Analytical Validation and Clinical Application of Rapid Serological Tests for SARS-CoV-2 Suitable for Large-Scale Screening

Abstract

Recently, large-scale screening for COVID-19 has presented a major challenge, limiting timely countermeasures. Therefore, the application of suitable rapid serological tests could provide useful information, however, little evidence regarding their robustness is currently available. In this work, we evaluated and compared the analytical performance of a rapid lateral-flow test (LFA) and a fast semiquantitative fluorescent immunoassay (FIA) for anti-nucleocapsid (anti-NC) antibodies, with the reverse transcriptase real-time PCR assay as the reference. In 222 patients, LFA showed poor sensitivity (55.9%) within two weeks from PCR, while later testing was more reliable (sensitivity of 85.7% and specificity of 93.1%). Moreover, in a subset of 100 patients, FIA showed high sensitivity (89.1%) and specificity (94.1%) after two weeks from PCR. The coupled application for the screening of 183 patients showed satisfactory concordance (K = 0.858). In conclusion, rapid serological tests were largely not useful for early diagnosis, but they showed good performance in later stages of infection. These could be useful for back-tracing and/or to identify potentially immune subjects.

Keywords: COVID-19; IgG; IgM; antibodies; immune response; microsampling; serological test.

Figure 1

Workflow of patient enrollment and of the evaluation of serological test performance.

Figure 2

Cumulative results of serological tests relative to the time from first COVID-19 diagnosis through rRT-PCR.

Figure 3

Age distribution among IgG-positive and IgG-negative patients 21 days the first molecular diagnosis of SARS-CoV-2.

Figure 4

IgM titers lower than six by FIA were associated with IgM false-negative cases with LFA rapid tests.