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.