“Thick blood” means your blood is more viscous than normal, making it harder for your heart to pump it through your vessels and more likely to form clots. Doctors measure this with a serum viscosity test: normal blood has a viscosity around 1.5 centipoise, and symptoms typically begin once it exceeds 4 centipoise. The thickness can come from having too many red blood cells, too many proteins floating in your plasma, or inherited conditions that make your blood clot too easily.
The term covers a range of conditions rather than a single diagnosis. Some are temporary and fixable, like dehydration. Others are chronic and need ongoing management, like polycythemia vera or genetic clotting disorders. What matters is figuring out why the blood is thick, because that determines what happens next.
What Makes Blood Thick
Blood viscosity depends on two main things: how many cells are packed into it and what’s dissolved in the liquid portion (plasma). When either of those increases beyond normal levels, the blood thickens.
The most straightforward cause is having too many red blood cells. Your hematocrit, the percentage of your blood volume made up of red blood cells, normally falls between 40 and 54% for men and 36 and 48% for women. When hematocrit climbs above those ranges, blood gets sluggish. Hemoglobin, the oxygen-carrying protein inside red blood cells, also rises in parallel. Normal hemoglobin runs 14 to 18 g/dL for men and 12 to 16 g/dL for women.
The other route to thick blood is an excess of proteins in the plasma. Certain blood cancers, particularly a type called Waldenström macroglobulinemia, cause the body to produce massive amounts of a single antibody protein. These proteins crowd the plasma and dramatically increase viscosity, sometimes pushing it above 6 centipoise, at which point most people are clearly symptomatic.
Common Causes
Polycythemia vera (PV) is one of the most well-known causes of thick blood. It’s a slow-growing blood cancer where the bone marrow overproduces red blood cells, white blood cells, and platelets all at once. About 95% of cases involve a specific gene mutation called JAK2, which essentially tells the bone marrow to keep producing cells when it should stop. PV is diagnosed when hemoglobin exceeds 16.5 g/dL in men or 16.0 g/dL in women, alongside that mutation and sometimes a bone marrow biopsy showing overactive cell production.
Inherited clotting disorders take a different path. Rather than making the blood physically thicker, they make it clot more aggressively. Factor V Leiden is the most common inherited clotting disorder worldwide. People who carry it produce a clotting protein that resists being shut off: it takes 10 to 20 times longer than normal for their body to deactivate this protein once clotting starts. The result is excess clot formation and a lifelong tendency toward blood clots, particularly deep vein thrombosis. Prevalence varies by region, running as high as 9 to 15% of the population in southeastern Europe and around 13% in the Middle East.
Everyday factors play a role too. Chronic dehydration reduces the liquid portion of your blood, concentrating the cells and proteins that remain. Smoking is another contributor. Carbon monoxide from tobacco binds to hemoglobin, reducing how much oxygen each red blood cell can carry. The body compensates by producing more red blood cells, which increases total red cell volume and thickens the blood over time. Research comparing smokers to nonsmokers has found significantly higher hemoglobin mass, total blood volume, and red cell volume in smokers.
Symptoms of Thick Blood
When blood is too thick, it struggles to flow through small vessels. The brain and eyes are especially vulnerable because they depend on extensive networks of tiny capillaries.
Neurological symptoms are often the first sign. Headaches, dizziness, and a general feeling of mental fogginess are common. In more severe cases, people experience hearing changes, difficulty with coordination, numbness or tingling in their extremities, or even seizures. Vision problems show up frequently too: blurred or double vision caused by tiny clots or bleeding in the blood vessels at the back of the eye.
Unexplained bleeding is the symptom that surprises most people. You’d expect thick blood to clot more, not less. But when the blood is extremely viscous, it damages the delicate lining of small vessels, particularly in mucous membranes. Nosebleeds, bleeding gums, and easy bruising can all result. The combination of bleeding, vision changes, and neurological symptoms is the classic presentation doctors look for.
Not everyone with mildly elevated viscosity notices symptoms. Many people with borderline-high hematocrit feel fine and only discover the issue through routine blood work. Symptoms tend to appear gradually and worsen as viscosity increases.
How It’s Diagnosed
A standard complete blood count (CBC) is usually the first step. This measures your red blood cell count, hemoglobin, and hematocrit. If your hematocrit is elevated, your doctor will want to figure out why.
A serum viscosity test directly measures how thick your blood is in centipoise units. Normal is around 1.5 cP. Symptoms generally start appearing above 4 cP, and most people are clearly symptomatic above 6 cP. This test helps determine how urgently treatment is needed.
From there, testing branches depending on what’s suspected. Genetic testing can identify mutations like JAK2 (for polycythemia vera) or Factor V Leiden (for inherited clotting disorders). A bone marrow biopsy may be needed to confirm a blood cancer diagnosis. Protein electrophoresis, a test that separates the proteins in your blood by type, can reveal the abnormal antibody overproduction seen in conditions like Waldenström macroglobulinemia.
Risks and Complications
The most serious risk of thick blood is abnormal clotting. Blood that moves slowly through vessels is more likely to form clots, and those clots can lodge in dangerous places. Deep vein thrombosis, where a clot forms in a leg vein, can break free and travel to the lungs (pulmonary embolism). Clots can also form in the brain’s blood vessels, causing a stroke. Blood disorders account for up to 8% of all ischemic strokes. People with polycythemia vera are at particularly high risk for these vascular events.
Over time, chronically thick blood also strains the heart. Pumping viscous blood requires more effort, which can contribute to high blood pressure and eventually heart enlargement or heart failure if left unmanaged.
How Thick Blood Is Treated
Treatment depends entirely on the underlying cause. For polycythemia vera, the primary tool is therapeutic phlebotomy, which is essentially a controlled blood draw. A technician removes about 500 mL (roughly one pint) at a time, a process that takes less than 10 minutes. This is repeated weekly to monthly until hematocrit drops below 50%. Over time, regular removal depletes the body’s iron stores, which naturally slows down red blood cell production. Some people need phlebotomy only a few times a year once their levels stabilize; others need it more frequently.
When thick blood is caused by excess proteins in the plasma, a procedure called plasmapheresis filters the blood. Your blood is drawn out, the protein-heavy plasma is separated and discarded, and the cleaned blood cells are returned with replacement fluid. This provides rapid relief but doesn’t address the underlying disease, so it’s typically combined with treatments that target the overproducing cells.
For inherited clotting disorders like Factor V Leiden, the approach focuses on preventing clots rather than thinning the blood itself. Blood-thinning medications reduce the blood’s ability to form clots. Not everyone with a clotting mutation needs lifelong medication. Many carriers only take blood thinners during high-risk periods, such as after surgery, during long flights, or during pregnancy.
Lifestyle changes matter across all causes. Staying well hydrated keeps the plasma volume adequate. Quitting smoking removes the carbon monoxide stimulus that drives excess red blood cell production. Regular movement prevents blood from pooling in the legs, where clots are most likely to form.