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Friday, May 16, 2003
Sars tests unreliable for controlling disease, WHO expert warns (9:33 am)
PARIS -- The current tests for detecting the Severe Acute Respiratory Syndrome (Sars) virus are time-consuming, flawed and should not be considered a tool for combating the spread of the disease, a UN health expert warned on Thursday.
Nations hit by the Sars epidemic are rushing to distribute kits and machines to labs to test samples taken from patients suspected to carry the virus.
These are useful but are not the simple and accurate dipstick tests routinely used by doctors, sometimes working in rugged conditions, to quickly check for common diseases and which are urgently needed in the effort to detect Sars carriers, the specialist said.
There are three kinds of tests at present: two of them test for the presence in blood of antibodies to the virus, and the other, polymerase chain reaction (PCR), amplifies a fragment of DNA to search for the virus' genetic "fingerprint."
All require a laboratory and trained staff as well as time to carry out the test, said Klaus Stohr, a project leader at the department of communicable disease at the World Health Organisation (WHO) in Geneva.
"These tests are good and robust in scientific laboratories ... but there are no quick tests yet available," he told AFP in a phone interview.
The three technologies also have flaws, he said.
PCR tests have "too low a sensitivity" and have a tendency towards so-called false negatives, he said.
In other words, they may reassuringly say that a patient is negative for the virus when in fact he is positive.
The current generation of Sars antibody tests, which use the immunofluorescence and Elisa techniques, also have a sensitivity problem.
Immunofluorescence can only detect the antibodies after a patient has carried the disease for at least 10 days, while for ELISA, about 20 days are needed.
During that time, the patient may have spread the virus to other people.
In that context, Stohr said, the tests are mainly useful after hospitalization, for confirming whether a patient has Sars.
"That's why these tests are not very useful frankly for infectious disease control. What we need is a bedside test, a quick on-the-spot test which allows the detection (from secretions) at an early stage of the disease or the detection of early antibodies, say from day five (of infection) onwards," he said.
"The most important thing now is to reduce the time between the onset of the disease and hospitalization. In Singapore it's 1.4 days, which is certainly excellent, what it should be, in other words, a relatively short time in which people have the chance to infect their neighbors, relatives.
"In Hong Kong, it's around three to four days, but this still suffices to control the disease in Hong Kong. The contribution that information and education can make to the control of the disease is much more significant than what the (present) diagnostic tests can do." AFP
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