What are SARS-CoV-2 Antibody Tests?
SARS-CoV-2 antibody tests provide qualitative detection of IgM and IgG antibodies against SARS- CoV-2 in individuals suspected of COVID-19. After infection with SARS-CoV-2, the virus antigen stimulates the immune system to produce antibodies that can be detected in the blood. Among these antibodies, SARS-CoV-2 IgM antibodies appears early and are mostly positive after 3-5 days of onset.
The SARS-CoV-2 IgM titers then decrease while the SARS-CoV-2 IgG antibody potency starts to rise rapidly.
During the recovery phase, the titer of the SARS-CoV-2 IgG antibody may increase four times or more compared to the acute phase1.
Covid-19 Test Background
A recently discovered Syndrome Coronavirus 2 (SARS-CoV-2) is causing an epidemic of respiratory disease known as COVID-19. The most common symptoms of COVID-19 are fever, shortness of breath, and dry cough. Some patients may have aches and pains, nasal congestion, runny nose, sore throat, or diarrhea. These symptoms are usually mild and begin gradually. Some people become infected but don’t develop any symptoms and don’t feel unwell. SARS-CoV-2 is transmitted through respiratory droplets and contact. When an infected person coughs or sneezes, droplets land on objects and surfaces. Other people may acquire SARS-CoV-2 by touching objects or surfaces, then touching their eyes, nose, or mouth.
Diagnosing COVID-19 currently relies on two major methodologies: Reverse Transcription Quantitative Polymerase Chain Reaction (RT-qPCR) and serological immunoassays that detect viral-specific antibodies (IgM and IgG).
Why SARS-CoV-2 Antibody Tests?
Combined with RT-qPCR, SARS-CoV-2 serological antibody tests offer a valuable diagnostic tool in identifying infected patients. According to recent studies, SARS-CoV-2 antibodies are not detectable before 3 days after onset of symptoms (or at least 7 to 10 days after infection)2-3. However, antibody tests can detect past infection because virus-specific antibodies can persist in the blood for several weeks/months after onset of symptoms. Since the exact time of infection is often unknown, combining RT-qPCR and IgM/IgG testing can improve the accuracy of the COVID-19 diagnosis.
Who can be tested?
SARS-CoV-2 antibody tests are recommended to be used on patients with at least 3 days after onset of symptoms or 7-10 days after infection with the virus.
Covid-19 Test Interpretation of Results
SARS-CoV-2 antibody assays are designed to complement RT-qPCR in the diagnosis of SARS-CoV-2 infections. Table1 shows the clinical interpretation of all possible scenarios that can be encountered when testing a patient with both RT-qPCR and an IgM/IgG serological test.