COVID-19 diagnostic and immunologic testing should guide community reopening

  • Fang FC & al.
  • Clin Infect Dis
  • 08/06/2020

  • Liz Scherer
  • Clinical Essentials
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  • 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.

Key points

  • 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.