It’s been reported that abnormal blood clotting may be contributing to the death of people with severe COVID-19. But what is known about this so far?

People with severe COVID-19 appear to have high levels of D-dimers in the blood in studies that have reached the news. D-dimers are strands of protein that come from blood clots. These could indicate that small clots are building up throughout the bloodstream which in turn could block small blood vessels in the lungs and other organs. It is thought this might contribute to low blood oxygen levels and perhaps explain other features of severe coronavirus disease.

In a study at one Irish hospital, every person admitted with COVID-19 was routinely tested for clotting factors. The study found that those who needed intensive care had abnormal clotting, including high D-dimer levels. On average these were at higher levels than found in those who didn’t need intensive care.

These interesting observations can’t yet tell us which clotting abnormalities are typical of COVID, and how traditional advice on the use of anti-clotting drug or combination might be changed if at all. Risks and benefits will both need to be considered; carefully designed randomised trials are underway and there is already provisional guidance for doctors managing these conditions.

Where did the story come from?

Science News reported on the study, which was led by researchers from the Royal College of Surgeons in Ireland and St James’ Hospital, Trinity College Dublin, and published last month in the British Journal of Haematology.

What is the basis for the claim?

Several studies from China also show that patients severely ill with COVID-19 often have abnormal blood clotting, including raised D-dimer and other blood markers. Post-mortem studies also found signs of tiny blood clots throughout the lung blood vessels in some people who died. Clotting disorders like deep vein thrombosis are common in immobilised people and is already known to differ between ethnic populations, the current study aimed to see whether the same was seen in a white European population.

Researchers reviewed all 83 patients admitted with COVID-19 to a Dublin hospital between 13th March and 10th April 2020. Patients were aged 64 years on average, 80% of white ethnicity and 80% had underlying illnesses. Two-thirds had raised D-dimer levels at the time of admission. All patients were given injections with the anti-clotting drug low molecular weight heparin for prevention of deep vein thrombosis.

About 60% recovered and were discharged without ever needing intensive care, 28% were admitted to intensive care, and 16% died. Compared with those who recovered, people who needed intensive care had higher D-dimer levels at admission, which remained high in the coming days.

It seems experts can’t draw definite conclusions on treatments yet. Randomised controlled trials are planned to look into this.

What do trusted sources say?

The International Society on Thrombosis and Haemostasis has issued guidance on regulation and management of clotting disorders in COVID-19. This highlights the evidence linking raised D-dimer levels with increased mortality. Measurement of D-dimer and other blood clotting markers is recommended on admission, with continued daily monitoring to assess the need for more intensive management.

‘Markedly raised D-dimer’ is highlighted as a risk factor and the authors of this guideline note that a cut-off level is not yet established. They suggest that a 3-4 fold increase may be considered significant.

Analysis by EIU Healthcare, supported by Reckitt Benckiser



  1. Fogarty H, Townsend L, et al. COVID‐19 Coagulopathy in Caucasian patients. British journal of haematology. 2020 Apr 2

Reading list

  1. Thachil J, Tang N, et al. ISTH interim guidance on recognition and management of coagulopathy in COVID‐19. Journal of Thrombosis and Haemostasis. 2020 May;18(5):1023-6.
  2. Tang N et al. Abnormal coagulation parameters are associated with poor prognosis in patients with novel coronavirus pneumonia. J Thromb Haemost. 2020;18(4):844–7.
  3. Wang D, Hu B, et al. Clinical characteristics of 138 hospitalized patients with 2019 novel coronavirus–infected pneumonia in Wuhan, China. Jama. 2020 Mar 17;323(11):1061-9.
  4. Bikdeli B, Madhavan MV, et al. COVID-19 and thrombotic or thromboembolic disease: implications for prevention, antithrombotic therapy, and follow-up. J Am Coll Cardiol. 2020.