Boris Johnson is likely to undergo tests to check his oxygen levels, white blood cell count, and liver and kidney function before he is released from hospital following his admission over coronavirus, according to doctors.
The Prime Minister is also likely to undergo an electrocardiogram to check his heart.
Mr Johnson tested positive for the virus 10 days ago, but spent Sunday night in hospital as his symptoms "continue to persist".
GP Dr Sarah Jarvis told the BBC Mr Johnson would also have his chest X-rayed and lungs scanned, particularly if he was found to be struggling for breath.
Dr Jarvis said around 80% of people who contract the virus only suffer from mild symptoms.
While the remaining 20% could suffer moderate to severe illness.
Dr Jarvis said the Prime Minister continues to lead Government so "probably has moderate disease".
She added: "The main focus though is going to be on his lungs."
Dr Jarvis continued: "The majority of people (with moderate or severe symptoms) are going to progress to have inflammation of the lungs and that inflammation can result in damage to lung tissue but also importantly can prevent oxygen being transferred into the bloodstream."
She said it would probably take a few hours for the tests to be conducted and the scans analysed by a consultant.
Dr Bharat Pankhania, senior clinical lecturer, University of Exeter Medical School, said the PM's doctors would be be playing it safe, because pneumonia can take hold very quickly.
Dr Pankhania said: "If a patient is developing pneumonia, it can get progressively worse very quickly and hence early admission upon the first signs of difficulty with breathing are very important".
Those with moderate or severe symptoms tend to become breathless very quickly and suffer from exhaustion, Dr Jarvis warned, and said it would be difficult for the Prime Minister to carry on working as a result.
"At the severe stage, they may become more breathless even at rest and find it difficult to speak more than a couple of sentences - those are the levels at which you should be calling for emergency help," she said.
"Given that we have been told that at this stage this is routine, his brain certainly sounds like it is getting enough oxygen, but it is clearly exhausting."
She said it was possible Mr Johnson had received a larger "dose" of the virus than Health Secretary Matt Hancock.
Mr Hancock was diagnosed at the same time and has since recovered, returning to host the Government's daily update on the outbreak.
Dr Jarvis said: "That may be why someone who lives in a house with someone who has been affected and is constantly exposed to the virus is more likely to get severe illness.
"Likewise there is something called anti-body dependent enhancement where your immune system may go into overdrive and that may account for some people getting very severe inflammation."
Coronavirus: Everything you need to know