Covid19 antibody testing has been a much talked about topic recently. Broadly speaking there are instant-result home testing and laboratory tests – both use different methods.
Regarding point of care / home testing most manufacturers report >95% accuracy, however there was hesitation to them being widely rolled out. This comes down to 2 main reasons;
False negatives – most tests have been validated on known COVID-19 patients that have been admitted into hospital. By default these patients will be quite unwell and have likely mounted a strong immune response so antibodies will be more easily detected.
In the community however some patients have mild symptoms or are even asymptomatic. These patients may produce lower antibody levels which may not be detectable by current point pf care tests.
False positives – there are other coronaviruses which have been circulating seasonally for many years – often causing mild cold / flu like symptoms. We are unsure whether these home testing kits are specific enough. So an individual may test as positive if they have been infected with a coronavirus that is not SARS-CoV-2, this is obviously a concern and can lead to individuals / families / companies making unwise decisions based on the assumption that they are immune.
New Laboratory testing using methods such as ELISA is more promising. They can provide quantitive antibody levels, rather than a negative or positive reading, so are able to identify those patients that had mild symptoms with lower antibody levels in the community.
The body produces different antibodies to different parts of the SARS-CoV-2 virus. Some antibody responses are stronger than others. The better tests now look for more than 1 antibody which also increases sensitivity.
Also many of the lab based tests report better no cross reactivity to other coronaviruses which increases specificity, this should mean less false positives.
In fact, the latest laboratory based antibody tests have sensitivity and specificity values of over 99%.
The big question however remains, what does a positive antibody result mean? Does is guarantee protection from COVID19 or a less severe infection in the future – hopefully yes, but unfortunately we don’t yet know for sure.
For the time being we need to interpret with caution and those testing positive for antibodies still need to take precautions.