Medical experts in March first flagged that Covid-19 increases the likelihood of blood clots in patients.

The formation of these blood clots can be dangerous as they can travel to the brain or heart, causing a stroke or heart attack.

Neurologists at University College London studied six Covid-19 patients who had acute stroke as a result of a large arterial blockage.

In five of the cases, the stroke occurred more than a week after the patients showed Covid-19 symptoms such as headache, cough and fever. One patient was pre-symptomatic when the stroke happened.

The researchers found all six patients had markedly raised blood levels of a protein fragment called D-dimer, which is associated with abnormal clotting.

The findings suggested that early testing for D-dimer could enable doctors to prescribe blood-thinning drugs to people at risk, reducing the chance of stroke or harmful clotting elsewhere in the body.

Professor Tan Huay Cheem, chairman of the Singapore Heart Foundation (SHF), said coronaviruses are well known to increase blood coagulability. This means blood clots tend to form more easily within the circulatory system, with serious complications.

A blood clot can form in an artery or vein in any part of the body. It can travel to other parts of the body. If it lodges in the heart or brain, blocking blood flow to these organs, it can result in a heart attack or stroke.

In deep vein thrombosis, a clot forms in a vein deep in the body, often in the leg. If the clot travels to the lung, it could cause a pulmonary embolism - blocking of a major blood vessel - and gangrenous toes, said Prof Tan, said. "It is important to monitor Covid-19 patients for such potential complications. Blood thinning medicines may be selectively used in high risk patients to prevent such clot formation."

Dr Mak Koon Hou, SHF assistant honorary secretary, said patients with severe illness, particularly those who are immobilised, are more likely to have an increased risk of clot formation.

"The risk is higher for those with more risk factors such as diabetes and heart disease. What is surprising is the extent of the problem and degree of clot formation, particularly for those who are not apparently at high risk of clot formation.

"Recognising this adverse manifestation of Covid-19 can lead to an earlier institution of clot prevention," added Dr Mak.