Severe Acute Respiratory Syndrome (SARS): Laboratory diagnostic tests
29 April 2003
Researchers in several countries are working towards developing fast and accurate laboratory diagnostic tests for the SARS coronavirus (SARS-CoV). However, until standardized reagents for virus and antibodies detection are available and methods have been adequately field tested, SARS diagnosis remains based on the clinical and epidemiological findings: acute febrile illness with respiratory symptoms not attributed to another cause and a history of exposure to a suspect or probable case of SARS or their respiratory secretions and other bodily fluids.
Those requirements are reflected in the current WHO case definitions for suspect or probable SARS. However in several countries (Canada, France, Germany, Hong Kong SAR, Italy, Japan, the Netherlands, Singapore, United Kingdom and the United States of America) samples from suspected and probable SARS cases are being tested for SARS-CoV.
Laboratory test result criteria for confirming or rejecting the diagnosis of SARS remain to be defined.
1. Molecular tests (PCR)
Polymerase chain reaction (PCR) can detect genetic material of the SARS-CoV in various specimens (blood, stool, respiratory secretions or body tissues Sampling for Severe Acute Respiratory Syndrome (SARS) diagnostic tests). Primers, which are the key pieces for a PCR test, have been made publicly available by WHO network laboratories on the WHO web site. A ready-to-use PCR test kit containing primers and positive and negative control has been developed. Testing of the kit by network members is expected to quickly yield the data needed to assess the test’s performance, in comparison with primers developed by other WHO network laboratories and in correlation with clinical and epidemiological data.
Principally, existing PCR tests are very specific but lack sensitivity. This means that negative tests cannot rule out the presence of the SARS virus in patients. Furthermore, contamination of samples in laboratories in the absence of laboratory quality control can lead to false positive results.
Positive PCR results, with the necessary quality control procedures in place. Recommendations for laboratories testing for SARS-coronavirus, are very specific and mean that there is genetic material (RNA) of the SARS-CoV in the sample. This does not mean that there is live virus present, or that it is present in a quantity large enough to infect another person.
Negative PCR results do not exclude SARS. SARS-CoV PCR can be negative for the following reasons:
- The patient is not infected with the SARS coronavirus; the illness is due to another infectious agent (virus, bacterium, fungus) or a non-infectious cause.
- The test results are incorrect (“false-negative”). Current tests need to be further developed to improve sensitivity.
- Specimens were not collected at a time when the virus or its genetic material was present. The virus and its genetic material may be present for a brief period only, depending on the type of specimen tested.
2. Antibody tests
These tests detect antibodies produced in response to the SARS coronavirus infection. Different types of antibodies (IgM and IgG) appear and change in level during the course of infection. They can be undetectable at the early stage of infection. IgG usually remains detectable after resolution of the illness.
The following test formats are being developed, but are not commercially available yet:
- ELISA (Enzyme Linked ImmunoSorbant Assay): a test detecting a mixture of IgM and IgG antibodies in the serum of SARS patients yields positive results reliably at around day 21 after the onset of illness.
– IFA (Immunofluorescence Assay): a test detecting IgM antibodies in serum of SARS patients yields positive results after about day 10 of illness. This test format is also used to test for IgG. This is a reliable test requiring the use of fixed SARS virus on an immunofluorescence microscope.
Positive antibody test results indicate a previous infection with SARS-CoV. Seroconversion from negative to positive or a four-fold rise in antibody titre from acute to convalescent serum indicates recent infection.
Negative antibody test results: No detection of antibody after 21 days from onset of illness seems to indicate that no infection with SARS-CoV took place.
3. Cell culture
Virus in specimens (such as respiratory secretions, blood or stool) from SARS patients can also be detected by inoculating cell cultures and growing the virus. Once isolated, the virus must be identified as the SARS virus with further tests. Cell culture is a very demanding test, but currently (with the exception of animal trials) only means to show the existence of a live virus.
Positive cell culture results indicate the presence of live SARS-CoV in the sample tested.
Negative cell culture results do not exclude SARS (see negative PCR test result).