SARS-CoV-2 IgG antibody test
SARS-CoV-2 IgG antibody test
The disposable serological SARS-CoV-2 IgG test is an enzyme immunoassay designed to identify IgG antibodies to the SARS-CoV-2 virus. Such tests allow a quantitative assessment of possible contact with the virus that causes COVID-19.
The IgG antibodies in the venous blood serum can be detected between 11 and 14 days after the first symptoms of the disease. They represent a “late-stage” infection accompanied by the humoral immune response that causes the antibody production confirming the contact with the SARS-CoV-2 virus. These antibodies can neutralize the virus, and their presence in blood confirms the immunity to COVID‑19.
Testing with the qualitative measuring of the concentration of neutralizing IgG antibodies to SARS-CoV-2 should be done for people who:
- met a person infected with Covid-19;
- recovered after the coronavirus (antibodies to the glycosylated protein, S protein that creates so-called “corona” of the virus, must be present in blood at least a few months after the recovery);
- got vaccinated with vaccines based on the S glycoprotein of the SARS-CoV-2 virus (it is Moderna, Astra Zeneca, Pfizer–BioNTech);
- recovered from a disease with similar symptoms (fever, cold, cough) without coronavirus testing;
- are healthy but would like to check if they could have had Covid-19 without or with light symptoms.
When is a test needed?
Convalescent patients should make the first test after at least two weeks after recovery. Vaccinated people should be tested 10-14 days after the first or second (or last) shot.
The test material is the patient’s venous blood (serum or plasma). The antibody conjugation reaction is performed on a supplied plastic plate with vitals covered with another antibody. The test results are calculated spectrophotometrically. The test doesn’t require any preparation. You don’t need to keep your stomach empty before the test.
Coronavirus antibody diagnosis
Positive results of serological tests suppose you are currently infected with or had recovered from SARS-CoV-2. But they can also confirm the cross-reaction of other coronavirus variants. False-positive results have about 3% of people vaccinated against influenza or having an autoimmune disease.
The highest accuracy is achieved through parallel testing for SARS-CoV-2 IgG and IgM antibodies. It would be 100% after the 10th day after the first symptoms and only 66% if the symptoms arose less than 10 days ago. For a final negation or confirmation of the SARS-CoV-2 infection, a polymerase chain reaction (PCR) test is needed.
Interpretation of test results
1. IgG negative (-):
- No contact with the SARS-CoV-2 virus.
- A test was made within the serological gap (it must be repeated in 2-4 weeks). 2. Doubtful result (IgG +-):
- The test must be repeated within 1-2 weeks. 3. IgG positive (+):
- Current or past infection.
- Maybe, it was caused by cross-reactions to coronavirus variants different from SARS-CoV-2.