Mechanism · Coagulation
Fibrinaloid microclots
One of the most discussed and most disputed ideas in long COVID is that tiny abnormal blood clots, too small to see on ordinary scans, clog the smallest vessels and starve tissue of oxygen. The underlying science is real, and so is the disagreement about what it means. This page lays out what is shown, what is proposed, and where serious researchers still argue.
Short version: in long COVID the clotting protein fibrin can misfold into clumps that resist the body's normal clot-clearing. Whether these microclots are a main cause of symptoms, a side effect, or hard to measure reliably is genuinely unsettled, and treatments based on them are not proven.
What a microclot is
When you bleed, the protein fibrin forms a mesh that holds a clot together, and once the job is done your body dissolves that mesh in a process called fibrinolysis. In long COVID, researchers have described fibrin folding into an abnormal, amyloid form: clumped, misshapen, and studded with other trapped proteins. These are the fibrinaloid microclots. The amyloid structure is the key feature, because it makes them resist the normal breakdown that should clear them away.1 Spike protein on its own, with no live virus, can push fibrin into this state in the laboratory.4
Why they might matter
The proposal is straightforward. Microclots too small to see on any standard scan could lodge in the smallest vessels, limit how much oxygen reaches tissue, and stir up inflammation. Because they resist breakdown, they would linger, and the injury they cause to the vessel lining would generate more clotting, a loop that sustains itself.1, 2
low-moderate contested
How they are detected
The clots are made visible by staining plasma with dyes that bind amyloid, such as thioflavin T, and counting them under a fluorescence microscope, with newer work using flow cytometry to count them faster and more objectively.3 As a group, people with long COVID show higher microclot counts than controls, but the ranges overlap widely, and there is no standardized, widely validated assay yet.2, 3
measurement is a research tool
Where serious researchers disagree
This is where honesty matters. The team that described these clots argues they sit on the disease pathway. Other researchers question whether the detection is reproducible across labs, whether the clots cause symptoms or are a downstream marker of inflammation, and whether the treatments built on the idea, intensive anticoagulation or blood-filtering apheresis, have ever been shown to help in a properly controlled trial. A published exchange between these groups, including a Cochrane review of apheresis, lays the dispute out in the open.2 The right reading today is that the phenomenon is real and interesting, and its role and usefulness are unsettled.
genuine scientific dispute
What this means for treatment
No microclot-targeting therapy is proven for long COVID. Strong anticoagulation and apheresis carry real risks, bleeding chief among them, and are not established treatments. They belong in monitored trials, not in self-directed care. If you have read about triple anticoagulation, that is the honest status: promising to some researchers, unproven, and not without danger.
What we don't know
Honest about the edges of the evidence. These are open questions, not settled answers.
- Whether microclots are a primary cause of symptoms, a downstream consequence of inflammation and clotting, or both.
- Whether the detection methods are reproducible enough across laboratories to serve as a diagnostic test.
- Whether removing or preventing microclots changes how people feel, which no adequately controlled trial has yet shown.
- How microclot counts relate, if at all, to the severity of any one person's symptoms.
- Why some studies find a clear separation from healthy controls and others find large overlap.
References
Every reference is free to read in full.
- Kell DB, Laubscher GJ, Pretorius E. A central role for amyloid fibrin microclots in long COVID/PASC. Biochemical Journal, 2022.
- Fibrinaloid microclots in long COVID: assessing the actual evidence properly. Research and Practice in Thrombosis and Haemostasis, 2024.
- Increased fibrinaloid microclot counts in platelet-poor plasma are associated with long COVID. medRxiv, 2024.
- SARS-CoV-2 spike protein amyloid fibrils impair fibrin formation and fibrinolysis. 2025.