High levels of two proteins have been found in patients who got Covid then later suffered cognitive problems, including ‘brain fog’ – revealing a clue as to one cause of their symptoms: blood clots.

Dr Max Taquet and colleagues from Oxford and Leicester universities looked at blood tests from 1,837 people who were hospitalised with Covid-19 to find biomarkers (proteins) linked to cognitive problems, including serious and persistent problems with thinking, focus and memory.

They identified two – the first was a protein called fibrinogen, and the second a protein fragment called D-dimer. High levels of both are likely to indicate blood clots. Dr Taquet says: “Both fibrinogen and D-dimer are involved in blood clotting, and so the results support the possibility that blood clots are a cause of post-Covid cognitive problems.

“Fibrinogen may be directly acting on the brain and its blood vessels, whereas D-dimer often reflects blood clots in the lungs.

“The problems in the brain therefore might be due to lack of oxygen. In line with this possibility, people who had high levels of D-dimer were not only at a higher risk of brain fog, but also at a higher risk of respiratory problems. The ultimate goal is to be able to prevent and reverse the cognitive problems seen in some people after Covid-19 infection.”

Professor Paul Harrison from the University of Oxford’s Department of Psychiatry, who supervised the study, said: “Identifying predictors and possible mechanisms is a key step in understanding post-Covid brain fog. This study provides some significant clues.”

The participants involved in this research were part of the Post-Hospitalisation Covid-19 Study (PHOSP), led by Leicester University. Their memory was assessed at six and 12 months after hospitalisation using both a formal test and personal questions about their memory.

One participant in the PHOSP trial, Geoffrey Hodgson, said: “Brain fog has been a symptom of Long Covid that I’ve really struggled with. I found things I used to do easily much more difficult, such as the software I used for work. Taking part in the research and hearing the findings around cognitive difficulties has really helped me. It’s given me an understanding I didn’t have and that definitely makes things easier.”

Dr Rachael Evans of Leicester University added: “Large detailed studies such as PHOSP are vital if we’re to understand the causes of the often debilitating symptoms associated with Long Covid and to urgently find treatments.”