© 2002 Occupational and Environmental Medicine
LETTER
Methodological problems in a case-referent study based on a register of occupational asthma
1 Department of Public Health and Clinical Medicine, Umea University, SE-901 85 Umea, Sweden; bengt.jarvholm@envmed.umu.se
Keywords: asthma; case-referent study
Meredith et al,1 performed a case-referent study to investigate asthma caused by isocyanates. They claimed that the results indicated that isocyanate asthma occurs at low 8 hour average exposure (around 1.5 ppb); for exposures above 1.125 ppb there was about a threefold increased risk, however, this was of limited significance (odds ratio (OR)=3.2, 95% confidence interval (95% CI) 0.96 to 10.6; p=0.06). They also concluded that their study, by contrast with other studies, had a higher risk of isocyanate asthma in smokers and people with atopy.
The study design is original as cases were recruited from a register of occupational asthma. A case-referent study based on a register of cases with both the disease and the exposure of interest is new. I think the design requires some discussion as it may introduce severe bias.
A typical case-referent study selects cases with a certain disease—for example, asthma—from a hospital register
2 MRC Clinical Trials Unit, 222 Euston Road, London NW1 2DA, UK
Correspondence to:
Correspondence to:
Dr S Meredith, Department of Preventive Medicine, Vanderbilt University, Nashville, Tennessee 37232–2637, USA;
sm@ctu.mrc.ac.uk
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