Several blood disorders directly cause your body to form clots when it shouldn’t — or make existing clots harder to dissolve. These conditions, collectively called thrombophilias, affect roughly 5–8% of the general population and are responsible for a significant share of strokes, heart attacks, pulmonary embolisms, and deep vein thromboses (DVTs) that occur each year.
Because inherited thrombophilias often reveal themselves long before middle age, clinicians pay close attention to causes of blood clots in young adults, where an unexplained DVT may signal an underlying disorder.
Some of these disorders are inherited — you’re born with a genetic mutation that tips your coagulation system toward clotting. Others are acquired, meaning they develop later in life due to autoimmune disease, cancer, or other triggers. Below, I’ll walk through the major types, how they’re diagnosed, and what treatment actually looks like in practice.
The 9 Major Blood Disorders That Cause Blood Clots
Not all clotting disorders carry the same level of risk. Some increase your lifetime clot risk by 3–5x; others push it to 50x or higher. Here’s a breakdown:
| Disorder | Inherited or Acquired | Prevalence (General Population) | Relative Clot Risk Increase |
|---|---|---|---|
| Factor V Leiden | Inherited | 3–7% (heterozygous) | 3–8x (heterozygous); 50–80x (homozygous) |
| Prothrombin Gene Mutation (G20210A) | Inherited | 1–3% | 2–5x |
| Antithrombin Deficiency | Inherited | 0.02–0.2% | 10–50x |
| Protein C Deficiency | Inherited | 0.2–0.5% | 7–10x |
| Protein S Deficiency | Inherited | 0.1–0.7% | 5–10x |
| Antiphospholipid Syndrome (APS) | Acquired (autoimmune) | 1–5% | 5–16x |
| Polycythemia Vera | Acquired (myeloproliferative) | ~0.02% | 3–5x |
| Essential Thrombocythemia | Acquired (myeloproliferative) | ~0.01% | 2–4x |
| Paroxysmal Nocturnal Hemoglobinuria (PNH) | Acquired (somatic mutation) | Very rare (~1 in 100,000) | Markedly elevated (thrombosis is leading cause of death) |
Inherited Clotting Disorders: The Big Five
Factor V Leiden
This is the most common inherited thrombophilia worldwide. A single point mutation makes Factor V resistant to inactivation by activated Protein C, so the clotting cascade stays “on” longer than it should. Carrying one copy (heterozygous) raises your venous clot risk about 3–8 times. Carrying two copies pushes that to 50–80 times normal — a level that often triggers clots before age 40.
Prothrombin Gene Mutation (G20210A)
The second most common inherited cause. This mutation leads to elevated prothrombin (Factor II) levels in the blood, which translates to more thrombin generation and easier clot formation. It’s frequently found alongside Factor V Leiden, and carrying both simultaneously compounds the risk significantly.
Antithrombin, Protein C, and Protein S Deficiencies
These three natural anticoagulant proteins act as brakes on your clotting system. When any of them is deficient — due to genetic mutations — the brakes fail. Antithrombin deficiency is the rarest but most dangerous of the three, with some studies showing a lifetime VTE risk above 50% by age 60 in affected individuals.
Acquired Blood Disorders That Cause Clots
Antiphospholipid Syndrome (APS)
APS is the most clinically significant acquired thrombophilia. Your immune system produces antibodies (anticardiolipin, anti-beta-2 glycoprotein I, lupus anticoagulant) that attack phospholipids on cell membranes, paradoxically promoting clot formation. APS can cause both arterial clots (strokes, heart attacks) and venous clots (DVT, PE) — a pattern that should immediately raise suspicion. It’s also a leading cause of recurrent pregnancy loss.
Myeloproliferative Neoplasms: Polycythemia Vera and Essential Thrombocythemia
Polycythemia vera (PV) causes your bone marrow to overproduce red blood cells, thickening the blood and promoting clots. Essential thrombocythemia (ET) does the same with platelets. In both conditions, thrombosis — not bleeding — is the primary threat, and a clot may actually be the first symptom that leads to diagnosis. Clots in unusual locations (portal vein, hepatic veins, cerebral sinuses) should always prompt workup for these disorders.
Paroxysmal Nocturnal Hemoglobinuria (PNH)
PNH is rare but worth knowing about because thrombosis accounts for roughly 40–67% of deaths in untreated patients. A somatic mutation causes red blood cells to lack protective surface proteins, leading to complement-mediated destruction and a hypercoagulable state.
Symptoms That Should Raise a Red Flag
A single DVT in a 70-year-old post-surgical patient doesn’t necessarily point to a blood disorder. But certain patterns strongly suggest an underlying thrombophilia:
- First clot before age 40–45 without obvious provocation (no surgery, immobility, or hormonal trigger)
- Recurrent clots despite adequate anticoagulation
- Clots in unusual sites: portal vein, mesenteric vein, cerebral venous sinuses, hepatic veins (Budd-Chiari syndrome)
- Family history of VTE, especially in first-degree relatives under 50
- Recurrent pregnancy loss (2 or more miscarriages) — classic for APS
- Warfarin-induced skin necrosis — a hallmark of Protein C or Protein S deficiency
Classic clot symptoms themselves include leg swelling, calf pain, warmth, and redness (DVT), or sudden shortness of breath, chest pain, and rapid heart rate (pulmonary embolism).
How These Disorders Are Diagnosed
If your doctor suspects a clotting disorder, here are the tests you should expect:
- D-dimer: A screening test — elevated levels suggest active clot formation, but it’s not specific
- Activated Protein C resistance assay → screens for Factor V Leiden
- Prothrombin gene mutation analysis → genetic test, simple blood draw
- Antithrombin activity level
- Protein C and Protein S activity levels (must be drawn when patient is NOT on warfarin and NOT acutely clotting, as both can falsely lower results)
- Antiphospholipid antibody panel: lupus anticoagulant, anticardiolipin IgG/IgM, anti-beta-2 glycoprotein I IgG/IgM — must be positive on two occasions at least 12 weeks apart to confirm APS
- CBC with differential → catches polycythemia vera (elevated hematocrit >49% men, >48% women) and essential thrombocythemia (platelets >450,000/µL)
- JAK2 mutation testing → confirmatory for myeloproliferative neoplasms
Timing matters. Many hematologists prefer to test at least 2–4 weeks after an acute clot and after anticoagulation is paused (when safe), because acute thrombosis and blood thinners can distort results for Protein C, Protein S, and antithrombin levels.
Treatment Overview
Treatment depends on the specific disorder, but anticoagulation is the backbone of management for most thrombophilias:
- Initial treatment of acute clots: Low-molecular-weight heparin (enoxaparin) or direct oral anticoagulants (DOACs) like rivaroxaban or apixaban
- Long-term prevention: Warfarin (INR target 2.0–3.0) or DOACs. APS is one condition where warfarin is often preferred over DOACs, as trials (notably TRAPS) showed higher clot recurrence rates with rivaroxaban in triple-positive APS patients.
- Myeloproliferative neoplasms: Low-dose aspirin, phlebotomy (for PV), cytoreductive therapy (hydroxyurea) when indicated
- PNH: Complement inhibitors (eculizumab, ravulizumab) have transformed outcomes
Duration of anticoagulation ranges from 3–6 months for a first provoked clot to indefinite therapy for unprovoked recurrent clots, high-risk thrombophilias (antithrombin deficiency, homozygous Factor V Leiden), or APS.
When to See a Doctor
Go to the emergency room immediately if you develop sudden leg swelling with pain, unexplained shortness of breath, or chest pain — these can signal DVT or pulmonary embolism, which can be fatal within hours if untreated.
Schedule a hematology consultation if you have:
- A blood clot before age 45 with no clear cause
- Two or more clots at any age
- A strong family history of clotting (especially if relatives had clots young)
- Recurrent miscarriages
- A clot in an unusual location
Frequently Asked Questions
Can blood clotting disorders be cured?
Inherited thrombophilias cannot be cured — the genetic mutation is permanent. However, they can be managed very effectively with anticoagulation, and many carriers never experience a single clot. Acquired conditions like APS are chronic but controllable, while PNH now has disease-modifying treatments with complement inhibitors.
Should I get tested for a clotting disorder if a family member has Factor V Leiden?
It’s reasonable to discuss testing with your doctor, especially if you’re a first-degree relative. That said, routine screening of asymptomatic relatives is debated among hematologists. Testing becomes more clearly beneficial if you’re planning pregnancy, starting hormonal contraception, or facing major surgery — situations that independently raise clot risk.
Can I take birth control pills if I have a clotting disorder?
Estrogen-containing contraceptives (combined oral contraceptive pills, patches, vaginal rings) are generally contraindicated in known thrombophilia because estrogen independently increases clot risk by 2–6x. Progestin-only methods (mini-pill, hormonal IUD, implant) are considered safer alternatives. This is a conversation to have with both your hematologist and gynecologist.
Does flying increase my risk of clots if I have a blood disorder?
Yes. Long-haul flights (>4 hours) raise VTE risk in the general population by about 2–4x, and that risk compounds if you have an underlying thrombophilia. Practical measures include staying hydrated, performing calf exercises hourly, wearing graduated compression stockings, and — for high-risk patients — discussing prophylactic anticoagulation with your doctor before travel.
What’s the difference between a clotting disorder and “thick blood”?
“Thick blood” isn’t a medical diagnosis, but patients often use it to describe hyperviscosity or hypercoagulability. True hyperviscosity (physically thicker blood) occurs in conditions like polycythemia vera or Waldenström’s macroglobulinemia. Hypercoagulability (blood that clots too easily) is the hallmark of thrombophilias like Factor V Leiden or APS. The treatments are different, so getting the right diagnosis matters.
