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Interpretation of diagnostic laboratory tests for severe acute respiratory syndrome: the Toronto experience

From the University of Toronto (Tang, Louie, Richardson, Simor, Jamieson, Fearon, Poutanen, Mazzulli, Tellier, McGeer, Low, Phillips, Rachlis, Matukas, Walmsley, Gold); Sunnybrook and Women's College Health Sciences Centre (Tang, Louie, Simor, Phillips, Rachlis, Matukas); The Hospital for Sick Children, Toronto, Ont. (Richardson,Tellier); McMaster University, Hamilton, Ont. (Smieja, Mahony, Loeb, Petrich, Chernesky); Saint Joseph's Hospital, Hamilton, Ont. (Smieja, Mahony, Petrich, Chernesky); Central Public Health Laboratory, Toronto, Ont. (Jamieson, Fearon); Mount Sinai Hospital and Toronto Medical Laboratories, Toronto, Ont. (Poutanen, Mazzulli, McGeer, Low); National Microbiology Laboratory, Winnipeg, Man. (Jones, Bastien, Li, Dick, Grolla, Fernando, Booth); Toronto Public Health (Henry); The Scarborough Hospital, Scarborough, Ont. (Rose, Lovinsky); University Health Network, Toronto, Ont. (Walmsley, Gold); and St. Joseph's Health Centre, Toronto, Ont. (Krajden)Additional members of the Ontario Laboratory Working Group for the Rapid Diagnosis of Emerging Infections: George Broukhanski, Jeff Fuller, Ilene Gugliemi, Aimin Li, Central Public Health Laboratory, Toronto; Grant Johnson, Anne Matlow, The Hospital for Sick Children, Toronto; JoAnne de Jager, Catherine Harlton-Strezov, Lorraine Moss, Laboratory Working Group; Barbara Willey, Mount Sinai Hospital and Toronto Medical Laboratories, Toronto; Anton Andonov, Michael Drebot, Heinz Feldmann, Amin Kabani, Frank Plummer and Graham A. Tipples, National Microbiology Laboratory, Winnipeg; Sylvia Chong, Sean Song, St. Joseph's Healthcare and Hamilton Health Sciences, Hamilton; Roslyn Devlin, St. Michael's Hospital, Toronto; Lisa Louie, Mona Loutfy, Mary Vearncombe, Sunnybrook and Women's College Health Sciences Centre, Toronto; Sheela Basrur, Michael Finkelstein, Marjolyn Pritchard, Barbara Yaffe, Toronto Public Health, Toronto

Correspondence to: Dr. Marie Louie, Department of Microbiology, Sunnybrook and Women's College Health Sciences Centre, Rm. B121, 2075 Bayview Ave., Toronto ON M4N 3M5

Background: An outbreak of severe acute respiratory syndrome (SARS) began in Canada in February 2003. The initial diagnosis of SARS was based on clinical and epidemiological criteria. During the outbreak, molecular and serologic tests for the SARS-associated coronavirus (SARS-CoV) became available. However, without a "gold standard," it was impossible to determine the usefulness of these tests. We describe how these tests were used during the first phase of the SARS outbreak in Toronto and offer some recommendations that may be useful if SARS returns.

Methods: We examined the results of all diagnostic laboratory tests used in 117 patients admitted to hospitals in Toronto who met the Health Canada criteria for suspect or probable SARS. Focusing on tests for SARS-CoV, we attempted to determine the optimal specimen types and timing of specimen collection.

Results: Diagnostic test results for SARS-CoV were available for 110 of the 117 patients. SARS-CoV was detected by means of reverse-transcriptase polymerase chain reaction (RT-PCR) in at least one specimen in 59 (54.1%) of 109 patients. Serologic test results of convalescent samples were positive in 50 (96.2%) of 52 patients for whom paired serum samples were collected during the acute and convalescent phases of the illness. Of the 110 patients, 78 (70.9%) had specimens that tested positive by means of RT-PCR, serologic testing or both methods. The proportion of RT-PCR test results that were positive was similar between patients who met the criteria for suspect SARS (50.8%, 95% confidence interval [CI] 38.4%–63.2%) and those who met the criteria for probable SARS (58.0%, 95% CI 44.2%–70.7%). SARS-CoV was detected in nasopharyngeal swabs in 33 (32.4%) of 102 patients, in stool specimens in 19 (63.3%) of 30 patients, and in specimens from the lower respiratory tract in 10 (58.8%) of 17 patients.

Interpretation: These findings suggest that the rapid diagnostic tests in use at the time of the initial outbreak lack sufficient sensitivity to be used clinically to rule out SARS. As tests for SARS-CoV continue to be optimized, evaluation of the clinical presentation and elucidation of a contact history must remain the cornerstone of SARS diagnosis. In patients with SARS, specimens taken from the lower respiratory tract and stool samples test positive by means of RT-PCR more often than do samples taken from other areas.

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