The effort to contain the deadly SARS virus was seemingly dealt a blow this week, as a test developed to diagnose the disease appeared far from foolproof.

The Centers for Disease Control and Prevention revealed that about seven of the 13 probable cases of SARS (search) that were tested came up negative.

The CDC also said that 32 cases considered "suspected" rather than "probable" were tested, all of them coming up negative.

"The fact that some of our probable SARS cases are not virologically positive is not surprising," said CDC director Dr. Julie Gerberding.

The World Health Organization also advised against putting too much trust in diagnostic exams, noting on its Web site that national authorities and medical staff should "understand the limitations of currently available tests."

Countries across the globe were struggling to contain varying outbreaks of the flu-like disease that has already taken the lives of more than 260 and infected more than 4,300.

Earlier Friday, Beijing city officials told 4,000 people suspected of being exposed to SARS to remain in their homes as quarantined medical workers in Taiwan held a raucous protest and Hong Kong hospitals were accused of not properly protecting frontline doctors from the deadly bug.

In Canada, officials expressed optimism Friday that the World Health Organization will lift its SARS travel warning for Toronto as early as next week, as three more people died and economic damage continued to mount. 

The Toronto area death toll from the disease is up to 19, out of more than 250 probable and suspected cases.

Prime Minister Jean Chretien, in his first public comments since the travel advisory was issued Wednesday, said he spoke Friday morning to WHO head Gro Harlem Brundtland and she agreed to review the situation next week.

The facts Canada will provide should get the travel warning rescinded, said the provincial commissioner of public health, Dr. Colin D'Cunha. "My sense is they've listened to some of the things that people have said in Canada."

In Beijing (search), officials sealed off Ditan Hospital -- the third medical center to be closed this month in the Chinese capital. Earlier this month, foreign reporters were allowed to tour the 500-bed Ditan, which officials considered to be a showcase of the government's efforts to battle SARS. The hospital specializes in infectious diseases.

So far, officials have reported 42 SARS deaths in Beijing and a total death toll of 115 nationwide. The country has reported a total of 2,422 cases, with more than 750 in Beijing.

Two days after invoking emergency powers to quarantine people, Beijing health officials ordered 4,000 people to stay at home because they had "intimate contact" with others showing SARS symptoms.

Guo Jiyong, deputy director general of the Beijing Health Bureau, didn't say who the people were or how long they had been ordered to stay home.

In the Taiwanese capital of Taipei, about 30 nurses and workers were getting fed up with being quarantined for two weeks at the Hoping Hospital, which reported 10 probable SARS cases earlier this week. They complained that confining people could expose healthy staffers to the deadly disease.

Some protested by tossing bottles and paper out of windows and displaying banners saying "Wrong Policy" and "Long, Long 14 days."

"This is ridiculous. Why can't I go home?" yelled a woman with a gauze mask tightly strapped to her mouth.

Taiwan has reported 41 probable cases, but no deaths.

In Hong Kong, local media accused the Hospital Authority of not providing enough protective gear for frontline workers -- hundreds of whom have been sickened by SARS on the job.

Medical workers have been lobbying for protective suits for weeks, according to the South China Morning Post. The newspaper solicited donations to help buy head-to-foot suits made of fabric that can shield wearers from microscopic substances.

Many staff working in the 14 hospitals treating SARS patients have been wearing paper gowns that don't offer enough protection when workers come into close contact with SARS patients, the report said.

"This is the scandalous part," Dr. Peter Tong Chun-yip was quoted as telling the Post. "The paper gowns are not waterproof, they are like tissue paper."

The Hospital Authority's acting chief executive, Dr. Ko Wing-man, admitted to problems in dealing with the crisis that has infected about 1,500 people in Hong Kong and killed 109.

"I must admit the performance of the whole management, especially under my guidance, does not reach many people's expectations," Ko said in a radio interview.

Vietnam is counting down to Monday, when the World Health Organization is prepared to announce that it is the first country to have a local outbreak and then rid itself of the deadly bug.

Five people died in Hanoi after an outbreak at a hospital stemming from an infected patient from Hong Kong, but no new cases of the disease have been reported in Vietnam since April 8.

In the Philippines, a flight to South Korea returned shortly after takeoff because of fears that a feverish passenger might have SARS. Philippine Airlines Flight PR416 took off again later after doctors ruled out SARS.

The Associated Press contributed to this report.