What Are Microclots? A Plain-Language Guide to Amyloid Microclots

"Microclots" went from a niche research term to a common Long COVID search in a couple of years. This guide explains what they are in plain language, how they form, which symptoms researchers connect to them, and what's still unproven. It's written for patients, but clinicians new to the topic should find it a quick orientation.

What are microclots?

A microclot is a clot too small to see without a microscope. Your body forms and clears tiny clots all the time, and that's normal. When researchers talk about microclots in Long COVID, though, they usually mean something more specific: fibrin amyloid microclots, also called fibrinaloid or amyloid microclots.

These are particles of fibrin, the protein that forms the mesh of a normal clot, that have folded into an amyloid-like structure. In a 2022 review, Pretorius, Kell and colleagues described how fibrinogen can clot into this anomalous amyloid form, which is relatively resistant to the body's clot-dissolving system. The research behind those findings is summarized in amyloid microclots: what the research shows.

How do microclots form?

Normal clotting is a two-step cycle. Thrombin turns soluble fibrinogen into fibrin strands that knit together, then plasmin breaks the mesh down once the job's done. Microclots form when that cycle goes wrong in two ways at once.

The fibrin misfolds. Under inflammatory conditions, fibrin can take on an amyloid shape. In the lab, adding isolated SARS-CoV-2 spike protein S1 to healthy plasma changed the structure of the resulting clots and made them harder to digest. That was a test-tube experiment, not a measurement in patients, but it suggested one route.

The breakdown stalls. A 2021 study of Long COVID plasma found the microclots contained raised levels of α2-antiplasmin, a natural brake on clot breakdown. A later proteomics study found more inflammatory molecules trapped inside, including von Willebrand factor and platelet factor 4.

Immune cells may be involved too. A 2025 study reported that circulating microclots are structurally associated with neutrophil extracellular traps, the DNA webs that neutrophils release, and that both were raised in Long COVID. Our article on microclots and NETs in Long COVID goes into that link.

What are the symptoms of microclots?

There isn't a symptom list unique to microclots, and anyone who offers one is overselling the evidence. What researchers propose is a mechanism. If amyloid microclots lodge in capillaries, they could reduce oxygen delivery to tissues. The Pretorius and Kell review argues this could contribute to symptoms common in Long COVID:

• Fatigue and post-exertional malaise
• Brain fog and trouble concentrating
• Breathlessness
• Muscle and joint pain
• Exercise intolerance

Every one of these has other possible causes. A 2023 review of Long COVID in Nature Reviews Microbiology lists clotting and endothelial problems as one of several proposed mechanisms, alongside viral persistence, autoimmunity and reactivated viruses. Microclots may be part of the picture for some people, all of it for none, or a marker rather than a cause. Our plain-language guide to microclots and Long COVID covers how this fits with the other theories.

Who gets microclots?

Most of the published data come from three groups:

• People with Long COVID, where the most research has been done.
• People with persistent symptoms after vaccination, a smaller and newer area. Our article on post-vaccination microclots covers what's been published.
• People with chronic inflammatory conditions such as type 2 diabetes, where the same group described amyloid fibrin before the pandemic.

Some healthy controls show low levels too. That's one reason researchers treat microclots as a sign of an inflamed clotting system rather than a marker of any single disease.

Is the microclot theory proven?

No, and it's worth being straight about that. The evidence so far is observational, comes mostly from one research network, and uses methods without an agreed cutoff.

In 2024, haematologists led by Beverley Hunt published a critique of the microclot hypothesis. They argued the conclusions went beyond the evidence and that unlicensed anticoagulant use and apheresis shouldn't be offered outside trials. Kell, Khan and Pretorius published a reply defending their data. Both sides agree on one thing: properly controlled trials are needed.

Next step

Curious whether microclots show up in your blood?

Our at-home amyloid fibrin test stains your sample with Thioflavin T and records video microscopy you can share with your doctor.

See the at-home test

How are microclots detected?

Standard clotting tests can't see them. D-dimer measures fragments released when clots dissolve, which is a different question, as we explain in D-dimer vs microclot test.

Detection relies on Thioflavin T, a dye that glows when it binds amyloid. A stained blood or plasma sample is imaged under a fluorescence microscope or run through an imaging flow cytometer. Our step-by-step guide on how to test for microclots compares the methods and where to get them, and the amyloid fibrin test pillar explains the staining method in detail.

If you'd rather test from home, our at-home amyloid test uses a finger-prick dried blood sample stained with Thioflavin T and returns a Videographic Microscopy Report. It isn't a diagnosis. It gives you observational data to review with your doctor.

Can microclots be treated?

Several approaches are being studied, including enzyme supplements like nattokinase, anticoagulant combinations and apheresis. None has been proven in a large randomized trial for microclots, and anything that affects clotting carries bleeding risk. Our review of nattokinase and lumbrokinase for microclots sets out what the evidence actually shows.

Don't start or stop blood thinners on your own. If you're looking for a clinician who knows this research, our doctor directory is a place to start.

Where to go next

For the research in more depth, read amyloid microclots: what the research shows. If you're ready to look at testing, start with how to test for microclots.