Unusual White Blood Clots: What the 2026 Research Actually Shows
Reports of unusual white, fibrous and rubber-like structures being removed from blood vessels during embalming have attracted widespread attention since around 2021. In 2026, researchers began formally characterizing some of these structures as anomalous intravascular casts (AICs). What do the studies actually show—and what remains unknown?
Unusual pale, elongated intravascular structures have been documented and are now being subjected to laboratory analysis. Early research suggests that at least some specimens are protein- and fibrin-rich and may have unusual structural characteristics. However, the available evidence is still limited and does not establish how common these structures are, exactly when they form, what causes them, or whether they increase the risk of disease or death.
The subject has become controversial partly because photographs and videos circulated before there was much formal scientific characterization. That makes it especially important to distinguish between what has been directly observed, what laboratory studies suggest, and what remains hypothetical.
This article reviews the emerging evidence and places the 2026 research in the context of established thrombosis and fibrin biology.
What Are the “Unusual White Blood Clots”?
Embalmers have reported finding long, pale, cohesive structures within blood vessels that can follow the branching geometry of the vascular system. Some have described them as relatively elastic or rubber-like compared with the familiar gelatinous or red postmortem clots they have encountered for many years.
The structures can be sufficiently long to form casts of blood vessels, including branching vessels. The term anomalous intravascular casts, or AICs, has been proposed for this phenomenon. A 2026 preprint described elongated lumen-filling structures, some reported to reach approximately 25 cm in the specimens studied, with branching morphology and elastic resistance. [1]
Importantly, AIC is a research term rather than an established clinical diagnosis. The available studies are exploratory and should not be interpreted as establishing a new disease category.
White Thrombi Are Not a New Medical Phenomenon
One of the most important points to clarify is that the phrase white blood clot does not refer to something that medicine had never seen before 2021.
Conventional thrombi can differ substantially in composition. Some are relatively rich in red blood cells and appear red or dark. Others contain proportionally more platelets and fibrin and can appear pale or white.
In a study of thrombi retrieved during primary PCI for ST-elevation myocardial infarction, investigators classified approximately 31% of the available specimens as white thrombi. These were associated with greater fibrin infiltration and lower red-blood-cell content than red thrombi. [2]
White thrombi have also been investigated in other vascular settings, including stroke and acute coronary syndromes.
| Feature | Conventional white thrombus | Reported AIC phenomenon |
|---|---|---|
| Recognized in medical literature? | Yes | Formal characterization is emerging |
| Typical composition | Platelet- and fibrin-rich | Early studies report substantial fibrin/protein content |
| Typical appearance | Pale/white | Pale, fibrous or rubber-like in reported cases |
| Long vessel-shaped cast | Not a defining feature | Frequently reported in AIC descriptions |
| Established clinical entity? | Yes | No |
| Cause established? | Depends on the underlying thrombotic condition | No |
The term “AIC” refers to the structures described in the 2026 exploratory literature and should not be treated as synonymous with all white thrombi.
Why Did the Subject Attract Attention After 2021?
Beginning around 2021, embalmers in different locations publicly described seeing unusually long and cohesive pale structures during postmortem preparation. Some reported that the material could interfere with normal embalming because it obstructed vascular channels.
These observations generated a legitimate scientific question:
Photography and video can document that a physical structure exists. They cannot by themselves establish its biochemical composition, time of formation, prevalence, cause or clinical significance.
That is why the 2026 laboratory studies are more important than the photographs alone.
What Did the First 2026 AIC Study Find?
A January 2026 preprint by Bruce Rapley and Matt Shelton examined the morphology and histology of anomalous intravascular casts. The authors described elongated structures with lumen-conforming geometry, branching, dense fibrinous lamination and relatively sparse cellular material. [1]
The authors interpreted some of the histological features as potentially consistent with formation during life under atypical haemodynamic conditions.
This study is a preprint and was not peer-reviewed at the time of publication. Its observations are hypothesis-generating rather than definitive proof that AICs are a previously unrecognized disease process.
Lines of Zahn: Why the Interpretation Matters
The discussion of Lines of Zahn is particularly important because alternating layers of platelets/fibrin and red-cell-rich material have traditionally been associated with thrombi formed in flowing blood.
However, modern pathology research shows that this issue is more nuanced than often presented online.
A comparative study of antemortem thrombi and postmortem clots found that thin fibrin strands can occur in both. The investigators proposed a narrower definition of Lines of Zahn based on thick bands of nested platelets surrounded by fibrin, which were much more characteristic of antemortem thrombus. [3]
Therefore, the presence of fibrin layering alone should not be presented as incontrovertible proof that a structure formed before death.
What Did the 2026 Proteomic Study Find?
A companion 2026 preprint used high-precision liquid chromatography and tandem mass spectrometry to investigate the protein composition of AIC specimens.
The investigators reported identification of 541 human proteins, with fibrin-family components prominent among the identified proteins. They also reported findings they interpreted as evidence of non-canonical fibrin architecture and impaired fibrinolysis. [4]
This is potentially interesting because the balance between fibrin formation and fibrinolysis is central to thrombosis.
A clot that forms readily but breaks down poorly could, in principle, become more persistent.
But that does not mean that these structures have been proven to be intrinsically resistant to fibrinolysis in patients. Functional experiments and larger, independently replicated datasets are needed to establish that.
| Finding | What it may indicate | What it does not prove |
|---|---|---|
| High fibrin-related protein content | Fibrin-rich matrix | A specific cause of formation |
| Unusual protein distribution | Possible atypical fibrin organization | A new disease mechanism |
| Low or altered fibrinolytic components in specimens | Possible impaired breakdown | System-wide fibrinolysis resistance |
| 541 identified human proteins | Complex biological matrix | Proof of causality |
Could These Structures Be Amyloid?
Another hypothesis concerns amyloid-like protein organization.
A 2026 Raman spectroscopy study examined two specimens and reported differences in protein secondary-structure signatures, including beta-sheet-associated features in one sample. [5]
That finding is interesting, but it should not be simplified to “the clots are amyloid.”
Beta-sheet structures occur naturally in many proteins. Demonstrating canonical amyloid requires more specific structural evidence and appropriate confirmatory assays.
The more scientifically accurate description is that some analyzed material showed protein structural features that raise the possibility of aggregated or amyloid-like organization.
How Does This Relate to Fibrin Microclots and Long COVID?
This question has attracted considerable attention because previous research has described abnormal fibrin-containing microclots in people with Long COVID.
A 2021 study reported persistent clotting-related abnormalities and fibrinolysis-resistant microclot material in samples from people with acute COVID-19 and Long COVID/PASC. The investigators reported increased alpha-2-antiplasmin and other proteins associated with the persistent deposits. [6]
A 2022 review and hypothesis paper subsequently proposed a potential role for amyloid-like fibrin microclots in Long COVID. [7]
More recently, a 2025 Biochemistry study investigated specific amyloidogenic SARS-CoV-2 spike-protein peptides using purified components of the coagulation system. One peptide, Spike685, produced fibrin networks that showed increased resistance to plasmin-mediated fibrinolysis in vitro. [8]
Findings involving COVID-19, spike protein, fibrin microclots and fibrinolysis do not establish that the large white intravascular casts recovered during embalming were caused by SARS-CoV-2 infection or COVID-19 vaccination.
Are the White Clots Caused by COVID-19 Vaccination?
This is one of the most common questions surrounding the subject.
At present, the available AIC studies do not establish a causal relationship between these structures and COVID-19 vaccination.
A causal investigation would need information such as:
| Variable | Why it matters |
|---|---|
| Vaccination history | Allows exposure classification |
| Timing of vaccination | Allows temporal analysis |
| Previous SARS-CoV-2 infection | Separates infection from vaccination effects |
| Age and sex | Controls for demographic differences |
| Cancer and cardiovascular disease | Major independent thrombotic risk factors |
| Diabetes and inflammatory disease | Can influence coagulation and vascular biology |
| Anticoagulant or antiplatelet medication | Can alter clot formation and persistence |
| Cause of death | Essential for clinical interpretation |
| Matched controls | Needed to estimate whether the finding is actually unusual |
Without those data, the observation of a white intravascular structure cannot identify its cause.
Could the Structures Simply Be Postmortem Clots?
This is another question that requires careful consideration.
Postmortem blood clots are well recognized and can have different appearances depending on the circumstances surrounding death and the postmortem interval.
Radiological and pathology research has demonstrated that postmortem blood clots can sometimes resemble thromboemboli, emphasizing the need for careful pathological differentiation. [9]
Therefore, demonstrating that a structure is unusual in gross appearance is not enough to determine whether it formed during life.
The question should be answered using a combination of:
- gross morphology
- histology
- immunohistochemistry
- proteomics
- ultrastructural analysis
- clinical history
- cause-of-death investigation
- appropriate control specimens
Why Haven't Pathologists Reported More of Them?
This is a reasonable question, but the answer is not necessarily that pathologists failed to notice an entirely new disease.
Embalmers and pathologists have different workflows.
During embalming, the vascular system is manipulated extensively and blood is removed before embalming fluid is introduced. A structure that obstructs a vessel or becomes trapped in a filtration system may therefore become particularly noticeable.
Conventional autopsy pathology, by contrast, is designed primarily around identifying disease, injury and cause of death.
Differences in sampling, documentation, terminology and publication can all influence what becomes visible in the medical literature.
Consequently, the apparent lack of historical reports is an important observation—but it is not proof that the structures did not exist previously.
What We Know vs. What We Don't Know
| Question | Current evidence |
|---|---|
| Do unusual pale intravascular structures exist? | Yes, reported specimens have been documented and studied. |
| Are all white clots new? | No. Conventional white thrombi are well established in medical literature. |
| Are some reported AICs fibrin-rich? | Yes, early laboratory studies support this. |
| Are they biologically identical to ordinary thrombi? | Unknown. |
| Are they definitely antemortem? | Not established. |
| Are they definitely postmortem? | Not established. |
| Are they amyloid? | Not established. Some findings raise the possibility of unusual protein structure. |
| How common are they? | Unknown. |
| Do they cause disease? | Unknown. |
| Do they cause death? | Not demonstrated. |
| Are they caused by COVID-19 infection? | Not established. |
| Are they caused by COVID-19 vaccination? | Not established. |
What Research Is Needed Next?
The next step should be systematic, prospective and independently replicated research rather than more anecdotal photography.
A useful research program would compare AIC specimens with:
- confirmed antemortem thrombi
- well-characterized postmortem clots
- routine white thrombi
- red thrombi
- samples from individuals with and without major thrombotic disease
Researchers could then investigate the material using a standardized panel of methods.
| Research question | Useful method |
|---|---|
| What is the structure? | Histology and electron microscopy |
| Is it primarily fibrin? | Proteomics and immunohistochemistry |
| Is there genuine amyloid? | Congo red, Thioflavin-T and ultrastructural confirmation |
| Does it resist fibrinolysis? | Direct plasmin/tPA degradation assays |
| Did it form during life? | Validated pathology markers plus clinical correlation |
| How common is it? | Prospective surveillance with a defined denominator |
| What causes it? | Controlled epidemiological and mechanistic studies |
| Does it affect outcomes? | Clinical, imaging and cause-of-death correlation |
What Does the Evidence Mean for the Public?
For the general public, the most important point is that the existence of unusual intravascular material should not be confused with proof of a new epidemic, a particular exposure or a specific medical treatment.
At the same time, dismissing the observations without investigation would also be inappropriate.
The correct scientific response is to characterize the specimens, establish appropriate controls, determine how often they occur, establish when they form, and investigate whether they have meaningful clinical consequences.
The phenomenon is worth studying precisely because important questions remain unanswered. Evidence can support a hypothesis without proving the hypothesis.
Bottom Line
Reports of unusual white, elongated and fibrous intravascular structures have evolved from largely photographic and anecdotal observations into an emerging area of laboratory investigation.
The 2026 AIC studies report unusual morphology and protein composition, including prominent fibrin-related material and structural features that may warrant further investigation. [1] [4]
However, these studies are early-stage research, and some are preprints rather than peer-reviewed publications. They do not yet establish the prevalence, cause, timing of formation or clinical consequences of these structures.
It is also important to distinguish these reported AICs from ordinary white thrombi, which have been recognized in cardiovascular medicine for many years. [2]
Similarly, research on fibrin microclots, Long COVID and SARS-CoV-2 spike-protein interactions with fibrin provides interesting mechanistic context, but it does not prove that the large white casts reported during embalming were caused by COVID-19 infection or vaccination. [6] [8]
The most defensible conclusion in 2026 is therefore:
Long COVID Research | COVID-19 Research | Cardiovascular Health | Medical Research
Frequently Asked Questions
What are unusual white blood clots?
The phrase generally refers to pale, fibrous or rubber-like intravascular structures reported by some embalmers. Some researchers are calling these structures anomalous intravascular casts (AICs). This term is still investigational and is not an established clinical diagnosis.
Are white blood clots a new phenomenon?
No. Conventional white thrombi have been described in medical literature for many years. What is newer is the specific report of long, cohesive and vessel-shaped fibrous structures being observed during embalming and investigated as a possible distinct phenomenon.
Are anomalous intravascular casts definitely blood clots?
Their composition appears to include substantial protein and fibrin-related material in the studies published so far, but the exact pathological classification remains under investigation.
Are the white clots caused by COVID-19?
There is not enough evidence to conclude that. COVID-19 has well-established effects on coagulation, and research has investigated fibrin microclots and fibrinolysis abnormalities, but those findings do not establish the cause of the larger structures described as AICs.
Are the white clots caused by COVID-19 vaccines?
A causal connection has not been established by the available AIC studies. Proper assessment would require matched clinical data, vaccination history, infection history and appropriate controls.
Are the structures made of amyloid?
Some early laboratory findings raise questions about unusual beta-sheet-rich protein organization, but that is not equivalent to proving that the material is canonical amyloid.
Can white clots cause stroke or heart attack?
Conventional thrombi can cause stroke, myocardial infarction and other vascular events. Whether the specific structures described as AICs cause clinically important vascular disease has not yet been established.
Why are researchers studying these clots?
Because the reported structures appear unusual in morphology and mechanical properties, and early laboratory work suggests that their protein and fibrin composition may differ from familiar clot material. Rigorous research is needed to determine whether this represents a distinct phenomenon.
Are the 2026 studies peer reviewed?
Some of the key 2026 AIC studies discussed here are preprints and explicitly state that they have not been peer reviewed. They should therefore be treated as preliminary evidence requiring independent replication.
References
1. Rapley B, Shelton M. Morphological and Histological Characterisation of Anomalous Intravascular Casts (AICs). Preprints. 2026. DOI: 10.20944/preprints202601.1846.v1 .
2. Quadros AS, et al. Red versus white thrombi in patients with ST-elevation myocardial infarction undergoing primary percutaneous coronary intervention: clinical and angiographic outcomes. Am Heart J. 2012;164(4):553-560. PMID: 23067914. DOI: 10.1016/j.ahj.2012.07.022 .
3. Krywanczyk AR, Tan CD, Rodriguez ER. Histologic and Immunohistochemical Features of Antemortem Thrombus Compared to Postmortem Clot: Updating the Definition of Lines of Zahn. Arch Pathol Lab Med. 2023;147(11):1241-1250. PMID: 36626295. DOI: 10.5858/arpa.2022-0147-OA .
4. Rapley B, Shelton M. Proteomic Characterisation of Anomalous Intravascular Casts Reveals Non-Canonical Fibrin Architecture and Impaired Fibrinolysis. Preprints. 2026. DOI: 10.20944/preprints202601.2319.v1 .
5. Santiago D, Harrison G, Veres M, File M, Planner D. Raman spectroscopic Characterization of Anomalous Intravascular Fibrous Casts: Evidence for Stage-Dependent β-sheet Enriched Protein Maturation. International Journal of Innovative Research in Medical Science. 2026;11(07):182-203. Article .
6. Pretorius E, et al. Persistent clotting protein pathology in Long COVID/Post-Acute Sequelae of COVID-19 (PASC) is accompanied by increased levels of antiplasmin. PMID: 34425843.
7. Kell DB, Laubscher GJ, Pretorius E. A central role for amyloid fibrin microclots in long COVID/PASC: origins and therapeutic implications. Biochem J. 2022;479(4):537-559. PMID: 35195253. DOI: 10.1042/BCJ20220016 .
8. Westman H, Hammarström P, Nyström S. SARS-CoV-2 Spike Protein Amyloid Fibrils Impair Fibrin Formation and Fibrinolysis. Biochemistry. 2025;64(24):4818-4829. PMID: 41295749. DOI: 10.1021/acs.biochem.5c00550 .
9. Total-body CT and MR features of postmortem change in in-hospital deaths. Discussion of pulmonary thromboembolism versus postmortem blood clot. PMID: 28953923.
10. Dr John Campbell. Unusual White Blood Clots. Substack. 2026: https://johninengland.substack.com/p/unusual-white-blood-clots.
11. Wang JJ, et al. Adenoviral Inciting Antigen and Somatic Hypermutation in VITT. NEJM. 2026.
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