FROM NEJEM
Testing: More Questions than Answers
Thomas Glück, MD reviewing
A serologic test
for antibodies against the SARS-CoV-2 nucleocapsid protein shows optimal
specificity and sensitivity at day 14, but its prognostic role is unclear.
Researchers present data on
the sensitivity and specificity of the Abbott SARS-CoV-2 immunoglobulin G (IgG)
test, which detects IgG antibodies against the SARS-CoV-2 nucleocapsid protein.
When the SARS-CoV-2 assay
was used on 1020 deidentified serum specimens from 1010 different individuals
submitted in 2018 and 2019 for serologic testing of other viral infections,
specificity was 99.9%. When the assay was used on 689 samples obtained from 125
patients with PCR-confirmed SARS-CoV-2 infection in March and April 2020,
sensitivity was 53.1% at 7 days after the estimated day of symptom onset, 82.4%
at 10 days, 96.9% at 14 days, and 100% at 17 days; sensitivity was 88.7% at 7
days after the date of PCR positivity, 97.2% at 10 days, 100% at 14 days, and
100% at 17 days. In a voluntary field test of anti-SARS-CoV-2 antibody seroprevalence
in Boise, Idaho, in late April
2020, 1.8% of 4856 individuals tested positive
with this test.
COMMENT
Given that >90% of adults aged 50 and older harbor antibodies
to other circulating coronaviruses, the serologic test evaluated here shows excellent
specificity and high sensitivity at 14 days after symptom onset. FDA emergency
authorization recently was provided for this and other serologic tests for
SARS-CoV-2. As Theel and colleagues recently outlined in a separate commentary
article, various non–FDA-authorized tests with undocumented sensitivity and
specificity are on the market, the results of which must be interpreted with
caution. However, even the FDA-authorized tests use different antigens and
formats. It is unclear to date which responses to which antigens occur first
and whether a positive serologic test indicates protective immunity against
reinfection or cessation of viral shedding (NEJM JW Infect Dis Jun 2020 and Clin Infect Dis 2020 Mar 21; [e-pub]; Clin Infect Dis 2020 Mar 28; [e-pub]; and J Med Virol 2020 Feb 27; [e-pub]).
Currently, approved serologic tests for SARS-CoV-2 can be used, adjunctive to
molecular tests but not alone, as diagnostic tools (Important
Information on the Use of Serological (Antibody) Tests for COVID-19 — Letter to
Health Care Providers. Silver Spring, MD: U.S. Food and Drug Administration;
2020 Apr 17). In the future we will need serologic tests for
epidemiologic studies and for measuring responses to vaccines.
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