Issue 8: Understanding testing for COVID-19 (APRIL 24, 2020)
Type of tests
Optimal testing methods for COVID19 should: directly or indirectly detect infection; have a short turnaround time; high throughput and batch capability; have the capability of being performed low infrastructure settings (e.g., cruise ships or remote communities). Lab-based molecular assays are the gold standard for COVID-19 diagnosis, but point-of-care (POC) technologies and serologic immunoassays are rapidly emerging. The FDA has categorized approved tests by complexity1. Waived tests are available for use at the point-of-care (POC), whereas moderate- and high-complexity tests must be performed in a laboratory. There are currently two broad categories of tests: (1) Those directed at detecting the virus; and (2) Those that detect the host’s response to the virus2.
1. Identification of viral RNA: Here, the reverse transcription polymerase chain reaction (RT-PCR)3 is used to identify presence of SARS-CoV RNA is collected samples obtained from swabs of the nasopharynx and/or oropharynx, with the former being more sensitive than the latter3. For patients with pneumonia, lower respiratory tract secretions (e.g., sputum, bronchoalveolar lavage fluid) are also collected. After collection, the swabs or samples are placed in an eluent solution to extract the RNA from cells. A predetermined potion of the collected RNA is then amplified via PCR by using specifically designed oligonucleotide primer sequences. Detection rates for RT-PCR tests approved by the FDA ≥ 95%4. RT-PCR detection of SARS-CoV-2 is the cornerstone of testing for COVID-19. Importantly, a negative test does not exclude infection; a positive test is most likely correct, barring erroneous cross-contamination; viral RNA does not necessarily indicate live virus presence or imminent transmission5.
2. Serology: These tests are designed to detect immunoglobulins (IgM, IgA, IgG) or the total amount of antibodies in blood of a known infected person5. Seroconversion typically occurs 7-11 days postexposure6-9. IgM and IgG antibodies become detectable after 4-5 days; ~70% of symptomatic patients are positive by days 8-14, and 90% by days 11-24. IgG reactivity reaches >98% after several weeks, but duration of this antibody response is not yet known. Conventional Enzyme-Linked Immuno-Sorbent Assays (ELISAs) or point-of-care (i.e., POC) devices are used to detect SARS-CoV-2 antibodies. Assessment of nine immune assays (3 ELISAs, 6 POC tests) found specificities of ELISAs to be 93, 96, and 100% with respective sensitivities of 90, 65, and 90%10. POC tests showed sensitivities of 93-100% and specificities of 80-100%. POC tests can produce results in <20 minutes, but tests may not be positive until the second week of infection. Sensitivity may also be lower after asymptomatic infection9. However, after eight days of illness, the sensitivity of serological assays surpasses that of nucleic acid testing10-13.
3. Who/When to test: The CDC recommends that physicians use discretion when testing, with priority given to three groups: hospitalized patients with COVID-19 symptoms, other symptomatic persons at risk for poor outcomes, and persons who had close contact with a suspected/confirmed COVID-19 case within 14 days of illness onset or travelled to an affected area14. A test well suited for one use case may be inadequate for another. Timing of testing is also key. Some use cases are shown in the table below. Combination testing could also help to improve diagnostic probability.
- Patel et al. mBio . 2020 Mar 26;11(2):e00722-20.
- Zou et al. 2020. N Engl J Med 382:1177–1179
- Beeching et al. BMJ. April 7 2020.
- Zhang et al. Emerg Microbes Infect 2020;9:386-9.
- Zhao et al. Clin Infect Dis 2020:ciaa344.
- Li et al. J Med Virol 2020.
- Okba et al. medRxiv 2020.03.18.20038059.
- Lassauière et al. medRxiv preprintdoi.org/10.1101/2020.04.09.20056325.
- Zhao et al. Clin Infect Dis. 2020. doi.org/10.1093/cid/ciaa344.
- Guo et al. bioRxiv 2020:2020.03.13.990226.
- Lui et al. VIEW. 2020;1:e4.
- Centers for Disease Control and Prevention. www.cdc.gov/coronavirus/2019-nCoV/hcp/clinical-criteria.html Accessed April 21 2020.