Diagnosing SARS

DOUBTS have been raised about the reliability of the diagnostic tests for Severe Acute Respiratory Syndrome (SARS) that are used by the National Institute of Virology (NIV), Pune, and the National Institute for Communicable Diseases (NICD), Delhi. According to sources in the World Health Organisation (WHO), the `tool kit' for the Polymerase Chain Reaction (PCR) test needs to be validated before the results based on it can be included in the WHO data in view of the May 1 case definition.

According to the new WHO case definition, all SARS `suspect' cases that test positive will be regarded as SARS `probable' cases. The WHO surveillance record includes only worldwide `probable' cases. If this definition were to be followed, given that India has 10 `suspect' cases that have tested PCR-positive, India should figure in the list, with 10 `probable' cases. One reason for the need to validate the laboratory procedures followed is the much greater proportion of PCR-positives in India than, say, in the United States. In the U.S., as of April 30, out of a total of 233 `suspect' cases, no one had tested positive. Even among the 56 clinically `probable' cases, a large fraction had not tested positive. Since both the U.S. and India were using the same `primers' for the test, this inconsistency needed to be resolved before PCR-based results could be used as the basis for surveillance data, the WHO source said.

R. Ramachandran