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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