The lingering effects of COVID-19 are undeniable. The virus came with some important effects on the body. Among these, you could be dealing with persistent fatigue, brain fog, or exercise intolerance. And if you’re wondering what the impact of this on blood clotting is, then you’ve probably come across two main blood tests: D-dimer and the newer microclot test.
Although they sound related because they both deal with blood clotting, they measure different things. Plus, they’re used for different purposes and have very different levels of clinical validation. Here’s a look at how they actually compare.
The short version
| D-Dimer | Microclot Test | |
| What it measures | A breakdown product left over after a clot dissolves | The actual number, size, and type of microclot particles in a blood sample |
| Clinical status | Standard, well-validated, used worldwide | Emerging; used mainly in research settings and specialty clinics |
| Best at | Ruling out acute, dangerous clots (DVT/PE) | Detecting the smaller, persistent clots associated with Long Covid |
| Availability | Any hospital or lab | A handful of research labs and specialty clinics such as the Apheresis Center |
| Turnaround | Hours (often same-day, used in emergency settings) | Days to weeks, depending on the lab |
What D-Dimer Actually Indicates
D-dimer is a protein fragment your body produces when a fibrin clot breaks down. It’s been used in medicine for decades, mainly to help doctors rule out serious clotting events like deep vein thrombosis (DVT) or pulmonary embolism (PE) in the emergency room.
A low D-dimer result, combined with a low clinical suspicion, is quite a reliable way of excluding these conditions.
But D-dimer has real limitations, especially for something like Long Covid:
- It’s non-specific: Levels can rise from infection, recent surgery, pregnancy, inflammation, cancer, or simply getting older, which makes an elevated result hard to interpret in isolation.
- It’s a “rule-out,” not a “rule-in,” test: A normal D-dimer helps exclude an acute clot, but an elevated one doesn’t tell you what its cause is. And it can’t localise a clot or tell you whether it’s a large clot in a vein or a scattering of much smaller ones.
- It’s not designed to detect microclots: Microclots are amyloid-like, fibrinogen-based particles that behave differently from the clots D-dimer was built to flag, so a person can have significant microclot burden while their D-dimer looks entirely normal.
What the Microclot Test Indicates
Microclot testing involves a different approach.
Rather than measuring a breakdown product, specialised labs use techniques like fluorescence microscopy or imaging flow cytometry to directly visualise and count clot-like particles in a blood or plasma sample.
Some versions also distinguish between amyloid-positive particles (ThT+) and amyloid-negative microparticles (ThT-), and assess particle size, which researchers believe may be clinically meaningful.
This matters for Long Covid, especially because recent research has repeatedly found these particles at significantly higher levels in Long Covid patients compared with healthy people. In some studies, for example, the results were around 20 times higher.
They’re also often larger in size and structurally bound up with neutrophil extracellular traps (NETs). The latter refers to sticky material that’s released by the immune system.
This has made microclots one of the more actively pursued explanations for symptoms like fatigue, brain fog, and exercise intolerance in Long Covid.
The trade-off is validation and access.
Microclot testing isn’t yet a standardised, widely available clinical test. It’s currently offered mainly through a small number of research laboratories and specialty clinics, and it hasn’t reached the same level of regulatory validation or diagnostic consensus as D-dimer.
Results can also be harder to interpret in terms of a clear “positive/negative” clinical threshold, as the field is still establishing reference ranges and treatment implications.
Which Test Should You Get?
These two tests are not really substitutes for each other because they answer different questions:
- If you have symptoms that could indicate an acute, dangerous clot such as sudden leg swelling, chest pain, or breathlessness, a D-dimer test (usually alongside imaging) is the appropriate, well-established first step, and it should be done urgently through a doctor or emergency department.
- If you have persistent post-Covid symptoms like fatigue, brain fog, or exercise intolerance, and standard workups haven’t explained them, a microclot test may yield useful information, particularly when combined with a clinical symptom assessment, since it’s looking for a different phenomenon than D-dimer is designed to catch.
Many clinicians treating Long Covid use both tests, together with other markers, rather than relying on either test in isolation.
D-dimer helps rule out the emergency scenario. Microclot testing (and related symptom screening tools) can help build a fuller picture of ongoing microvascular and coagulation issues that a standard clotting panel wouldn’t catch.
The Bottom Line
D-dimer is the established, widely available test for excluding dangerous acute clots. It’s fast and well-validated, but is non-specific and not built to detect microclots.
Microclot testing is newer and more specialised, aiming directly at the persistent, amyloid-like particles increasingly linked to Long Covid symptoms, but it’s less standardised and harder to access.
Neither test replaces a full clinical evaluation, and if you’re dealing with concerning symptoms, especially anything suggesting an acute clot, that always comes first.
Medical Review Note
Reviewed by Dr Yvonne Tuszing, Medical Director at The Apheresis Centre, who has over 10 years of experience in chronic illness care and post-viral recovery medicine.
Sources
- Cleveland Clinic — D-Dimer Test: What It Is, What It Is Used For, Risks & Results
- StatPearls (NCBI Bookshelf) — D-Dimer Test
- Mayo Clinic — Thrombophlebitis: Diagnosis & Treatment
- Harvard Health Publishing — Superficial Thrombophlebitis
- ScienceDaily / Stellenbosch University — Strange Microscopic Structures Found in Long COVID Blood
- Thierry, A. et al., Journal of Medical Virology (2025) — Circulating Microclots Are Structurally Associated With Neutrophil Extracellular Traps and Their Amounts Are Elevated in Long COVID Patients
- ScienceAlert — Strange Structures Found Lurking in the Blood of People With Long COVID
