A COVID-19 Antibody Test: The Beckman Access SARS-COV-2 IgG II Assay
The Access SARS-CoV-2 IgG II assay is a two-step enzyme chemiluminescent immunoassay using paramagnetic particles. The assay is intended to qualitatively detect IgG antibodies to the SARS-Cov-2 virus in human serum. The Access SARS-CoV-2 IgG assay is intended for use as an aid in identifying individuals with an adaptive immune response to SARS-CoV-2, indicating recent or prior infection.
IgG antibodies to SARS-CoV-2 are generally detectable in the blood several days after initial infection, although the length of time antibodies may be present post-infection is not currently well characterized.
The assay is initiated by adding a patient’s sample to a reaction vessel that contains buffer and paramagnetic particles coated with recombinant SARS-CoV-2 protein specific for the receptor-binding domain (RBD) of the S protein. The sample is incubated in the reaction vessel for a given time, and then the materials bound to the solid phase are held in a magnetic field while the unbound materials are washed away. A monoclonal anti-human IgG alkaline phosphatase conjugate is added to the reaction vessel, where the conjugate binds to the IgG antibodies captured on the particles. A second separation and wash step removes unbound conjugate. Then, a chemiluminescent substrate is added to the vessel, and light generated by the reaction is measured with a luminometer. The light production is compared to the cut-off value defined during the calibration of the instrument. The benchtop Beckman Access 2 immunoassay system is commonly used as an analyzer to perform the assay and to measure the chemiluminescence generated by the assay.
The assay system produces a semi-quantitative result that is reported in arbitrary units (AU) from 2.00–450 AU/mL. Based on the AU, a qualitative result is reported as a non-reactive (negative) result, an equivocal result, or a reactive (positive) result in the following manner:
Test result of ≤ 0.80 is reported as non-reactive for SARS-CoV-2 IgG antibodies
Test result of 0.80 to < 1.00 is reported as equivocal for SARS-CoV-2 IgG antibodies.
Test result of > 1.00 is reported as reactive for SARS-CoV-2 IgG antibodies.
Based on clinical studies, the company indicates that the test has a 96% positive percent agreement or sensitivity at 8–14 days post symptom onset and a 99.9% negative percent agreement or specificity.
Interpretation of Results and Limitations:
The test results for the Access SARS-CoV-2 IgG II test should be interpreted based on the total clinical presentation of the patient, including symptoms, clinical history, data from additional tests, and other appropriate information. In addition, the results should not be used as the sole basis to diagnose or exclude SARS-CoV-2 infection or to inform infection status.
A negative or non-reactive result does not necessarily preclude acute SARS-CoV-2 infection. Since the IgG antibodies may not be at adequate levels for detection in the first few days of infection, the sensitivity of the Access SARS-CoV-2 IgG II assay early after infection may be low. If an acute infection is suspected, direct testing for the SARS-CoV-2 virus using a molecular-based assay may be necessary.
A positive result may not indicate previous SARS-CoV-2 infection. A false positive result can occur due to cross-reactivity from pre-existing antibodies or other possible causes, such as a past or present infection with other coronavirus strains.
