There has been so much media frenzy over rapid point-of-care blood tests for COVID-19 IgM/IgG lately. Emirates Airlines is the first airline in the world to introduce pre-boarding rapid blood tests for COVID-19 , despite strong scientific evidence against the use of such tests as a screening or early diagnostic tool. Many media reports are applauding the move as a way to boost passenger confidence and convince governments to end travel and entry restrictions [2-4]. The CNN report is the only one I could find that included any information on the limitations of the rapid blood test for COVID-19 . I have summarised the scientific evidence about the limitations of the rapid blood tests below for COVID-19.
Disclaimer: I am a 4th year Registrar in Anaesthesia in Australia. I am not an infectious disease or diagnostic testing expert, but I do have a Masters in Clinical Epidemiology and skills to critically appraise scientific literature. Also, obligatory mention that I am posting from mobile, so apologies for formatting.
Rapid point-of-care serological tests for COVID-19 detect IgM and IgG antibodies that develop in response to the infection, not the SARS-CoV-2 virus itself . There is a significant time lag between infection, +/- onset of symptoms and formation of antibodies (seroconversion). The median time to seroconversion in COVID-19 is 12 and 14 days, respectively . This is the primary reason why the Royal College of Pathologists of Australasia (RCPA) explicitly advises AGAINST the use of COVID-19 IgM/IgG rapid blood tests to screen for early infection .
Notably, the first and largest study to date of rapid point-of care serological testing for COVID-19, on which most commercial rapid blood test kits are based, was conducted 8-33 days AFTER the onset of symptoms in PCR-confirmed positive controls . The reported sensitivity of the rapid blood test (i.e. the proportion of confirmed positive COVID-19 cases correctly detected by the rapid blood test) in this setting was 89%. It must be recognised that this result only applies to this setting and population (people who have been symptomatic for at least 1 week, if not more, with time for seroconversion).
The sensitivity of the rapid blood test will be much lower when the test is used in people with new onset of symptoms, and possibly lower yet in asymptomatic people. In one study, the rapid blood test was only able to correctly detect 1 out of 12 PCR-confirmed positive COVID-19 cases at the time of presentation to the emergency department, and 7 out of 38 hospitalised cases at 1 week after initial diagnosis – i.e. it missed >80% of cases in relatively early stages of infection .
The rapid blood test introduced by Emirates will fail to detect early stage infections, especially prior to and within the first week of symptom onset, when an infected person is at greatest risk of spreading SARS-CoV-2 to others. Infectiousness of COVID-19 has been estimated to begin 2 days before symptom onset, peak at 1 day before symptom onset and decline