What do Microclots Look Like?

And how can we test for them?

In the past few posts, I’ve talked about the hypothesis that microclots are the primary event that causes Long Covid & it’s associated symptoms and about the mechanism of how microclots form.

Today, I want to address two more questions that seem to be coming up a lot:

1. How do we test for microclots in Long Covid patients?

2. What do microclots really look like in the blood?

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How do we test for microclots in Long Covid Patients?

Currently, there are no clinical tests to detect microclots in Long Covid or acute Covid.

What everyone would really like is a test where a patient can get their blood drawn and then either a clinical testing laboratory or a pathology laboratory could determine if the blood sample contains microclots. Alternatively, people would love a point-of-care device, like a glucose meter or lactic acid meter, that could be used in a physician’s office, a pharmacy, or even at home. Unfortunately, none of those tests exist.

Right now, we can only detect microclots using tests in the research laboratory. The most common technique for detecting microclots is using fluorescence microscopy. The first figure below shows what microclots look like using this technique. Some patients do seem to be having success getting fluorescence microscopy testing done on their blood by some of the larger labs engaged in Covid research. But fluorescence microscopy is still a low-throughput method and it’s not available on a mass scale.

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What do microclots really look like in the blood?

In the literature, I’ve found data visualizing microclots in Long Covid using three techniques - Fluorescence Microscopy, Bright-Field Microscopy, and Scanning Electron Microscopy (SEM). Each of these visualizes the microclots in a different way and gives us different information about the microclots.

Fluorescence Microscopy

Fluorescence Microscopy was the first technique used in the research laboratory to visualize Long Covid microclots. In a rather simple procedure, a whole blood sample from a patient is centrifuged to isolate the platelet poor plasma (PPP), which is then treated with the fluorescent dye Thioflavin T (ThT). This allows visualization of the microclots as fluorescent green structures under the microscope (see below). (Ref. 1) This method is important because ThT only binds to and stains proteins in the beta-sheet conformation. So this experiment not only allows us to visualize and detect the microclots in a blood sample, but also tells us that the microclot is in the abnormal, amyloid beta-sheet conformation.

Bright-Field Microscopy

A second technique that can be used to visualize microclots is Bright-Field Microscopy. This is the simplest of microscopy techniques that just uses light plus magnification to observe the sample. After the PPP sample is loaded on a slide, the microclots appear as the dark patches seen under magnification (panel A below). (Ref 2) This particular sample has also been treated with ThT, so the microclot can be observed by fluorescence microscopy as well (panel B). An overlay of the two (panel C) shows that the microclot seen by bright-field microscopy aligns with and matches the microclot observed by fluorescence microscopy.

Scanning Electron Microscopy (SEM)

The last technique to visualize microclots formed in Long Covid is using Scanning Electron Microscopy (SEM). SEM is a high-resolution technique that creates an imaging by scanning a surface with a focused beam of electrons. SEM is the closest thing to a photograph we can get on cellular scale with microscopy.

Below are SEM images of whole blood samples from healthy volunteers that were treated with Covid spike protein. (Ref 3) Please note that the images are at different magnifications, as seen by the scale bars. Treatment of the whole blood samples with spike protein results in the formation of abnormal microclots, which are amyloids of fibrin. The amyloid microclots are “sticky” and able to bind other structures in the blood circulation like red blood cells (RBCs, round circles, panels E, F, & H) and platelets (panel H). Overall, these SEM images allow us to visualize the large scale of the microclot and the fact that the microclot binds other structures and even proteins (not shown, but as detected by proteomics experiments described in Ref 3).

Impact of Microclots in Long Covid

Now that we understand that the Covid spike protein causes microclots, that we can see how large the microclots are, and that we understand that microclots bind other structures (like RBC and platelets) and proteins, it’s easier to comprehend how these microclots cause a lot of symptoms of Long Covid. For example, binding of RBCs to the microclots prevents the RBCs from transporting oxygen into cells, causing fatigue and muscle weakness, and potentially a buildup of lactic acid. The microclot complexes can also cause inflammation and immune response through the binding and hyperactivation of platelets and through autoantibodies generated by the immune system detecting the proteins bound in the microclot complex.

What’s Next?

The next questions that come to mind for me are:

• Will anti-coagulants prevent microclots and treat Long Covid?

• Do other aspects of the microclot complex, like platelet hyperactivation, contribute to Long Covid and cause symptoms?

Stay tuned for answers to these questions and more.

Leave me a comment below and let me know what you think of this post.

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References:

1. Persistent clotting protein pathology in Long COVID/Post-Acute Sequelae of COVID-19 (PASC) is accompanied by increased levels of antiplasmin. Cardiovasc. Diabetol. 2021, 20, 172.

2. Prevalence of symptoms, comorbidities, fibrin amyloid microclots and platelet pathology in individuals with Long COVID/Post-Acute Sequelae of COVID-19 (PASC). Cardiovasc. Diabetol. 2022, 21, 148.

3. SARS-CoV-2 spike protein S1 induces fibrin(ogen) resistant to fibrinolysis: implications for microclot formation in COVID-19. Bioscience Reports 2021, 41, BSR20210611.