Microclots and Long COVID: A Plain-Language Guide to Amyloid Clots

This guide is for anyone who's heard the word "microclots" and wants a clear explanation without the jargon. By the end you'll know what microclots are, why researchers think they might matter in Long COVID, and which parts of the story are solid versus still being tested.

What are microclots?

Microclots are tiny clumps of fibrin, the protein your body uses to build blood clots, that have folded into an unusual, amyloid-like shape. That shape makes them stickier, denser and much harder for the body to clear. Our guide to what microclots are covers how they form and which symptoms researchers link to them.

Think of your blood as a river. When you're injured, your body throws a temporary net across the wound, then cuts it apart once the job's done. Microclots are like nets that came out twisted and sticky, so the scissors can't cut them.

What "amyloid" means

Amyloid isn't one substance. It's a way proteins can misfold, stacking into rigid sheets that clump together and resist being broken down.

• A healthy protein is like a shirt folded neatly in a drawer.
• An amyloid-like protein is like a shirt twisted up, damp and stuck to five other shirts.
• The more they stick together, the harder the mess is to clean up.

Amyloid isn't always harmful. The body uses some amyloid structures on purpose, including as part of immune defence, which our article on amyloid beta as an immune defense explores.

Why fibrin matters

Fibrin is made from fibrinogen, a protein that circulates in everyone's blood. Normally the body follows three steps: spot damage, build a temporary patch, then dissolve the patch with an enzyme called plasmin. That dissolving process is called fibrinolysis.

Microclots are a problem in step three. In lab studies, the amyloid-like fibrin in Long COVID samples resisted fibrinolysis and trapped proteins that block plasmin (Pretorius et al., 2021). The detailed findings are summarized in fibrin amyloid microclots research.

Why would tiny clots matter for blood flow?

Your smallest blood vessels, capillaries, are only about 5 to 10 micrometres wide. Many microclots are that size or larger (Kell et al., 2022). Scans that find big clots can't see particles this small.

Picture a city where the highways are open but thousands of side streets are partly blocked. Deliveries still arrive, just slower and less reliably. The hypothesis is that if enough capillaries are affected, oxygen delivery to muscles and the brain could drop, which might feed fatigue and brain fog. That's a proposal, not a proven mechanism.

What the glowing microscope pictures show

Researchers add a dye called Thioflavin T to blood plasma. It sticks to amyloid-shaped material and glows under the right light. More glowing clumps suggests more amyloid-like fibrin in the sample.

In published studies, plasma from people with Long COVID showed more and larger glowing deposits than plasma from healthy controls (Pretorius et al., 2021). A bright image is a reason to look further, not proof of what's causing someone's symptoms. If you're curious how the stain works, the amyloid fibril test guide explains it.

Where does the spike protein fit in?

Lab experiments at Linköping University showed that fragments of the SARS-CoV-2 spike protein can form amyloid fibrils on their own (Nyström and Hammarström, 2022). Later work from the same group found that these spike amyloid fibrils changed how fibrin formed and made clots harder to dissolve (Westman et al., 2025).

In plain terms: spike fragments may be present, they may interact with clotting proteins, and the resulting clot may be tougher to clear. All of this comes from test-tube work. The fuller story is in spike protein amyloid fibrils and fibrinolysis.

Clinical next step

Want to see what your own blood sample shows?

Explore our at-home amyloid fibrin test and discuss the report with a qualified clinician.

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What has actually been shown so far

It helps to sort the evidence by where it comes from.

In the lab

• Certain spike protein fragments can form amyloid-like fibrils.
• Spike amyloid can make fibrin clots that resist breakdown.
• Amyloid dyes light up these structures in blood-derived samples.

In patients

• Some people with Long COVID show more amyloid-like microclot material than healthy controls.
• In some small studies, higher microclot burden went with worse symptoms.
• Standard clotting tests often don't reflect these structural changes, as explained in D-dimer vs microclot testing.

Still unresolved

• How common this is across everyone with Long COVID.
• Whether microclots drive symptoms or are one sign of a wider injury.
• Which tests are reliable enough for routine clinical use.
• Which treatments actually improve outcomes.

Why don't routine blood tests find microclots?

A routine clotting test asks whether blood clots too fast or too slowly, or how much clot breakdown product is around. Microclot research asks a different question: what shape is the clot material in, and does it resist clean-up? Those are different measurements, so a normal routine result doesn't answer the microclot question.

That's why testing relies on fluorescence microscopy, the method described in the amyloid fibrin test guide. For every route to a result, from clinics to home kits, see how to test for microclots. Our own amyloid test uses the same staining principle on a finger-prick sample you collect at home. It gives you observational microscopy data, not a diagnosis.

Do microclots explain all of Long COVID?

Almost certainly not. Researchers are studying several overlapping mechanisms, including viral persistence, immune dysregulation, autoantibodies, nerve and autonomic problems, and blood vessel damage. Different people likely have different mixes.

Microclots may be central for some people, a side issue for others and absent in some. The immune side of the clotting story, especially neutrophil extracellular traps, is covered in microclots and NETs in Long COVID.

Microclots after vaccination

Because vaccines instruct cells to make spike protein, researchers are asking whether spike-related clotting changes could contribute to symptoms in some people after vaccination. The evidence is limited, and how often this might happen isn't known. Our article on post-vaccination microclots goes through what's been published.

A traffic-light view of the evidence

Sorting claims by confidence keeps things honest.

What can you do with this information?

If microclots sound relevant to you, the most useful next steps are practical. Keep a symptom log, rule out urgent problems with your doctor, and decide whether a microclot test would add useful information. The Long COVID survival guide covers day-to-day pacing and tracking, and our review of nattokinase for microclots explains what's known about enzyme supplements.

The people living with this illness are dealing with something real. The most trustworthy approach takes that seriously while staying honest about what the trials have and haven't shown yet.