A top NHS official has urged caution over the use of antibody tests, warning anyone who has one not to change their social distancing behaviours.

NHS medical director Professor Stephen Powis said the tests show whether or not someone has had Covid-19 but it is not yet clear whether this means people will not contract the virus again.

He would not be drawn on whether people would be given access to antibody tests for free on the NHS in the future.

It comes as Superdrug became the first high street retailer to offer a DIY kit to test for Covid-19 antibodies.

The testing kit costs £69 and users need to take a blood sample at home which is sent off to an accredited laboratory for testing.

Following a question on antibody tests from the PA news agency at the daily Downing Street press briefing, Prof Powis said: “Public Health England have been evaluating the new antibody tests – the commercial tests that are becoming available – and I have most confidence in that evaluation process, because I think that gives the stamp that we need in order to roll these types on throughout the NHS.

“So as those types are evaluated and become available they will be rolled out through health and social care settings.

“Initially their use will be in those settings, and also for surveillance within the community so that we get some information on how many people in the population may have been infected by the virus.

“I would caution against using any tests that might be made available without knowing quite how good those tests are. Public Health England as I say is evaluating them for the NHS so I would caution people against being tempted to have those tests.”

He added: “I think one other point to make about the antibody test, the antibody test shows you that you have had the virus. Once you have the virus the body’s immune system develops antibodies against it and it is those antibodies that are detected typically a number of weeks after you’ve had the virus so it tells you you’ve had it.

“What we don’t absolutely know at the moment is whether having antibodies, and having the antibodies that are tested in those tests, means that you won’t get the virus again.

Any antibody test at present can only provide a partial picture.

“I wouldn’t want people to think just because you test positive for the antibody that it necessarily means that you can do something different in terms of social distancing or the way you behave.

“Because until we are absolutely sure about the relationship between the positive antibody tests and immunity, I think we as scientists would say we need to tread cautiously.”

When asked whether everyone in the country will get access to an antibody test via the NHS, he said: “When I say it is the early phase of these tests and where we will use them first, is in health, and probably social care settings for patients, obviously, but also for staff in those settings where it is most important that we understand about infection.”

Superdrug said it was “confident” in the accuracy and reliability of the test.

People who use the test will need to take a finger prick blood sample at home and then post the sample to a lab.

Results are posted through Superdrug’s Online Doctor portal 24 hours after reaching the lab.

It said the test detects the IgG, which is the protein that develops after infection. If positive, it means that the person tested had the virus at some point.

Those who have recently developed symptoms should not take the test until at least 14 days afterwards, as the antibodies may not be apparent before that point, it added.

Superdrug said the test has a sensitivity of 97.5%, which means that it will detect positive antibodies 97.5% of the time.

It has a specificity of 100%, which means a positive result is specific to the SARS-CoV-2 virus – the novel coronavirus which causes the Covid-19 disease.

But the science behind the level of protection provided by antibodies is still in its early stages – and the virus has not been around long enough to know what level of protection any antibodies give.

Commenting on the availability of the test from Superdrug, as well as one released by healthcare company Babylon for the same price, Professor Gino Martini, chief scientific officer at the Royal Pharmaceutical Society, added: “Any antibody test at present can only provide a partial picture.

“The real issue is that no-one knows the level of immunity that is conferred by having antibodies to coronavirus, how long it might last, and if you can become re-infected. We need much more information and data on immunity before we can understand the importance of having antibodies to the virus.”