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http://bmj.com/cgi/content/full/326/7401/1265-a
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BMJ 2003;326:1265 (7 June)
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EDITOR—The article by Helms et al raises the importance of case selection and clinical data on the estimates of short and long term mortality from clinical illness.1 Clinicians' understanding of an illness entails identifying symptoms and signs, in relation to age, sex, and geography, the three variables for which the authors selected controls. But clinicians also decide on the necessity for diagnostic laboratory tests. Results deemed unlikely to influence patients' management or outcome affect the likelihood of investigation.2 Variations in laboratory testing protocols and methods affect what is detected and reported.3
Medical epidemiologists are aware that the infecting dose affects severity. In general, the larger the infecting dose, the more severe the illness and the more likely the patient is to present to a clinician,4 so that severe illnesses are more likely to be represented in those studied.
Medical microbiologists are aware, as acknowledged by the authors, that among the several thousand different salmonellas, some, such as Salmonella typhi, S choleraesuis, S dublin, and S virchow seem predisposed to severe illness and bacteraemia. Similar variation in severity occurs with Shigella and Campylobacter.
One way to deal with the estimate of short and long term mortality is to obtain clinical information concerning mortality. Helms et al think that deaths occurring within one year may relate to the bacterial cause of illness. If a review of the death certificates might give an incomplete picture, examining hospital records could reduce the difficulty.
The method used, as suggested by Evans's commentary,1 relates to estimates of an exposure that affects mortality, making full use of the extraordinary data available in Denmark and other Scandinavian countries. Incorporating clinical information about cause of death might clarify whether the observations concern clinically severe cases, or represent other factors, not related to the gastrointestinal infection.
Sarah J O'Brien, head of gastrointestinal diseases division
Health Protection Agency, Communicable Disease Surveillance Centre, London NW9 5EQ sarah.o'brien@hpa.org.uk
Roger A Feldman, emeritus professor of clinical epidemiology
Barts and the London, Queen Mary School of Medicine and Dentistry, University of London E1 2AD
Competing interests: RAF reviewed published and unpublished epidemiological data relating to campylobacter on behalf of Bayer for an administrative hearing involving Bayer and the US Food and Drug Administration's centre for veterinary medicine.
References
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