TORONTO — In a global climate of rumoured cases and persistent fear, SARS experts from Canada, the United States and the World Health Organization are gathering in Ottawa this week to try to answer a pressing question: How do you conduct surveillance for a hard-to-spot disease that may not be coming back?

With a possible new SARS case in Singapore, regular rumours of outbreaks in nursing homes and hospitals in former SARS hot spots and flu season around the corner, public health officials need to figure out how and when physicians and hospitals should ring alarm bells, experts say.

To that end, Health Canada is calling together officials from Ontario and British Columbia - the two provinces which had SARS outbreaks - as well as the U.S. Centers for Disease Control and the WHO to try to establish guidelines for SARS surveillance for the coming cold and influenza season.

Coming up with workable framework isn't going to be easy, admitted Dr. Arlene King, director of Health Canada's immunization and respiratory infections division.

"It's challenging. There's no question about it. This is not an easy subject," she said in an interview Monday.

One of the key questions that will be on the table during the meeting - to be held Wednesday and Thursday in Ottawa - is when should doctors order SARS testing to be done?

Tests for the SARS coronavirus are still new and have not been validated. It remains unkown how often they produce false positives - or false negatives.

But a positive SARS test demands instant action and draws worldwide attention. That's something public health officials in British Columbia learned to their dismay last month when they were trying to figure out what was behind an outbreak of mild respiratory illness at a seniors' home in Surrey, south of Vancouver.

The illness didn't look like SARS, which has a mortality rate of 50 per cent in people aged 60 and older. The average age of residents of the lodge was 82, but most of those who became ill just experienced cold-like symptoms.

Still, when an answer to what was ailing them couldn't be found, someone sent off samples to Health Canada's National Microbiology Laboratory in Winnipeg to be tested for the virus that causes SARS.

Those initial tests came back positive. Within days, the WHO had dispatched an expert to Winnipeg to study the findings and some Asian countries started musing about imposing a travel ban on Vancouver.

SARS has since been ruled out as the cause of the outbreak. But the incident points out how crucial it is that doctors and public health authorities across the country get some guidance about when SARS testing should be conducted, said Dr. Donald Low, a leading SARS expert.

"We've got to be careful. We can't take these tests in isolation. They have to be put in the context of the clinical picture. And only when we do that is it going to be valuable to us," said Low, chief microbiologist at Toronto's Mount Sinai Hospital.

"The mistake that happened out of Surrey but has to be recognized in the future is that this test is only one piece of the puzzle. And if you're going to do it for any other reason - that is if you don't have a good clinical indication - then it should be research."

King agrees guidance on this issue is key.

"I think we have to be guided. . . by our clinical and our epidemiologic sense. There are serious public health consequences of both over calling and under calling," she said.

"I think that will really be a lot of the discussion."

Health Canada's goal is that whatever is fleshed out at the meeting will conform, as much as is possible given differences in health-care systems, with positions taken by the WHO and the CDC, King said.

"We want to ensure that we're all on the same song sheet."

In addition to setting out criteria for when SARS tests should be conducted, the experts at the meeting are expected to finalize new Canadian case definitions for suspect and probable SARS.

New definitions are needed because the existing ones say only people who have been known to have been in contact with a SARS patient or who have recently visited an area where there is a SARS outbreak can have the disease.

As there are no SARS outbreaks anywhere at this point, those definitions are a classic Catch-22: By definition you can't have SARS because there was no one for you to catch it from.