Comparison of serological assays used across the globe

CITF-funded scientists from Canadian Blood Services and Héma-Québec participated in an international study which looked to compare the different serological assays used across 27 countries to estimate seroprevalence. The results were recently published in Vox Sanguinis, highlighting the diversity of assays used, with the Abbott SARS-CoV-2 Ig assay being the most common.

Main Conclusions:

• Seroprevalence estimates of 5.6% or less were reported in Europe, North America and Asia, whereas seroprevalence estimates of 13.5% or more were reported in Brazil and Iran. The seroprevalence rates appear to correlate with the number of COVID-19 cases in these areas.
• Several blood suppliers worldwide used the Abbott SARS-CoV-2 Ig assay.

Being able to compare seroprevalence among countries is valuable in informing policy makers, however, not every blood supplier is using the same assay to allow for this comparison. In this international study, surveys were conducted to determine the assays used, the detected immunoglobulins and target antigen, performance characteristics (sensitivity and specificity), and seroprevalence estimates in their region. Out of the suppliers contacted, 38 provided at least partial responses, representing 27 countries. Thirteen provided seroprevalence results.

The Abbott SARS-CoV-2 IgG assay was the most common kit, used by 15 blood suppliers. This was followed by the EUROIMMUN Anti-SARS-CoV-2 ELISA, and the Beijing Wantai Biological Pharmacy SARS-CoV-2 Total Ab ELISA, to name a few. Most of the assays used detect IgG, underlining the importance of IgG in immunity against SARS-CoV-2 as it declines less quickly than IgM or IgA. Of the assays, 68.2% targeted the SARS-CoV-2 spike protein and 50% targeted the nucleocapsid protein, while 18.2% targeted both viral proteins. The sensitivity of IgG-specific assays ranged from 71.9% to 100%, and the specificity ranged from 96.2% to 100%.

Among the 13 blood suppliers who provided seroprevalence estimates, 10 reported values of 5.6% or less (in Europe, North America and Asia), whereas the other four reported seroprevalence estimates of 13.5% or more (in regions of Brazil and Iran) (Table 1). The seroprevalence rates appear to correlate with the number of COVID-19 cases in these areas.

The authors highlight that it is important to consider geographic region within each country, stratification by age, and consistency in interpretation of test results when estimating seroprevalence. As there is heterogeneity in assays used, standardization of methodology is needed to allow for comparison of seroprevalence studies.

Table 1. Seroprevalence Results

Both Héma-Québec and the Canadian Blood Services (CBS) collaborate with CITF to estimate seroprevalence rates for Canada and provinces served by CBS. Read more about the latest findings here.