Case definition versus screening tool for SARS

Letters

CJEM 2004;6(1):8-9

To the Editor: In the November issue of CJEM, Wong Wing Nam and colleagues published an excellent study in which they compared physician judgement to the WHO case definition and concluded that the latter is an ineffective screening tool for SARS.¹ Other researchers^2,3 have made similar criticisms, which may be unfair. The WHO criteria were not meant to be a triage screening tool. Rather, they were intended to "describe the epidemiology of SARS and to monitor the magnitude and spread of this disease, in order to provide advice on spread and control."⁴ It may therefore be inappropriate to apply these criteria in the ED.

In a subsequent study (see page 12), which was also published as an early online release, these authors identified clinical predictors helpful in the diagnosis of SARS.⁵ Not surprisingly, chest radiography was the strongest of these. Given that emergency physicians were able to use chest radiography in their diagnostic decisions, and that the WHO "suspect case" definition does not include radiographic findings, it is no wonder that physician judgement was more accurate. It would have been fairer to compare physician judgement with the WHO "probable case" definition, which includes radiographic evidence.⁴

Finally, the WHO criteria had poor sensitivity for ED screening because fever and respiratory symptoms are often delayed, in some cases appearing after radiographic changes.² In the Wong Wing Nam study, a patient who presented with a fever of 37.8°C, a positive contact history and radiographic changes would most likely have been correctly admitted as a suspected SARS case according to physician judgement, but would be considered a "miss" by the WHO criteria, even if the patient later progressed to develop a higher temperature (>38°C) and respiratory symptoms. In such a case, the ED physician was accurate, and the WHO criteria fulfilled its surveillance function. It is important to recognize the distinction between "screening tool" and "case definition." Misunderstanding may lead to unnecessary discredit to the WHO.

Stewart S. Chan, MBBS(Syd), FRCSEd, FHKAM (EM)
Honorary Clinical Assistant Professor
The Chinese University of Hong Kong
Emergency Physician
Accident & Emergency Medicine
Academic Unit
Prince of Wales Hospital
30-32 Ngan Shing St.
Shatin, New Territories, Hong Kong

References

1. Wong WN, Sek ACH, Lau RFL, Li KM, Leung JKS, Tse ML, et al. Accuracy of clinical diagnosis versus the World Health Organization case definition in the Amoy Garden SARS cohort. Can J Emerg Med 2003;5(6):384-91. Epub 2003 Oct 22.
2. Rainer HT, Cameron PA, Smit D, Ong KL, Hung AN, Nin DC, et al. Evaluation of WHO criteria for identifying patients with severe acute respiratory syndrome out of hospital: prospective observational study. BMJ 2003;323:1354-8.
3. Thompson J. SARS: finding a deadly needle in the haystack [editorial]. Can J Emerg Med 2003;5(6):392-3.
4. World Health Organization. Case definitions for surveillance of severe acute respiratory syndrome (SARS). Geneva: The Organization; 2003. Available:www.who.int/csr/sars/casedefinition/en(accessed 2003 Dec 5).
5. Wong WN, Sek ACH, Lau RFL, Li KM, Leung JKS, Tse ML, et al. Early clinical predictors of severe acute respiratory syndrome in the emergency department. Can J Emerg Med 2004;6(1):12-21. Epub 2003 Dec 2.