ABSTRACT
Background: The use of COVID-19 convalescent plasma (CCP) as experimental therapy during the SARS-CoV-2 outbreak has led to the need to quantify neutralizing antibodies (Abs) to SARS-CoV-2 to make it suitable for clinical use. In this respect, the National Blood Centre (CNS) and the National Centre for the Control and Evaluation of Medicines (CNCF) of the Istituto Superiore di Sanit(ISS) have organized an External Quality Assessment (EQA) exercise on serological methods for CCP Ab screening. Aims: To present the results deriving from the anti-SARS-COV-2 EQA program. Methods: Blood donations with anti-SARS-CoV-2 Abs at different concentrations were used as positive samples. Negative samples were prepared using a pool of plasma donations negative for anti-SARS-CoV-2 Abs and tested negative for HBV, HCV, HIV and Syphilis markers. A panel of 10 samples (two negative and eight positive) has been sent to each participating laboratory. Production, storage and distribution of the panels were carried out according to pre-established procedures to ensure the homogeneity and stability of the samples. The samples were tested for the presence of neutralizing Abs using a neutralization assay based on the use of lentiviral particles pseudotyped with the SARS-CoV-2 Spike glycoprotein, and expressing luciferase as a detection system infectivity in VERO E6 cells1.On the basis of anti- SARS-CoV-2 Ab content, the 10 panel samples were divided into four groups: negative (2 samples, ID90 < 20), low positive (2 samples ID90 ~ 50), medium positive (three samples, ID90 100-200) and high positive (three samples, ID90 300-800). 28 laboratories participated to the EQA study using five different methods. In particular, three laboratories participated with two different methods. Results: 30 out of 31 panels were correctly identified: one laboratory erroneously identified a negative sample as reactive. The re-test of the panel gave the correct result. Quantitative results (14 laboratories) were converted in Binding Antibody Unit (BAU/ml) using the conversion factor indicated by the kits' manufacturer. The conversion in BAU/ml allowed to standardize the results from different methods: in fact, after conversion, results seem to be more closed even if some differences are still present in two samples with a high content of IgG. Summary/Conclusions: The EQA exercise provided the participants a valid tool to test the laboratory performance and to highlight any critical issues concerning a not-routinely performed test, used by several blood establishments for the qualification of CCP. Moreover, the use of a common measure unit (BAU/ml) using the WHO international standard for SARS-CoV-2 immunoglobulin seems to reduce the differences among methods improving the analytical and diagnostic comparability. However, it is not true for all samples tested. The intrinsic Ab composition of each samples tested (presence of different type of Abs, the Abs' avidity and specificity, time of recovery from the infection) may contribute to the persistence of differences of the results across methods. 1. Dispinseri S, Nat. Commun. 2021.