Validation of a modified rapid antibody test to identify COVID-19 patients with low neutralizing antibody titers in an outpatient setting

Background

Primary aim of the presented project was to establish an assay, which allows rapid identification of patients with low anti-SARS-CoV-2 IgG titers in an outpatient setting. Such patients may be eligible for recruitment into the COVIC-19 study and may benefit from treatment with high-titer COVID-19 convalescent plasma.

Methods

For the validation, the rapid test from Canea for qualitative detection of neutralizing antibodies against SARS-CoV-2 in human serum, plasma or whole blood samples was used. The test cohort included 80 samples from 35 SARS-CoV-2-positive patients. Whole blood (EDTA) and corresponding serum samples were acquired before and on days 3, 14 and 28 after convalescent plasma administration. Modification of the test included a defined pre-dilution of whole blood samples with D-PBS and the introduction of a 5-level grading system to visually evaluate the test results. The main acceptance criterion was a negative predictive value of > 90% to detect a serum antibody concentration of ≤ 700 BAU/ml in the samples.

Results

With 94.6%, the negative predictive value met the required main acceptance criterion. The overall concordance rate between quantitative anti-spike IgG titers and the results of the modified Canea COVID-19 rapid test was 95%. Furthermore, the analysis showed a highly significant correlation between the rapid test results (5-level grading) and the quantitative antibody measurements (BAU/ml), thereby confirming the semi-quantitative validity of the modified rapid test system (Fig. 1, Pearson, *** p < 0.0001).

Conclusion

The modified test system from Canea was successfully validated and allows patients with anti-SARS-CoV-2 IgG titers < 700 BAU/ml to be recognized with minimal equipment using whole blood samples. Based on this test method, potential subjects benefiting from a treatment with high-titer convalescent plasma can be easily identified in an outpatient setting.

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