- Diagnostic testing for SARS-CoV-2 and postinfection confirmation of specific antibodies are key for local reopenings.
Why this matters
- Familiarity with testing limitations is essential for assessing COVID-19 infections and immunity within the community, especially in light of anticipated waves.
- RT-PCR testing assays have high analytical sensitivity (estimated limit of detection, 100-1000 copies) as well as high specificity.
- Nasopharyngeal swab may promote patient coughing; wear appropriate personal protective equipment during testing.
- Alternative samples: nasal, mid-turbinate, oropharyngeal swabs, and saliva; comparability relies on viral load at time of infection.
- Negative specimens do not rule out COVID-19; repeat testing is warranted when clinical suspicion is high, especially in higher-prevalence settings.
- PCR-positive results are possible in a range from 1 week (mild illness) to several weeks or months (severe illness) after symptom onset, although infectivity in the latter is not established.
- Although patients may become PCR-positive after testing negative, data regarding true virological/clinical relapse are scarce.
- Inconclusive results warrant confirmation with an alternative assay.
- Serologic test performances vary; positive serology with a low-specificity assay likelier than not represents a false positive.
- IgM, IgG against SARS-CoV-2 appear as early as 3-6 days after symptom onset.
- Almost all patients will seroconvert by 3 weeks.
- Antibodies persist for at least 2 months (longer term outcomes remain unknown, given the limited time since the virus appeared).
- Quantitative antibody titer cutoff correlating with protective immunity is currently undefined.