The hunt is on for a coronavirus antibody test that tells people whether they have had Covid-19 and are immune.

But before a test can be rolled out to the public, authorities have to make sure it is accurate enough.

A number of antibody tests, called serology tests, have already failed to meet requirements, and laboratories across the UK are continuing to evaluate others.

Health Secretary Matt Hancock said around 500 highly accurate tests were being performed each day by Porton Down, the military’s science laboratory.

– So how do the tests work?

They look similar to a pregnancy test and would involve a single prick blood test to analyse whether there are any Covid-19 antibodies present.

– What are some of the companies involved in producing tests, and who is testing them?

Liverpool School of Tropical Medicine (LSTM) is validating a test created by Bedfordshire-based Mologic.

The Covid-19 tests will be jointly manufactured in the UK at Mologic and in Senegal at Institut Pasteur de Dakar.

Once ready, the device will allow users to test for exposure to the virus at home, in the community, or in the clinic, providing a result within 10 minutes.

A test by biotechnology company Attomarker is being tested at St Thomas’s Hospital in London.

Derby firm SureScreen has created a finger-prick test which they say takes 10 minutes to return a result and is 98% accurate.

Professor Sharon Peacock, director of the National Infection Service at Public Health England (PHE) has said some tests are being evaluated in an Oxford laboratory.

– When can people be tested for antibodies?

Prof John Newton, the director of public health improvement for PHE, has said antibody tests work best 28 days after an infection.

Dr James Gill, locum GP and honorary clinical lecturer, Warwick Medical School, explained: “The antibody IgM is the body’s first response to an infection, normally within five to 10 days of an infection taking hold, peaking at 21 days after the infection.

“That time frame is crucial, if you have just developed symptoms you think are coronavirus, it will take approximately a week for your body to raise IgM antibodies to the virus.

“Now whilst with coronavirus we do have evidence of IgM being presenting the blood within one day of symptoms, but that isn’t going to be a reliable test at that stage, as the likely won’t be large amounts of IgM to detect.”

He added that the other antibody, IgG, is also detected by the coronavirus tests.

Dr Gill said: “IgG shows that someone has had the virus and is now protected from the virus, this can be detected in a patient’s blood 10-14 days after infection.

“Perhaps one way of looking at this is the IgM is the fast reaction force, whereas IgG is the main, slower, but more powerful army, which acts to keep the body safe afterwards as well.”

A huge volume of IgG is raised to combat the virus within 24-48 hours of re-exposure and will then hopefully prevent a new infection.

Dr Gill said: “Thus if a patient is found to have IgM, that suggests that they may be within the first week of an infection.

“If they have BOTH IgM and IgG, that suggests they will be within the first month of infection, and should hopefully be protected from repeat infection.”

– Have any antibody tests been approved anywhere?

This week the US Food and Drug Administration issued its first emergency use authorisation from a manufacturer called Cellex Inc.

It requires blood to be collected through a vein, and the test itself can only be performed in a certified lab.

– What could the availability of antibody tests mean for the UK lockdown?

Mr Hancock told BBC’s Today Programme: “The antibody tests are important, the scientists say they are more important later as we are trying to come out of the extraordinary social distancing measures, the lockdown, because it is on the way out that you then know how many people have the immunity.

“At the moment, the most important thing for getting out of this as soon as possible is for people to follow the social distancing rules.”

But he added the lifting of restrictions was not dependant on the availability of a test that could be used by the public at large.