Monoxide*

Results From Two Population-Based Epidemiologic Studies in the North of England

Mark S. Pearce, PhD; Louise Hayes, MSc; on behalf of the Newcastle Heart Project and the Newcastle Thousand Families Study

* From the School of Clinical Medical Sciences (Dr. Pearce), and the School of Population and Health Sciences (Ms. Hayes), University of Newcastle, Newcastle Upon Tyne, UK.

Correspondence to: Mark S. Pearce, PhD, Sir James Spence Institute, University of Newcastle Upon Tyne, Royal Victoria Infirmary, Newcastle Upon Tyne, NE1 4LP, United Kingdom; e-mail: m.s.pearce{at}ncl.ac.uk


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Study objectives: To investigate the validity of self-reported responses to questions on current smoking in two cohorts based in Northern England.

Design: A cross-sectional population-based study (the Newcastle Heart Project [NHP]) and a follow-up of the Newcastle Thousand Families birth cohort established in 1947.

Patients or participants: Participants included 1,189 members of the NHP and 410 members of the Newcastle Thousand Families cohort who completed a health and lifestyle questionnaire, including questions on current smoking, and attended a clinical examination, including testing for exhaled carbon monoxide between April 1993 and December 1998.

Results: The number of self-reporting smokers for whom very low (ie, < 6 ppm) exhaled carbon monoxide levels were recorded varied between 9% in the Newcastle Thousand Families Study and 26% among the members of the NHP who were of South Asian origin. Using a cutoff of 8 ppm, 80% of self-reported smokers were identified in both the Newcastle Thousand Families study and in the NHP European population, but only 60% were identified in the NHP South Asian population. In each population, < 7% of nonsmokers had exhaled carbon monoxide measurements of > 6 ppm, with nonsmoking men more likely to have higher levels than nonsmoking women. Among the nonsmokers, the levels of exhaled carbon monoxide did not vary with respect to the smoking status of a partner or socioeconomic status.

Conclusions: Using a cutoff value of 6 ppm would potentially miss a large number of smokers, although this may vary with ethnicity. Epidemiologic studies should continue to use biochemical markers to validate responses to smoking surveys. However, the use of exhaled carbon monoxide measurements as a method of assessing the validity of self-reported smoking status may require additional analyses of whether the cutoff level should vary for different populations.

Key Words: carbon monoxide • cohort studies • ethnicity • smoking


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Smoking is known to be a risk factor for many diseases in adulthood. However, the effect of smoking status must be adjusted for in epidemiologic studies that attempt to identify additional risk factors for disease. Smoking habits are most often assessed by questionnaire, although this is obviously prone to inaccurate responses and has been shown to underestimate the true prevalence of cigarette smoking.12 This has prompted a number of studies to attempt to validate the responses to such questionnaires by measuring the exhaled carbon monoxide, nicotine, cotinine, or thiocyanate levels in plasma or urine or by using the bogus pipeline approach.

This study aimed at investigating the validity of responses to the questions on smoking in three groups of individuals who were contributing to two epidemiologic studies in the North of England: the Newcastle Thousand Families Study; and two groups from the Newcastle Heart Project (NHP). We also investigated the effect of altering the cutoff level of exhaled carbon monoxide on the ability to distinguish between smokers and nonsmokers.


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
The NHP
The NHP was a cross-sectional, population-based study designed to investigate the risk factors for cardiovascular disease and diabetes in different ethnic groups who reside in Newcastle Upon Tyne in northern England.34567 European subjects in the NHP (n = 6,448) were people aged 25 to 74 years who were sampled from the Family Health Services Authority Register for a previous study, the Newcastle Health and Lifestyle Survey.8 South Asian subjects were identified by South Asian-sounding names on the full Family Health Services Authority Register.9 Both groups of potential participants were stratified by gender and 10-year age groups, and equal numbers from each stratum were selected at random.

Europeans were screened between April 1993 and October 1994, and South Asians were screened between May 1995 and March 1997. For both groups, information was collected using both a clinical examination and a questionnaire, which included sections on smoking and socioeconomic status.4 The European group self-completed the questionnaire at the time of the clinical examination. The South Asian group completed their questionnaires by interview in their homes, in their preferred language, by South Asian interviewers of the same gender as the participant. The questionnaire used for the NHP European group was translated from English into four South Asian languages and translated back into English by a different individual. These interviews were conducted prior to the clinical examination of the subjects (which usually took place between 1 and 4 weeks after the interview). The socioeconomic status, for both groups, was based on the occupation of the head of the household or the previous occupation if the head of the household had retired.

The Newcastle Thousand Families Study
The Newcastle Thousand Families cohort originally consisted of all 1,142 children born in May and June 1947 to mothers who resided within the city of Newcastle Upon Tyne. The study population is predominantly of European origin. Two thirds of these children were followed up prospectively to age 15 years, with detailed information collected on their health, growth, and socioeconomic circumstances.10 These early life data have been used, together with more recent data on adult health and lifestyle collected at age 50 years, to quantify the direct and indirect effects of the characteristics of fetal life, infancy, childhood, and adulthood on health in adulthood.101112131415161718

Participants in the follow-up at age 50 years were members of the original cohort who were either traced through the National Health Service Central Register or who contacted the study team in response to media publicity. Between October 1996 and December 1998, self-completion questionnaires on health and lifestyle were sent out. Clinical examinations, including the assessment of exhaled carbon monoxide, took place in the Royal Victoria Infirmary, Newcastle Upon Tyne, during the same time period.

The occupational social class (including I [the most advantaged], II, III [nonmanual], III [manual], IV, and V) of the main wage earner in the household, and the information on smoking habits (present and at ages 15, 25, and 35 years) of the study member and other members of the household were derived from the returned self-completion questionnaire data at age 50 years.12 The participants of both studies were asked the question "Do (or did) you smoke cigarettes?" (with possible answers "no," "occasionally [ie, not every day]," or "daily"), and, if the participants answered "yes," they were asked to "please write in the average number you smoke(d) per day." The current smokers were defined as the individuals who currently smoked at least one cigarette per day.

For both studies, the exhaled carbon monoxide was measured as part of the clinical examination using a carbon monoxide monitor (Smokerlyzer; Bedfont Scientific; Rochester, Kent, UK).19 The instructions of the manufacturer were followed for quality control. The measurement of exhaled carbon monoxide levels in parts per million was based on the conversion of carbon monoxide to carbon dioxide over a catalytically active electrode.20 The level of carbon monoxide is generally compared with an a priori-defined cut point that is used to identify the individuals who smoke but fail to report that they smoke. Historically,2122 an exhaled carbon monoxide level of 10 ppm has been used as the cutoff between smokers and nonsmokers. However, evidence2023 suggests that 10 ppm is too high for use as a screening method, with reduced sensitivity for identifying smokers, and that a cutoff at 6 or 8 ppm would be more appropriate. Accordingly, the suggested cutoff level for exhaled carbon monoxide monitoring devices has been lowered to 8 ppm.24

Statistical Analysis
Statistical analyses were performed using a statistical software package (Stata Statistical Software, version 8.0; Stata Corporation; College Station, TX). Because of the skewed nature of exhaled carbon monoxide measurements, summaries are presented as medians, interquartile ranges (IQRs), and absolute ranges. Wilcoxon rank sum tests were used to investigate the differences in exhaled carbon monoxide levels between self-reported smokers and nonsmokers in each of the three groups. Kruskal-Wallis tests were used to assess the differences in smoking habits among the three populations. Spearman rank correlation coefficients were calculated to assess the relationships in self-reported smokers between their exhaled carbon monoxide levels and the number of cigarettes they smoked and, in nonsmokers, the relationship between their exhaled carbon monoxide levels and their social class. Ethical approval for both studies was obtained from local research ethics committees, and all of the participants gave their informed consent.


   Results
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Of the 1,744 people sampled from the Newcastle Health and Lifestyle Survey, 1,308 were contacted, and 840 people (64%) agreed to participate in the study. Of these, 15 people were later found to be of non-European origin and were excluded from additional analyses. Of the 1,050 eligible South Asian persons who were contacted, 709 (68%) agreed to participate. Of these, 684 South Asians (96%) classed themselves as Pakistani, Indian, or Bangladeshi. The information on exhaled carbon monoxide levels and current smoking status was available for 514 members of the European group (135 men) and 675 members of the South Asian group (328 men).

Of the original 1,142 members of the Newcastle Thousand Families Study, 832 members (89% of the surviving sample of 932 children whose families remained in Newcastle Upon Tyne for at least the first year of the study) were traced at age 50 years. Of these, 574 study members completed the health and lifestyle questionnaire, and 412 study members (180 men and 232 women) attended the clinical examination. The information on exhaled carbon monoxide levels and current smoking status was available for all but two of the study members (both male) who attended the clinical examination.

Participants from the Newcastle Thousand Families Study were all approximately 50 years of age at the time of both the self-report questionnaire and the clinical examination. Participants from the NHP were between the ages of 26 and 77 years (mean age, 51.5 years) [Table 1 ].


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  		Table 1.. Number of Participants Included From Each Study Population, by Age, Gender, and Socioeconomic Status

 
As expected, there were highly significant differences in the exhaled carbon monoxide levels between smokers and nonsmokers in all three of the groups (p < 0.0001) [Table 2 ]. Similar differences were seen when comparing the exhaled carbon monoxide levels between current smokers and former smokers (p < 0.0001).


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  		Table 2.. Summaries of Exhaled Carbon Monoxide Levels by Reported Smoking Status, Gender, and Cohort*

 
Among the self-reported smokers, the median number of cigarettes smoked per day was higher in the Newcastle Thousand Families Study (median, 20 cigarettes; IQR, 10 to 20 cigarettes) compared with both the NHP European population (median, 15 cigarettes; IQR, 9 to 20 cigarettes) and the South Asian population (median, 10 cigarettes; IQR, 6 to 20 cigarettes), with a p value of 0.001. Of the South Asian women included in the study, only eight were reported to be smokers. In all three of the groups, the number of cigarettes smoked was significantly associated with the exhaled carbon monoxide levels among nonsmokers (the Newcastle Thousand Families Study, r = 0.3 and p = 0.001; both populations within the NHP, r = 0.4 and p < 0.0001).

The number of self-reported smokers for whom very low (ie, < 6 ppm) exhaled carbon monoxide levels were recorded varied between 8.8% in the Newcastle Thousand Families Study and 26% in the NHP South Asian population (Table 3 ). Although this included mainly light smokers (median number of cigarettes smoked per day, 6), > 25% of this group smoked at least 10 cigarettes per day. A cutoff at 8 ppm identified around 80% of the confirmed smokers in both the Newcastle Thousand Families Study and the NHP European population but only 60% in the NHP South Asian population. As expected, the sensitivity of using exhaled carbon monoxide measurements to identify smokers fell as the cutoff was raised for each group (Table 4 ). However, appreciable differences in sensitivity were seen at all of the cutoff points between the groups, particularly using a cutoff of 10 ppm for which sensitivity was only 47% among the NHP Asian population.


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  		Table 3.. Numbers in Each Exhaled Carbon Monoxide Level Group by Study Population, Gender, and Self-Reported Smoking Status*

 

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  		Table 4.. Sensitivity for Detecting Self-Reported Smokers Using Different Cutoffs for Exhaled Carbon Monoxide*

 
In the Newcastle Thousand Families Study, only 10 study members (3.4%) reporting as nonsmokers had exhaled carbon monoxide levels of > 6 ppm (range, 6 to 23 ppm). However, the participants in the NHP were more likely to have conflicting smoking status and exhaled carbon monoxide levels (6.8% of the NHP Europeans and 6.5% of the NHP South Asians who reported to be nonsmokers had exhaled carbon monoxide levels of > 6 ppm, with a maximum measurement of 91 ppm). In each population, the men who self-reported as nonsmokers were more likely to have an exhaled carbon monoxide level that was 6 ppm more than the female self-reported nonsmokers, although these figures are based on small numbers.

Among the nonsmokers, the exhaled carbon monoxide measurements did not vary depending on whether the individual’s partner smoked (p > 0.15) or socioeconomic status (p > 0.09).


   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Self-reported smoking surveys in epidemiologic studies are a crucial and common method used to obtain information on a major risk factor for many diseases. The results of this study suggest that such surveys are valid but that smoking status should be additionally assessed by the use of one of a number of biochemical markers. The actual choice of biochemical marker to be used will often depend on factors other than the scientific validity of the marker but more on the feasibility of obtaining the biological material and how crucial the accurate determination of smoking status is. Measures based on tests of blood are invasive and are unable to provide an immediate assessment. Less invasive are urine tests, which, for the measurement of urinary cotinine levels, can provide an almost immediate measure of smoking status. The measurement of exhaled carbon monoxide is noninvasive and, in addition, provides results almost immediately. For smoking cessation studies, exhaled carbon monoxide measurements are usually sufficient, particularly if accompanied by a bogus pipeline approach. As the accurate measurement of smoking status becomes more crucial, other biomarkers, such as levels of cotinine or even nicotine, should be considered. Although carbon monoxide exposure can occur from a variety of sources, such as environmental pollution, occupational exposures, or faulty heating systems, the major cause of high levels of carbon monoxide exposure is smoking.

Exhaled carbon monoxide levels were higher in self-reported smokers than in nonsmokers and were also higher than those seen among former smokers. There were also significant correlations between the number of cigarettes smoked per day and the exhaled carbon monoxide measurements, confirming previous findings.2526

Although many studies, both epidemiologic and otherwise, have traditionally used a cutoff level of 10 ppm to discriminate between current smokers and nonsmokers, evidence has emerged to suggest that this cutoff level is too high.20 In this investigation, approximately 30% of self-reported smokers in the two cohorts of European origin and > 50% of self-reported smokers in the South Asian group had levels of < 10 ppm. In 2001, the American Thoracic Society published an article23 suggesting that 6 ppm was the optimal cutoff, although this may vary when individuals have received certain diagnoses for some diseases. This cutoff has since been shown to be too low for patients with asthma or COPD,24 in whom exhaled carbon monoxide levels may potentially be affected by airway inflammation. It is not known how many of the participants in this study had received disease diagnoses that may have affected their exhaled carbon monoxide levels.

The results of this investigation, using data from three populations, suggest that even a cutoff at 6 ppm would potentially miss a large number of smokers. However, the relationship between exhaled carbon monoxide levels and self-reported smoking status appears to vary with ethnicity. A smaller proportion of self-reported smokers would have been correctly identified using a cutoff of 6 ppm among the NHP South Asian population than in the two European populations. The self-reported smokers in the NHP South Asian population, in general, smoked fewer cigarettes per day than did participants in the other two groups. Therefore, it is possible that ethnic differences in both the prevalence of cigarette smoking and in the types of cigarettes smoked may affect the ability of a validation study using exhaled carbon monoxide levels to accurately identify smokers. There may also be differences in the way in which people of different genders or different ethnic groups may respond to questions regarding their smoking status. This may account for the lower sensitivity for detecting self-reported smokers in the NHP South Asian population compared with the other two groups. However, this may also be at least partly attributable to the use of an interview-based questionnaire for the South Asian population, through which more valid responses may have been expected when compared with the self-administered questionnaires used for the NHP European subjects and for subjects in the Newcastle Thousand Families Study.

Where questionnaires are used to ascertain smoking status, it is useful for a time frame to be included within the questionnaire. For example, the 2003 US National Health Interview Survey27 combined questions on current smoking status (ie, yes or no) with questions on smoking habits (ie, the number of days they had smoked and the average number of cigarettes they had smoked per day) during the 30 days immediately prior to responding to the questionnaire. This is of particular use where questionnaires are administered at the time of any biomarker measurement but is an important consideration for all smoking validation studies, including epidemiologic studies in which smoking validation is often used to alter the responses to questionnaires when self-reported smoking and biomarker measurements may give conflicting results.

In two of the three populations (the Newcastle Thousand Families Study and the NHP South Asian population), there was an interval between the completion of the questionnaire and the clinical examination (smoking status was not ascertained at the time of the clinical examination, except for the NHP European group). This may have resulted in some reported smokers ceasing to smoke or, for some, most likely former smokers, to begin smoking. However, this is unlikely to have a large effect on our findings.

The choice of a validation method will depend on a number of factors, one of which should be the level of a biomarker chosen to suggest that an individual is a smoker. Even then, there must be a balance between sensitivity and specificity, which will itself depend on the situation for which the validation of self-reporting smoking status is designed. Specificity was not available for this investigation, but although lowering the cutoff level to <6 ppm may have increased the sensitivity to a more reasonable percentage in the South Asian population, it is unknown how many true nonsmokers would have been incorrectly classified as smokers by additionally lowering the level.

In conclusion, epidemiologic studies should continue to use biochemical markers to validate the responses to smoking surveys. However, the use of exhaled carbon monoxide measurements as a method of assessing the validity of self-reported smoking status may require additional analyses of whether the cutoff level should vary for different populations and how this may affect sensitivity and specificity.


   Acknowledgements
 
We thank the participants of both studies for taking part and the study teams past and present, in particular, Sir George Alberti, Raj Bhopal, Sir Alan Craft, Louise Parker, Dr. Nigel Unwin, and Dr. Martin White.


   Footnotes
 
Abbreviations: IQR = interquartile range; NHP = Newcastle Heart Project.

The Newcastle Thousand Families Study was supported by the Wellcome Trust, the Sir John Knott Trust, the Special Trustees of the Newcastle Hospitals, and the Minnie Henderson Trust. The NHP was supported by the Barclay Trust, the British Diabetic Association, Newcastle Health Authority, research and development directorate of the Northern Regional Health Authority, the Department of Health, and the British Heart Foundation.

Received for publication November 25, 2004. Accepted for publication January 26, 2005.


   References
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 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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