Researchers from the RCSI University of Medicine and Health Sciences, in Dublin, followed up research on dangerous clotting in patients with severe COVID-19 to assess whether clotting could also be a factor in long COVID.
Studies have suggested that more than 2m people in England may have experienced long COVID - equivalent to around 400 patients per average GP practice.
In the latest study, published in the Journal of Thrombosis and Haemostasis, researchers examined 50 patients with symptoms of long COVID syndrome to look at whether abnormal blood clotting was involved.
The researchers found that clotting markers were 'significantly elevated in the blood of patients with long COVID syndrome compared with healthy controls'.
While the clotting markers were higher in patients hospitalised with their initial COVID-19 infection, patients who managed their illness at home also had persistently high clotting markers, the study found.
Higher clotting was also found to be 'directly related to other symptoms of long COVID syndrome, such as reduced physical fitness and fatigue' - and the researchers found that while inflammation markers had returned to normal levels, increased clotting potential remained present in long COVID patients.
Long COVID symptoms
Lead author Dr Helen Fogarty said: 'Because clotting markers were elevated while inflammation markers had returned to normal, our results suggest that the clotting system may be involved in the root cause of long COVID syndrome.'
Director of the Irish Centre for Vascular Biology at the RCSI Professor James O’Donnell added: 'Understanding the root cause of a disease is the first step toward developing effective treatments.
'Millions of people are already dealing with the symptoms of long COVID syndrome, and more people will develop long COVID as the infections among the unvaccinated continue to occur. It is imperative that we continue to study this condition and develop effective treatments.'