CDC Laboratory Testing for Middle East Respiratory Syndrome Coronavirus (MERS-CoV)
CDC is working closely with state and local public health departments, travel industry partners, and others in an ongoing response to two recently imported laboratory-confirmed MERS cases in the United States. As part of this response, CDC is conducting several different laboratory tests to identify MERS-CoV infection.
In general, these lab tests fall into two categories:
- Molecular tests, which look for evidence of active infection; and
- Serology tests, which look for previous infection by detecting antibodies to MERS-CoV. Serology tests are for surveillance or investigational purposes and not for diagnostic purposes.
Molecular tests are used to diagnose active infection (presence of MERS-CoV) in people who are thought to be infected with MERS-CoV based on their clinical symptoms and having links to places where MERS has been reported.
- Real-time reverse-transcription polymerase chain reaction (rRT-PCR) assays are molecular tests that can be used to detect viral RNA in clinical samples. CDC's current case definition for laboratory confirmation of MERS-CoV infection requires either a positive rRT-PCR result for at least two specific genomic targets, or a single positive target with sequencing of a second target.
- Most state laboratories in the United States are approved to test for MERS-CoV by using an rRT-PCR assay developed by CDC. This test is done under authority of an Emergency Use Authorization because there are no FDA-cleared/approved tests available for this purpose in the United States.
- The success of rRT-PCR testing depends on several factors, including the experience and expertise of laboratory personnel, laboratory environment (e.g., avoidance of contamination), and the type and condition of specimens being tested. For this rRT-PCR assay, CDC recommends collecting multiple specimens, including lower (bronchalveolar lavage, sputum and tracheal aspirates) and upper (e.g., nasopharyngeal and oropharyngeal swabs) respiratory samples, serum, and stool specimens.
- CDC considers a patient under investigation to be negative for active MERS-CoV infection following one negative rRT-PCR test on the recommended specimens. Since a single negative result does not completely rule out MERS-CoV infection, in some circumstances additional specimens may be tested.
- CDC considers a known MERS patient to be negative for active MERS-CoV infection following two consecutive negative rRT-PCR tests on all specimens.
Serology testing is used to detect previous infection (antibodies to MERS-CoV) in people who may have been exposed to the virus. Antibodies are proteins produced by the body's immune system to attack and kill viruses, bacteria, and other microbes during infection. The presence of antibodies to MERS-CoV indicates that a person had been previously infected with the virus and developed an immune response.
- Evidence to date suggests there may be a broader range of MERS disease than was initially thought. For example, public health investigators have identified individuals who are PCR-positive but have no MERS symptoms; we do not know if MERS-CoV can be spread by these people. For this reason, public health scientists are working to learn more about how the virus is transmitted. One way to do this is through voluntary testing of blood samples from people who had close contact with people known to have MERS.
- CDC has a two-phase approach for serology testing, using one screening test and two confirmatory tests to detect antibodies to MERS-CoV.
- ELISA, or enzyme-linked immunosorbent assay, is a screening test used to detect the presence and concentration of specific antibodies that bind to a viral protein. ELISAs usually produce results within a few hours.
- If a clinical sample is determined to be antibody-positive by ELISA, CDC then uses the IFA assay and/or microneutralization assay to confirm the positive result.
- IFA, or immunofluorescence assay, is a confirmatory test in which specific antibodies, if present in the person’s blood, attach to virus-infected cells fixed on a glass slide. These attached antibodies are detected by adding a secondary antibody labeled with a compound that makes them glow an apple-green color when viewed under a special microscope. This secondary antibody will bind to any antibodies which are present in the blood and have attached to the virus-infected cells. Like the ELISA results, IFA results can also be obtained in a few hours.
- If a clinical sample is positive by both ELISA and IFA, the specimen is determined to be positive.
- If a clinical sample is positive by ELISA but indeterminate or negative by IFA, CDC then performs additional confirmatory testing.
- The microneutralization assay is a highly specific confirmatory test used to measure neutralizing antibodies, or antibodies that can neutralize virus. This method is considered the gold standard for detection of specific antibodies in serum samples. However, compared with the ELISA and IFA, the microneutralization assay is labor-intensive and time-consuming, requiring at least 5 days before results are available.
- If a clinical sample is positive by ELISA, indeterminate by IFA, and positive by microneutralization, the specimen is determined to be positive.
- If a clinical sample is positive by ELISA, indeterminate or negative by IFA, and negative by microneutralization, the sample is determined to be negative.
- In the end, a final determination of a positive serology result requires a positive ELISA test and confirmation by IFA or microneutralization assay.
- In general, CDC will not release MERS-CoV serology results until all appropriate tests have been completed. However, preliminary findings (e.g., based on ELISA/IFA results only) may be reported if CDC investigators determine that making available these initial results will help protect the public’s safety.
- MERS-CoV serology tests are for surveillance or investigational purposes and not for diagnostic purposes – they are tools developed in response to the MERS-CoV outbreak.
- Information is limited about MERS-CoV and how the virus is spread. As public health scientists learn more about MERS-CoV, the approach to conducting these types of laboratory tests might change.