16th February 2021
The UK-RTC, a consortium of UK diagnostic companies including Abingdon Health, CIGA Healthcare, BBI Solutions and Omega Diagnostics, welcomes a pre-print publication, funded by Public Health England (PHE), reviewing performance data on rapid antibody tests, including the AbC-19TM Rapid Test.
The study was performed by scientists at PHE, and the Universities of Bristol, Warwick and Cambridge and examined the performance of four lateral flow rapid antibody tests using the same cohort of samples. Although the study remains subject to peer review, it shows that the AbC-19TM Rapid Test is highly accurate when tested on samples from individuals previously positive for the virus by PCR and from samples collected pre-pandemic.
- AbC-19TM is within a class of immunoassays which are designed to detect IgG to the full spike of the SARS-CoV-2 virus including neutralising antibodies to the spike protein of the COVID-19 virus. These neutralising antibodies may be produced as part of the body’s immune response following COVID-19 infection or post-vaccination; and they not only bind to a virus but they bind in a manner that may block future infection.
- Rapid antibody tests such as the AbC-19TM that measure IgG alone give easily interpretable results and are different to those detecting IgG and IgM on different lines.
Professor Chris Molloy, Chair of the UK-RTC said, “This research on the different use cases for lateral flow antibody tests is welcome. The AbC-19TM Rapid Test is an important weapon in the global fight against COVID-19. It allows us to see the neutralising IgG antibodies produced in response to infection, or after vaccination”.
A detailed commentary based on analysis of the pre-print follows:
- The AbC-19TM rapid antibody test displayed high accuracy (a combination of sensitivity and specificity) on samples from patients who had a self-declared positive PCR and presumed negative samples collected pre-pandemic. The provision of full data sets in the pre-print is useful and allows us to consider other parameters such as accuracy*. When looking at these positive PCR and pre-pandemic negatives, all devices tested had accuracy of >92% with AbC-19TM showing the highest accuracy at 97.3%. Accuracy is an important value which is derived from the number of false positive and false negative results in the population being tested combined with prevalence. No test is 100% accurate – tests with high sensitivity will have a tendency for lower specificity and tests geared towards high specificity may show lower sensitivity. (When calculating accuracy for all devices we used the positive prevalence of the sample set (11.8%) and accuracy will change depending on prevalence).
- The paper stimulates welcome discussion about different types of antibody to SARS-CoV-2 requiring different types of tests. For example, assays detecting antibodies to nucleocapsid protein (NP) will show previous infection whereas assays, such as the AbC-19TM Rapid Test, detecting IgG to the spike, show the presence of antibodies capable of neutralising the virus and capable of creating immunity. IgG antibodies to the spike include those which neutralise the virus by preventing the spike being able to bind to the ACE2 receptor and enter the human cells.
- Comparison of IgG spike assays to those detecting antibodies to NP on a sample set of unknown status showed lower sensitivity for those which are targeted at anti-spike IgG (84.7% for AbC-19TM Rapid Test). Such side-by-side comparisons of assays with different target analytes are difficult in the absence of reference standards and these differences between tests for different antibody types (e.g. NP versus spike and/or IgM versus IgG) with different use cases are not surprising. Suitable reference standards would aid the continued evaluation of these important tools.
- The AbC-19TM test, detects IgG antibodies which bind to the full trimeric spike protein (which also incorporates the receptor binding domain) and is ideally suited to show the type of antibodies produced to create immunity post infection. These same type of IgG antibodies to the spike protein are generated to produce immunity following vaccination (with use of the devices alongside vaccines requiring further evaluation).
- Rapid antibody tests such as the AbC-19TM that measure IgG alone give very clear results and are different to dual line tests detecting IgG and IgM where IgM may alter interpretation. In our assessment of the results, we have compared complete test results rather than separating results from individual lines for IgM and IgG. This is because, although providing extra information, an end user has to follow the written instructions and interpret all test lines for a dual line test kit.
- The paper highlights the need for discussion on the potential to use of lateral flow devices which detect IgG to the spike protein and how they have potential to be used to monitor for immunity post infection or after vaccination. This will require further evaluation.
This article is available as pre-print via medRxiv: https://www.medrxiv.org/content/10.1101/2021.01.30.21250777v1.full.pdf
*Accuracy shows the overall probability that a patient is correctly classified and is calculated as;
Accuracy = Sensitivity × Prevalence + Specificity × (1 − Prevalence)