The test on leaving hospital was standard medical practice, the spokesperson said, according to the Press Association news agency.
Johnson spent a week in hospital including several days in intensive care after contracting the virus, and said he owed his life to the medical staff who treated him.
With the number of coronavirus deaths in Britain hitting 11,329 – a rise of 717 compared to the previous day – the Ministry of Defence said it was deploying 200 military personnel across the country in support of ambulance services, PA cited the prime minister’s office as saying.
Downing Street said the soldiers would carry out a variety of roles including driving vehicles, taking calls, manning response centres and supporting paramedic crews.
Interim chief medical officer Jason Leitch meanwhile urged families of those who have died during the outbreak not to delay funerals, in a bid to stop a backlog.
Excluding Northern Ireland, 88,621 people have tested positive for the coronavirus in Britain as of Monday morning.