COVID-19 briefing 03/06/20 – a guide to testing

stethoscope and blood sample tube

Researchers from across BHP are collaborating to analyse and summarise the latest COVID-19 literature to help inform clinical colleagues, and today’s update explains the differences between the two main types of coronavirus test, including accuracy rates, next steps, and what the tests don’t tell us. 

The infographic can be downloaded as follows:


The Antigen PCR test looks for the presence of COVID-19 virus in a patient, and is done via nose and/or throat swab. The ideal time to test is between two days before symptoms appear, and seven days afterwards.

If positive, continue PPE use and follow Trust policies in terms of patient pathway, if admitted.

However, the test does not tell us how ill the patient will become, or when they contracted the virus.

The test is accurate in 75% of cases, so if symptomatic patients return a negative result, re-test them in 2-3 days. If there are repeated negatives with a high suspicion of coronavirus, consider the antibody test.

The antibody test tells us whether a patient has been previously infected with the virus, and whether they have had an immune response.

The best time to test is at least 14 days after symptoms first appeared, and is done via blood test.

A positive result only tells us that the patient has indeed had coronavirus – it doesn’t tell us whether they are ‘immune’ and will not become ill if re-infected. It also doesn’t tell us whether they can act as a carrier to the virus and thus infect others.

The test is accurate in 80% of mild disease cases, and accuracy increases with severe illness. False negatives can also occur.

As a result, all social distancing, handwashing and PPE should still be used – the patient may still be susceptible to the virus.