Here is how clinical labs test for COVID-19

(Created using information from the CDC.gov website)

The CDC works closely with state and local public health departments, travel industry partners, and others to identify and test people who may be infected with COVID-19. There are  several different laboratory tests to detect COVID-19 infection. The testing strategy is based on experience from the SARS and MERS outbreaks.

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 COVID-19. Serology tests are for surveillance or investigational purposes and not for diagnostic purposes.

Molecular Tests

Molecular tests are used to diagnose active infection (presence of COVID-19) in people who are thought to be infected based on their clinical symptoms and having links to places where COVID-19 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 COVID-19 infection requires a positive rRT-PCR result for at least two specific genomic targets. Sequencing may also be used as a confirmatory test.
• Increasingly laboratories in the United States are approved to test for COVID-19 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 person under investigation to be negative for active COVID-19 infection following one negative rRT-PCR test on the recommended specimens. Since a single negative result does not completely rule out infection, in some circumstances additional specimens may be tested.
• CDC considers a known the patient to be negative for active COVID-19 infection following two consecutive negative rRT-PCR tests on all specimens.

Serology Tests

Serology testing is used to detect previous infection (antibodies to COVID-19) 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 COVID-19 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 COVID-19 disease than was initially thought. 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 COVID-19.
• CDC has a two-phase approach for serology testing, using two screening tests and one confirmatory test 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.
• The microneutralization assay is a highly specific confirmatory test used to measure neutralizing antibodies, or antibodies that can neutralize virus. This method is considered a gold standard for detection of specific antibodies in serum samples. However, compared with the ELISA, 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 either ELISA, and positive by microneutralization, the specimen is determined to be confirmed positive.
• If a clinical sample is positive by both ELISAs, and negative by microneutralization, the sample is determined to be indeterminate.
• If a clinical sample is positive by only one ELISA, and negative by microneutralization, the sample is determined to be negative.
• If a clinical sample is negative by both ELISAS, the sample is determined negative.
• In the end, a final determination of a confirmed positive serology result requires a positive ELISA test and confirmation by microneutralization assay.