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What Are Microclots?
Microclots are abnormally formed, microscopic clot-like structures found in the blood of many patients suffering from long COVID (post-acute sequelae of SARS-CoV-2, or PASC). Unlike normal blood clots that form in response to injury and dissolve through the body’s natural fibrinolytic processes, these microclots are resistant to breakdown and persist in the circulation, where they obstruct the smallest blood vessels and impair oxygen and nutrient delivery to tissues throughout the body.
This discovery has fundamentally advanced our understanding of why long COVID causes such a diverse and debilitating array of symptoms — and it has opened the door to targeted treatment strategies. At St. George Hospital (Klinik St. Georg) in Bad Aibling, Germany, we have been at the forefront of translating microclot research into clinical care for post-COVID patients from around the world.
Dr. Beate Jaeger’s Research: A Breakthrough in Understanding
Dr. Beate Jaeger, a researcher collaborating closely with St. George Hospital, has been instrumental in advancing our understanding of microclots and their role in long COVID. Her work builds upon foundational research by Professor Etheresia Pretorius at Stellenbosch University in South Africa, who first identified the presence of anomalous amyloid fibrin microclots in COVID-19 patients.
Key Findings
Dr. Jaeger’s research has documented several critical observations:
- Persistent microclots: Long COVID patients harbor microclots in their blood months — and in some cases years — after the initial SARS-CoV-2 infection.
- Fibrin amyloid formation: The SARS-CoV-2 spike protein interacts with fibrinogen (a clotting protein) to produce fibrin that misfolds into an amyloid form. This amyloid fibrin is structurally abnormal and highly resistant to the body’s normal clot-dissolving mechanisms.
- Trapped inflammatory molecules: These microclots entrap inflammatory cytokines, complement proteins, and even fragments of the spike protein itself, creating a self-perpetuating cycle of inflammation and clotting.
- Impaired microcirculation: The microclots physically obstruct capillaries and small blood vessels, reducing oxygen delivery to tissues. This impaired microcirculation is a plausible unifying mechanism for many long COVID symptoms.
It is important to frame these findings as an important research direction. While the evidence is compelling and growing, the microclot hypothesis is still being validated through larger clinical studies. Dr. Julian Douwes emphasizes that “we approach this research with scientific rigor — the data are promising, but we must continue to study these mechanisms carefully while offering our patients the best available care.”
How Microclots Impair Microcirculation
The microcirculation — the vast network of capillaries, arterioles, and venules that permeates every organ — is where oxygen, nutrients, and waste products are exchanged between blood and tissue. When microclots obstruct this network, the consequences are widespread:
Organ-Specific Effects
- Brain: Impaired cerebral microcirculation may explain the cognitive dysfunction (“brain fog”), concentration difficulties, and headaches that are among the most common long COVID symptoms.
- Heart: Microvascular obstruction in the cardiac muscle can cause chest pain, palpitations, and exercise intolerance — even when standard cardiac imaging appears normal.
- Lungs: Capillary-level obstruction contributes to persistent breathlessness and reduced oxygen exchange, even when pulmonary function tests are normal or near-normal.
- Muscles: Impaired oxygen delivery to skeletal muscle explains the profound fatigue and post-exertional malaise that characterize long COVID.
- Peripheral nerves: Microvascular compromise affecting nerve blood supply may contribute to neuropathy, tingling, and numbness.
The Endothelial Connection
Microclots do not act in isolation. They are closely linked to endothelial dysfunction — damage to the thin layer of cells lining all blood vessels. SARS-CoV-2 directly infects endothelial cells via the ACE2 receptor, causing inflammation and dysfunction. Damaged endothelium becomes more “sticky,” promoting further microclot adhesion and creating a vicious cycle of obstruction and inflammation (Pretorius et al., 2021).
Detecting Microclots
Standard blood tests — including routine coagulation panels (PT, PTT, INR) and D-dimer — often appear normal in long COVID patients, which has historically led to the dismissal of a clotting component in their illness. Microclots require specialized detection methods:
- Fluorescence microscopy: Using the dye thioflavin T, which binds specifically to amyloid structures, microclots can be visualized and quantified in blood samples.
- Viscoelastic testing (TEG/ROTEM): These tests measure the physical properties of clot formation and may reveal hypercoagulable patterns not detected by standard tests.
- Dark-field microscopy: Live blood analysis can visualize abnormal red blood cell aggregation and platelet behavior associated with microclot presence.
- Platelet activation markers: Elevated markers of platelet hyperactivation (P-selectin, platelet factor 4) support the diagnosis of a prothrombotic state.
At St. George Hospital, our diagnostic workup for post-COVID patients includes specialized assessments of microcirculation and coagulation status beyond what standard laboratory panels offer.
Apheresis as a Treatment Approach
One of the most promising treatment strategies for microclots is therapeutic apheresis — a blood filtration technique that can physically remove pathological substances from the bloodstream.
How Apheresis Targets Microclots
Several apheresis techniques are relevant to microclot removal:
- H.E.L.P. apheresis: (Heparin-induced Extracorporeal LDL Precipitation) removes fibrinogen, LDL cholesterol, and inflammatory mediators. By reducing fibrinogen levels, it addresses the raw material from which microclots are formed.
- Double filtration plasmapheresis (DFPP): Separates plasma components by molecular weight, removing large inflammatory molecules and abnormal clotting factors.
- Immunoadsorption: Targets specific inflammatory molecules and autoantibodies that may perpetuate endothelial damage and microclot formation.
Clinical Observations
Dr. Jaeger and colleagues at St. George Hospital have observed significant clinical improvements in long COVID patients following apheresis treatment:
- Reduced fatigue and improved exercise tolerance
- Improved cognitive function and reduced brain fog
- Resolution or improvement of chest pain and palpitations
- Measurable improvement in microcirculation parameters
- Reduction in inflammatory markers
These observations, while clinically significant and consistent with the microclot hypothesis, are being rigorously documented to build a robust evidence base. Dr. Julian Douwes emphasizes that “we are committed to honest reporting of our outcomes. The improvements we see in many patients are encouraging, but we continue to study which patients respond best and how to optimize our protocols.”
Beyond Apheresis: A Comprehensive Approach
At St. George Hospital, microclot-targeted apheresis is part of a broader post-COVID treatment strategy that includes:
- Anti-inflammatory protocols: Targeted infusion therapies to reduce systemic inflammation.
- Ozone therapy: Major autohemotherapy to improve oxygen delivery and reduce oxidative stress.
- Endothelial repair support: Specific nutrients and infusions that promote healing of the vascular lining.
- NAD+ infusions: To support cellular energy production, which is often impaired in long COVID.
- Immune modulation: Addressing the dysregulated immune response that perpetuates post-COVID pathology.
- Whole-body hyperthermia: Fever-range protocols that may help reactivate and resolve dormant viral reservoirs and stimulate immune function.
Frequently Asked Questions
Do all long COVID patients have microclots?
Research suggests that a significant proportion of long COVID patients have detectable microclots, but not all. Long COVID likely involves multiple overlapping mechanisms — including viral persistence, autoimmunity, autonomic dysfunction, and mitochondrial impairment — of which microclots represent one important component. Our diagnostic approach assesses each patient individually to determine which mechanisms are most active in their case.
Can microclots be detected through standard blood tests?
Unfortunately, no. Routine coagulation tests (PT, PTT, INR, D-dimer) typically appear normal in long COVID patients with microclots. Specialized testing such as fluorescence microscopy with thioflavin T staining or advanced viscoelastic testing is required. At St. George Hospital, we use specialized diagnostic protocols to evaluate for microclots and impaired microcirculation.
How many apheresis sessions are typically needed?
The number of sessions varies by patient. Some patients experience significant improvement after 2–3 sessions, while others require 5–10 or more. The treatment course is guided by clinical response, laboratory markers, and microcirculation assessments. Dr. Julian Douwes and the post-COVID team develop individualized treatment plans and adjust them based on response.
Is the microclot theory proven?
The microclot hypothesis is supported by a growing body of peer-reviewed research, including work by Pretorius et al. and clinical observations by Dr. Beate Jaeger and others. It is increasingly recognized as a plausible and important mechanism in long COVID. However, large-scale randomized controlled trials are still underway. We present this framework to patients as an important research direction with promising clinical results, not as established medical certainty (Pretorius et al., 2021).
Can I receive apheresis treatment for long COVID at St. George Hospital?
Yes. Our post-COVID treatment program includes apheresis alongside comprehensive diagnostics and multimodal therapy. Patients travel to our hospital from around the world for these treatments. Contact our patient coordination team to discuss your case and arrange an initial consultation.
Take the Next Step
If you are living with long COVID and struggling with persistent symptoms, the microclot pathway may offer answers — and actionable treatment options. Our team at St. George Hospital is ready to evaluate your case and develop a personalized treatment plan.
St. George Hospital (Klinik St. Georg)
Rosenheimer Str. 6–8, 83043 Bad Aibling, Germany
Phone: +49 (0)8061 398-0
Email: info@clinicum-stgeorg.de
Request a consultation — We support patients from over 90 countries.
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