Patients in clinical practice often falsely report their smoking status. To see if this was so in occupational surveys we have validated smoking histories (using a serum thiocyanate assay) in 206 randomly sampled office workers who completed a smoking questionnaire administered by a doctor. Past and present cigarette consumption was determined with an assessment of exposure to passive cigarette smoke at home and at work in the non-smokers. Serum thiocyanate concentrations were measured by the ferric nitrate method. All smoking groups showed mean thiocyanate concentrations greater than non-smokers except those smoking five or fewer cigarettes a day. There was a significant increase in plasma thiocyanate with increasing smoking (p less than 0.01). Non-smokers with and without exposure to passive smoke could not be separated by thiocyanate concentration. In our hands serum thiocyanate concentrations identified moderate and heavy smokers but could not distinguish between non-smokers, light smokers, and passive smokers. Fourteen non-smokers had serum thiocyanate concentrations higher than 70 mumol/l which were still raised on a repeat sample. On a further questionnaire two admitted to smoking. To help confirm non-smoking status, expired carbon monoxide levels were also checked in this group. One person had a level of 22 ppm and subsequently admitted to smoking. In the others the levels were less than or equal to 10 ppm. Using a combination of serum thiocyanate assay and exhaled breath carbon monoxide levels, non-smoking was confirmed in 98% of those stating that they were non-smokers. In non-smokers exposure to passive cigarette smoke was much more likely to occur at work than at home.
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