Tiny clots starving tissues of oxygen, triggering the fog
In the long aftermath of Covid infection, hundreds of thousands of people have found themselves stranded in a fog they cannot name or escape — unable to think, unable to rest, unable to return to themselves. British researchers have now found a molecular trace in the blood that may explain why: elevated clotting proteins, present at the moment of infection, appear to predict who will later lose cognitive clarity and endurance. The discovery, published in Nature Medicine, does not yet offer a cure, but it offers something almost as rare in medicine — a reason, and perhaps a path.
- Two blood proteins — fibrinogen and D-dimer — were found at elevated levels in Covid patients who later developed brain fog, memory loss, and fatigue, suggesting clotting activity may be silently damaging the brain and lungs.
- Tiny clots lodged in lung or brain tissue may be starving the body of oxygen, offering a physical mechanism for neurological symptoms that have long been dismissed or left unexplained.
- Clinical trials are already underway testing whether blood thinners can relieve these symptoms, and the new biomarkers could become a screening tool to identify which patients need early intervention.
- Experts urge caution: the proteins do not account for every long Covid case, treatments must still clear rigorous trials, and a single universal mechanism remains out of reach.
- Over 700,000 new long Covid cases accumulated during the Omicron wave in the UK alone, yet healthcare systems have largely moved on, leaving patients with shrinking support and almost no treatment access.
British researchers have identified a molecular pattern in the blood that may explain one of long Covid's most disabling features: the persistent fog over the mind, the exhaustion that refuses to lift. Examining samples from more than 1,800 hospitalised Covid patients, scientists found that those who later developed lasting cognitive problems were significantly more likely to have had elevated levels of two clotting proteins — fibrinogen and D-dimer — at the time of their initial infection.
The study, published in Nature Medicine, points to two possible mechanisms. Fibrinogen may be forming tiny clots within brain tissue, directly impairing cognitive function, or interfering with how the brain operates at a chemical level. D-dimer, meanwhile, correlated with fatigue and breathing difficulties as well as brain fog, suggesting clots in the lungs that reduce oxygen reaching the brain and body. Max Taquet of the University of Oxford, one of the study's authors, noted that patients with elevated D-dimer faced heightened risk of both neurological and respiratory complications.
The findings carry real clinical promise. Several trials, including one called Stimulate-ICP, are already testing anticoagulants as a treatment for long Covid symptoms. If successful, a simple blood test measuring these proteins could help identify which patients are most likely to benefit from early intervention — a way to direct care toward those who need it most. But researchers caution against overreach: the biomarkers do not capture every at-risk patient, and experts including Dr. Michael Zandi of UCL warn that no single mechanism yet explains the full picture.
The research arrives against a troubling backdrop. More than 700,000 new long Covid cases accumulated in the UK during the Omicron period, even as public health attention moved elsewhere. Co-author Chris Brightling of the University of Leicester expressed frustration that many patients remain without support or treatment, caught between an unresolved illness and a healthcare system stretched thin. The discovery of these blood markers offers a potential path forward — but researchers and clinicians alike warn that knowledge alone, without sustained resources and attention, may not be enough to reach the people still waiting.
British researchers have found a molecular fingerprint in the blood that points toward a physical explanation for one of long Covid's most debilitating symptoms: the fog that settles over the mind, the exhaustion that won't lift, the difficulty thinking clearly months after infection.
The discovery centers on two proteins that signal blood clotting. Scientists examining blood samples from more than 1,800 hospitalized Covid patients noticed something consistent: those who later developed long-lasting cognitive problems were significantly more likely to have elevated levels of fibrinogen or D-dimer at the time of their initial infection. Both proteins are markers of clotting in the body. The work, published in Nature Medicine, suggests that tiny clots—possibly lodged in the lungs or brain itself—may be starving tissues of oxygen and triggering the neurological symptoms that have left hundreds of thousands unable to work or think clearly.
Max Taquet, one of the study's authors at the University of Oxford, outlined two possible mechanisms. Fibrinogen could be forming actual clots within the brain tissue, directly damaging cognitive function. Alternatively, the protein itself might be interfering with how the brain works. D-dimer, the second marker, tells a slightly different story. Elevated levels correlated not just with brain fog but with fatigue and breathing difficulties, suggesting clots in the lungs that reduce oxygen flow to the brain and body alike. "Individuals with elevated D-dimer levels were not only more prone to brain fog but also displayed a heightened risk of respiratory challenges," Taquet said.
The implications could reshape how long Covid is treated. Several clinical trials, including one called Stimulate-ICP, are already testing whether anticoagulants—blood thinners—can ease brain fog and fatigue in long Covid patients. If those trials succeed, a simple blood test measuring fibrinogen and D-dimer levels could become a tool for identifying which patients are most likely to benefit from early treatment. It would be a way to stratify risk, to separate those who need intervention from those who may recover without it.
But experts are cautious about overstating what this discovery means. Danny Altmann, an immunology professor at Imperial College London and a leading long Covid researcher, called the work "an important step forward" in understanding the condition's underlying mechanisms. Yet he and others stressed that the biomarkers won't catch every patient at risk. Dr. Michael Zandi at UCL's Queen Square Institute of Neurology warned that "the associations do not give us yet a clear one-size-fits-all mechanism, and treatments need to pass through rigorous clinical trial first."
The timing of this research arrives as the scale of long Covid's burden becomes clearer. During the period of milder Omicron infections, when public health measures were largely abandoned, more than 700,000 new cases of long Covid accumulated in the UK alone. Yet the healthcare system has largely moved on. Chris Brightling, a respiratory medicine professor at the University of Leicester and co-author of the study, expressed frustration that many patients remain untreated and unsupported. "There are still a lot of patients that are suffering who haven't fully recovered and we don't know how long it will take for them to recover or whether they will need treatment," he said. "Covid is in competition with all the other needs of the healthcare system, meaning there's a risk patients with long Covid will not have the attention that is required."
Altmann echoed the concern, noting that most long Covid patients report dwindling support and almost no access to treatment options. The discovery of these blood protein markers offers a potential pathway forward—a way to identify who needs help and how to help them. But without sustained clinical attention and resources, the finding risks becoming another piece of knowledge that doesn't reach the people who need it most.
Notable Quotes
Individuals with elevated D-dimer levels were not only more prone to brain fog but also displayed a heightened risk of respiratory challenges— Max Taquet, University of Oxford
There are still a lot of patients that are suffering who haven't fully recovered and we don't know how long it will take for them to recover or whether they will need treatment— Chris Brightling, University of Leicester
The Hearth Conversation Another angle on the story
So these proteins—fibrinogen and D-dimer—they're not unique to long Covid, right? They show up in all kinds of clotting situations.
Exactly. They're general markers of clotting activity in the body. What's significant here is that researchers found them elevated in Covid patients who specifically went on to develop brain fog and fatigue, not in those who recovered normally.
And the theory is that tiny clots are physically blocking blood flow to the brain?
That's one theory. But it's more complicated. The clots might be in the lungs, reducing oxygen delivery. Or they could be in the brain itself. Or the proteins themselves might be triggering inflammation or damaging tissue directly. We don't have a single mechanism yet.
Why does that matter for treatment?
Because if you know a patient has elevated D-dimer, you might start them on anticoagulants early, before symptoms get worse. It's preventive. But you also need to know which patients will actually benefit—and that's what the clinical trials are trying to figure out.
What worries you most about this research?
That it becomes another finding that doesn't reach patients. Seven hundred thousand people accumulated long Covid during Omicron. Most of them aren't in clinical trials. Most don't have access to any treatment at all. A blood test is only useful if there's something to do with the result.