Update on Diagnostic Testing for SARS

Guidelines on diagnostic testing for severe acute respiratory syndrome (SARS) have been issued by the US Centers for Disease Control and Prevention (Atlanta, GA, USA).

According to these guidelines, initial diagnostic testing should include chest radiograph, pulse oximetry, blood cultures, sputum Gram's stain and culture, and testing for viral respiratory pathogens, notably influenza A and B and respiratory syncytial virus. A specimen for Legionella and pneumococcal urinary antigen testing should also be considered. Clinicians should save any available clinical specimens (respiratory, blood, serum) for additional testing until a specific diagnosis is made. Acute and convalescent (more than 21 days after onset of symptoms) serum samples should be collected from each patient diagnosed with SARS.

The CDC recommends that clinicians evaluating suspected cases use standard precautions (including hand hygiene) together with airborne (e.g., N-95 respirator) and contact (gowns and gloves) precautions. Until the mode of transmission has been defined more precisely, eye protection should also be worn for all patient contact.

Assays for laboratory diagnosis of SARS-CoV (coronavirus) infection include enzyme-linked immunosorbent assay (ELISA), indirect fluorescent-antibody assay, and reverse transcription polymerase chain reaction (RT-PCR) assays of appropriately collected specimens. However, warns the CDC, the absence of SARS-CoV antibody from serum obtained less than 21 days after illness onset, a negative PCR test, or a negative viral culture does not exclude coronavirus infection and is not considered a definitive laboratory result. In these cases, a convalescent serum specimen obtained more than 21 days after illness is needed to determine infection.

All SARS diagnostic assays are still under evaluation and none has been approved for a number of reasons. In the first days of illness, SARS patients produce only a small amount of the virus, unlike other viruses. This means a test has to be very sensitive. Also, before any test can be reliable, it has to be tested on many specimens known to contain the virus in order to be certain it is accurate. In the meantime, the CDC is offering an experimental PCR-based diagnostic test for SARS to about 100 specialized US laboratories around the country to gauge the effectiveness of the test. Patients will be asked to provide written consent before testing.

"The quick reaction to SARS by the CDC, at the global level, has been exceptional,” said Jared Schwartz, M.D., Ph.D., secretary-treasurer of the College of American Pathologists. "This most recent announcement of an experimental polymerase chain reaction (PCR) diagnostic test for SARS illustrates the CDC's aggressive reaction to this emerging disease.”




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Centers for Disease Control