The Covid-19 antibody test kits were approved by the SA Health Products Regulatory Authority (SAHPRA) to be used as part of SA’s fight against the pandemic.
The antibody test is different from the polymerase chain reaction (PCR) test being used in the country.
According to the authority, the difference between the molecular PCR and the antibody test is that the PCR tests are able to detect and diagnose whether one has been infected with Covid-19 and thus give a clinical diagnosis.
Antibody tests can detect if one has developed antibodies for Covid-19. This means the tests cannot be used for clinical diagnosis. The antibody test involves collecting a small blood sample from a patient by using a device that pricks their finger.
Recommendation for use
According to the health department, these tests may be used to:
- Diagnose Covid-19 retrospectively in patients who have recovered from a Covid-19 compatible illness and are negative based on results from the SARS-CoV-2 (the virus that causes Covid-19) PCR test.
- Diagnose Covid-19 in patients who are admitted with suspected SARS-CoV-2 infection but who test negative for RT-PCR (reverse transcription-polymerase chain reaction) as an ancillary investigation. This will include children with suspected multisystem inflammatory syndrome who may test negative with SARS-CoV-2 PCR.
- Identify past exposure to SARS-CoV-2 in individuals optimally at 21 days post-infection.
- To assess SARS-CoV-2 vaccine responses.
- To identify potential convalescent plasma donors.
A negative antibody test result does not reliably rule out prior infection. Possible causes would be:
- Insufficient sensitivity of antibody test.
- Acute phase testing (specifically within 14 days post-symptom onset).
- Some patients may not form detectable antibodies, especially following asymptomatic SARS-CoV-2 infection.
- Waning of antibodies over time, in as soon as one to two months in asymptomatic or mild cases.
A positive antibody test result does not reliably prove prior infection. Possible causes would be:
- Insufficient specificity of the antibody test.
- Cross-reacting antibodies, for example, those directed against other human coronaviruses.
The department warned that even though you may have Covid-19 antibodies you may not be immune to the virus and you should still maintain health and safety measures.
“A positive antibody test result therefore should not be regarded as proof of immunity and must not be used to reduce or abandon protective measures,” said the department.
“Antibodies detected by different assays do not necessarily represent neutralising antibodies that are assumed to be the best measure of humoral immunity and protection against infection and/or disease.”