Skip to main content



Covid-19-induced blood clots: What to know if you have diabetes, hypertension or other health issues

SINGAPORE — As doctors and researchers race to learn more about Covid-19, mounting evidence suggests that the disease is more than just a respiratory condition.

An illustration of a blood clot in a blood vessel. Some people with pre-existing chronic diseases such as diabetes or who are obese are more likely to require intensive care treatment and tend to have worse outcomes when they get Covid-19.

An illustration of a blood clot in a blood vessel. Some people with pre-existing chronic diseases such as diabetes or who are obese are more likely to require intensive care treatment and tend to have worse outcomes when they get Covid-19.

Follow us on Instagram and Tiktok, and join our Telegram channel for the latest updates.

SINGAPORE — As doctors and researchers race to learn more about Covid-19, mounting evidence suggests that the disease is more than just a respiratory condition.

Dr Shawn Vasoo, clinical director of National Centre for Infectious Diseases (NCID), said that Covid-19 has been associated with a “hypercoagulable” state, which refers to the tendency to form blood clots.

Blood clotting is the body’s mechanism to stop bleeding after an injury. However, clots that are formed can be life-threatening if they travel and affect blood flow to organs such as the lung, heart or brain and cause a stroke or heart attack, for example. 

Dr Vasoo said that blood complications certainly make it more challenging to manage Covid-19.

While excessive blood clotting has been seen in young and previously healthy people diagnosed with Covid-19, reports suggest that those living with pre-existing chronic health conditions that affect the blood vessels — such as diabetes, hypertension and a history of venous thromboembolism (blood clots in the veins) — may be at an even higher risk.

In patients infected with the Sars-CoV-2 coronavirus, clots in the blood vessels in the legs, organs such as the lungs, kidneys and heart and even fingertips have been reported.

In Singapore, the phenomenon has been observed in critically ill Covid-19 patients, too.

TODAY asked the medical experts to shed light on the potentially deadly Covid-19 complication, and how people living with pre-existing conditions can protect themselves.


Dr Yap Eng Soo, senior consultant at the department of haematology-oncology at the National University Cancer Institute, Singapore, said that several overseas papers on Covid-19 patients who were admitted to intensive care units found that 14 to 46 per cent of them get blood clots.

The complication has also been observed in some patients with less severe symptoms. Dr Yap said that a small number of studies done on Covid-19 patients admitted to general wards overseas found that 1 to 4 per cent of them developed blood clots.

However, he pointed out that the data is preliminary and the majority of the studies are from Caucasian patients.

“Caucasians are known to have a higher risk of blood clots compared to Chinese, Malays and South Indians,” he said. 

Having said that, Dr Vasoo from NCID said that some critically ill Covid-19 patients in Singapore have had such complications. Dr Yap said that he himself has encountered five patients who developed blood clots.


As with many aspects of Covid-19, the exact mechanism causing the blood clots in some patients is still a mystery.

Dr Vasoo said that blood clots have been reported with other viral infections such as in patients with the human immunodeficiency virus (HIV) or those infected with cytomegalovirus, a common virus related to the viruses that cause chickenpox and herpes.

Unlike these other viral infections though, the mechanism causing blood clots in Covid-19 patients appears more pronounced, he added.

A theory is that the coronavirus damages the lining of the blood vessels and also increases blood clotting protein levels, thus leading to the formation of blood clots, Dr Yap said. He added that being hospitalised and less able to move around also increase the risk of blood clots forming.

Dr Sriram Narayanan, senior consultant and a vascular and endovascular surgeon at The Harley Street Heart and Vascular Centre, said: “The virus appears to have a particular affinity for infecting the endothelium (the inside lining of the blood vessels). We also see that the veins are significantly more affected than arteries, but again, we don’t yet know why.”


Even without Covid-19, Dr Sriram said that some people with pre-existing chronic diseases are already at an increased risk of blood clot formation.

They include people who are obese, have diabetes and have been diagnosed with venous thromboembolism (VTE), a condition where a blood clot typically forms in the deep veins of the legs or groin.

These patients are more likely to require intensive care treatment and tend to have worse outcomes when they get Covid-19, Dr Sriram added

“We don’t really know why yet. It may be that their pre-existing conditions already cause inflammation of the blood vessels.”

Dr Vasoo said that in general, people with diabetes and hypertension have higher rates of atherosclerosis — the buildup of fats, cholesterol and other substances in and on artery walls — and this can narrow blood vessels and cause abnormalities in the vessels that predispose them to clots.

Dr Yap said: “It is postulated that the linings of their blood vessels are damaged, and this makes it easier for blood clots to form.”

Dr Sriram warned that reduced mobility and lack of exercise due to the need to stay home to limit the spread of the community spread of the disease may also increase the risk of blood clotting in this group. 

Safe distancing and self-isolation measures can also lead to frequent snacking at home, causing weight gain, which is another risk factor.

“I advise everyone, whether in a risk group for VTE or not, to make sure they stay hydrated, eat more healthily and move around regularly even if it is within their homes,” he added.

Being physically active can help improve blood circulation. Dehydration is a risk factor for the formation of blood clots.


To prevent complications from blood clots in Covid-19 patients here, Dr Vasoo and Dr Yap said that those who are hospitalised will be assessed.

In critically ill patients, this assessment may involve the use of laboratory markers, in particular a test called D-dimer, that may signal the presence of blood clots, Dr Vasoo said.

Dr Yap said: “If the (hospital patients) are assessed and deemed to be at high risk of developing blood clots (due to risk factors including immobility, active cancer, previous history of VTE or recent surgery), then doctors will start them on blood thinners.”

For Covid-19 patients who are severely ill and admitted to receive intensive care, they would all be started on preventive blood thinners, he added.  

Dr Vasoo said that a typical example of such a class of medications are low-molecular weight heparin, which are given as an injection.

If a patient is suspected to have a blood clot, certain imaging studies such as an ultrasound of the blood vessels or a CT scan of the blood vessels in the lungs may be carried out, he added.

Dr Sriram said that some countries are reporting successful outcomes from early and preventative use of anticoagulants such as heparin and direct-acting oral anticoagulants such as rivaroxaban.

Direct-acting oral anticoagulants are a group of new anticoagulants that inhibit the blood's ability to form blood clots.

Although they are commonly called blood thinners, Dr Sriram explained that anticoagulants do not actually “thin” the blood but make it less prone to clotting.

They are used to treat or prevent blood clots and embolism, which refer to clots that have detached from the lining of the blood vessel and are travelling in the blood stream, he said.

In patients who have recovered from Covid-19 and are discharged from the hospital, he added that studies have begun to continue treatment for them with lower doses of anticoagulants, although it is presently unclear how long this treatment should continue. 

It is not a straightforward course of action to just give all Covid-19 patients in hospitals preventive blood thinners.

Dr Yap said that it is a controversial topic. In the West, it is recommended that all hospitalised Covid-19 patients are given preventive doses of blood thinning medications, regardless of their underlying medical condition.

Pointing out the risk of bleeding when blood thinners are used, Dr Yap said: “This risk must be balanced against the benefits of preventing blood clots.

“Although there appears to be a high rate of development of blood clots in Covid-19, we are still unsure if this is true for the (Singapore) population and we do not want to harm patients by causing them to bleed with blood thinners.”  

Therefore, hospitalised Covid-19 patients here are started on preventive blood thinning medications only after they are assessed and deemed to be at high risk of developing blood clots.

Dr Yap said that Covid-19 patients with underlying diabetes and hypertension who are not hospitalised, such as those with mild symptoms at community care facilities, do not need to start blood thinning medications.

They should be well-hydrated and move around more.


For patients with a history of VTE who are already on blood thinning medications, Dr Yap said that their risk would be low.

“However, if they have stopped taking their medication, then their risk would be very high.

“Similarly, for those with hypertension and diabetes, if their disease is poorly controlled, it would increase their risk,” he said.

Dr Yap said that even without Covid-19, patients with diabetes and hypertension should comply with instructions on taking their medications, to ensure good control of their conditions.

“This is not merely to reduce the risk of blood clots should they contract Covid-19, but also to prevent general complications from a worsening of their chronic conditions, as a result of non-compliance to medication,” he said.

Dr Sriram advised patients with chronic conditions to keep to their doctor appointments, unless specifically advised by their doctor or clinic not to do so.

From June 2, hospital-based specialist outpatient services, medical procedures and allied health services for patients with higher needs have resumed after the containment measures for the past two months were lifted.

Chronic disease management have also resumed for patients, prioritised by healthcare providers based on medical necessity and available capacity.

Some clinics offer alternative options such as telemedicine and teleconsultations to patients who have less urgent conditions or require prescription renewals, Dr Sriram added.

However, he advised seeking urgent medical attention if there are the following symptoms:

  • chest pain or palpitations

  • acute limb pain

  • skin swelling or discolouration

  • breathlessness

  • facial drooping or slurred speech

Related topics

Covid-19 coronavirus blood clots NCID diabetes hypertension

Read more of the latest in



Stay in the know. Anytime. Anywhere.

Subscribe to get daily news updates, insights and must reads delivered straight to your inbox.

By clicking subscribe, I agree for my personal data to be used to send me TODAY newsletters, promotional offers and for research and analysis.