SARS: Availability and use of laboratory testing
8 April 2003
SARS diagnosis is still being made on clinical grounds and exposure only (Case Definitions for Surveillance of Severe Acute Respiratory Syndrome (SARS). However, several countries and areas (Canada, France, Germany, Hong Kong SAR, Japan, the Netherlands, Singapore, United Kingdom, United States of America) are beginning to routinely conduct laboratory testing of SARS suspect and probable cases. Tests are still being developed and improved, but using them now helps to understand how SARS and human coronavirus are connected.
Status of currently available diagnostic tests
- Antibody tests
- ELISA (IGM/IGA) detects antibodies in the serum of SARS patients reliably as from about day 20 after onset of clinical signs. Antibodies already are detected in some patients between 14-21 days.
- Immunoflorescence Assays (detecting Immunoglobulin M) in SARS virus infected VERO cells detects antibodies in serum after about day 10 of onset of disease. This is a reliable but demanding test requiring amongst others, use of live virus in cell culture and an immunofluorescence microscope. - Molecular tests
- PCR can detect genetic material of coronavirus in various specimens (blood, stool or respiratory secretions). For this test, several primers have been developed by Centers for Disease Control and Prevention, United States of America, Federal Laboratories for Health Canada, Hong Kong University, and the Governmental Viral Unit of Hong Kong, Canada and they have been shared on the WHO web site. Some laboratories in the WHO multi-centre collaborative network on SARS are currently comparing these tests and first results are will be forthcoming shortly. - Cell culture
- Virus in specimens (respiratory secretions, blood) from SARS patients can also be detected by infecting cell cultures such as VERO cells.
- Positive test results indicate that SARS patients are, or recently were, infected with the coronavirus. A negative coronavirus test does not mean that the patient does not have SARS.
- Negative test results:
SARS is diagnosed not on the basis of laboratory tests but on clinical evaluation and possible past exposures.
Reasons for negative test results in a patient with SARS may be:
- Tests are incorrect (“false-negative”). Current tests need to be further developed to improve sensitivity.
- Patient is not infected with the new coronavirus. The cause of infection could be another pathogen (virus, bacteria, etc.) causing respiratory symptoms, and pneumonia.
- Specimens were not collected at a time when virus or its genetic material was excreted. Excretions may be for a brief period only, depending on the type of specimen tested.