As England moves away from confirmatory PCR testing following a positive lateral flow test in the absence of COVID-19 symptoms, it’s a good time to look at what these two different testing strategies can offer us. There’s an excellent short review in NEJM combined with a case study to help illustrate the impact of pre-test probability plays out. Both lateral flow testing and PCR testing have their place, and in some ways lateral flow testing is a better correlate for infectivity (as well as being cheaper and easier!).
The image below from the article provides a superb overview of how levels of virus, antigen, and antibodies change over time. (PCR detects viral RNA, lateral flow tests detect viral antigen, and blood tests detect antibodies.)
The period during which culturable virus can be detected seems to be a pretty good correlate with the period during which a patient is infectious. This is typically for a couple of days before the onset of symptoms (if there are any) until about 10 days after symptoms. As you can see from the chart, lateral flow testing is a better fit for this period than PCR testing, although won’t detect early infectivity. For me, this means that if a lateral flow test is positive, then the person is likely to be shedding culturable virus and to be infectious (to some degree). If a lateral flow is negative but a PCR is positive, you need to know when you are at the start of the end of the disease to inform decision making about infectivity (repeated PCR tests, an evaluation of CT values, and antibody testing can help here). An assessment of ‘infectiousness’ is more than what our diagnostic tests are telling us – we need to consider patient and environmental factors too. But both PCR and lateral flow testing are useful tools.