London, Sep 1 : Researchers have found that blood clots after a Covid infection may be due to ongoing cognitive problems, commonly referred to as 'brain fog'.  Brain fog is common and debilitating, and can affect the ability to work, yet their biological underpinnings remain unknown. It can affect people's thinking, concentration or memory for at least six months. Researchers from the University of Oxford, UK said that high levels of two proteins at the time of Covid-19 have been found in patients who later experienced cognitive problems, including 'brain fog'. The study gave a major clue as to one cause of their symptoms: blood clots, said researchers from the University of Oxford, UK. The team looked at blood tests from 1,837 people who had been hospitalised with Covid to find potential proteins (biomarkers) associated with subsequent cognitive problems, with symptoms including serious and persistent problems with thinking, concentration and memory. Their memory was assessed at six and 12 months after hospitalisation using both a formal test and by asking them their own subjective view about their memory. In a new paper published in Nature Medicine, they identified two separate profiles of biomarkers. The first was having a high level of a protein called fibrinogen, and the second was a raised level of a protein fragment called D-dimer. 'Both fibrinogen and D-dimer are involved in blood clotting, and so the results support the hypothesis 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 and the problems in the brain might be due to lack of oxygen,' Dr Max Taquet and colleagues from the varsity. '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,' Taquet said. Other aspects of the profiles suggested they are likely to reflect blood clots. The main findings were replicated using electronic health records ina separate population. Professor Paul Harrison, from the University 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 ultimate goal is to be able to prevent and reverse the cognitive problems seen in some people after Covid-19 infection. Although our results are a significant advance in understanding the basis of these symptoms, more research is needed into the causes and effects before we propose and test interventions,' Taquet said. /IANS