IDSA published guidelines on serological diagnosis of COVID on September 18, 2020. The main recommendations were:
- When to test? IDSA recommended against antibody testing in the initial 2 weeks of illness considering the low sensitivity of both IgM and IgG in the first 2 weeks (33% for IgM and 23% for IgG in week 1), (73% for IgM and 68% for IgG in week 2). The ideal timing of test is three to four weeks after the symptom onset.
- What antibody to be tested? The panel recommended IgG (95% sensitive,99% specific) OR total antibody testing (98% sensitive, 100% specific) 3-4 weeks after symptom onset. There was no recommendation for or against IgM. It was noted that IgM even though specific, had lower sensitivity than IgG in 3rd and 4th week and did not have a rise in titre as weeks progressed as seen with IgG. IgA testing resulted in high false positive rates owing to low specificity and was not recommended for serological diagnosis of COVID. Combination of IgG or IgM testing was not recommended against as the ‘”either/or” positive interpretation conferred lower specificity as compared to testing individual antibody classes alone.
- Can serology be used for diagnosis of acute infection? IDSA advices the use of serology in patients with high clinical suspicion but has repeatedly negative PCRs, to provide evidence of infection with SARS CoV 2. Studies showed an improved diagnostic yield when serology was combined to PCR after 14 days of symptoms.
- In children with MIS-C, antibody testing is to be combined with NAAT to improve detection rates. This was based on the analysis of 9 case series of MIS-C, where NAAT positivity was 30-50% whereas antibody positivity was more than 80%.
- Kimberly E. Hanson, A. (2020, September 18). COVID-19 Guideline, Part 4: Serology. https://www.idsociety.org/practice-guideline/covid-19-guideline-serology