1. Not all antibody tests are the same.
There are currently over a hundred different antibody tests – in different phases of development. Only a small subset of these tests has FDA approval for diagnostic purposes and emergency use authorization. A majority of these tests are approved for research and surveillance purposes only (some pending FDA approval). Many are still in development and are not FDA approved yet.
Some of these tests involve only a fingerprick and yield results in 10 to 20 minutes. Others, which use whole blood, plasma, or serum samples, require more sophisticated laboratory equipment and can take several hours to yield results.
2. Not all antibody tests have the same accuracy.
These tests all have differing sensitivities (true positive rate) and specificities (true negative rate). Since these tests are new, and this virus is new, these tests were validated on only a few hundred samples. Rates of sensitivity and specificity are continuously changing as time passes, and more data is gathered.
3. The antibody test is different from the nasopharyngeal or oropharyngeal swab.
Antibody testing identifies exposure to SARS-CoV-2 (the virus which causes COVID-19) by looking for antibodies generated by the immune response. Most SARS-CoV-2 antibody tests detect IgM and/or IgG. IgM is the first antibody the body builds when fighting a new infection and may indicate you are still infected or recently recovered. IgG, on the other hand, take 7 to 10 days to develop, and indicate you have previously been infected and recovered.
The PCR tests with the nasopharyngeal or oropharyngeal swab detect the presence of viral material during an active infection. The PCR tests, therefore, diagnose active infection but does not indicate if a person has been infected in the past and recovered.
4. Testing positive for IgG does not guarantee immunity and mean a “free pass.” You still have to be careful.
Even if you test positive for IgG, which indicates you have contracted and recovered from the virus, the data is of limited use. This coronavirus is new, and no one knows how long these IgG antibodies last for. There is a possibility of the virus mutating and, therefore, a risk of reinfection. Furthermore, you can still carry the virus and spread it to others. It is still important to follow safety precautions, including wearing a mask and social distancing.
Christine Lau is a physician.
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