Blood thickness, known medically as blood viscosity, depends on two main things: how many red blood cells you have and how much protein is dissolved in your plasma. When either of those rises beyond normal levels, blood flows more sluggishly through your vessels, forcing the heart to work harder and raising the risk of clots. Some causes are as simple as not drinking enough water, while others point to serious underlying conditions.
What Actually Determines Blood Thickness
Blood is not a uniform liquid. It’s a suspension of cells floating in plasma, and its thickness changes depending on what’s in it. Four factors control how easily blood flows: the volume of red blood cells (measured as hematocrit), the viscosity of the plasma itself, how flexible your red blood cells are as they squeeze through tiny capillaries, and how much your red blood cells clump together.
Of these, hematocrit has the biggest influence. Blood viscosity rises in a nearly straight line as the percentage of red blood cells increases. In a healthy person, red blood cells make up roughly 36 to 50 percent of total blood volume, depending on sex. Push that number higher and blood becomes measurably thicker.
Plasma viscosity is the other major driver. Plasma on its own is mostly water, but it contains dissolved proteins, especially fibrinogen and various globulins. In healthy people, fibrinogen circulates at concentrations of 2 to 5 milligrams per milliliter. During acute inflammation or infection, fibrinogen levels can spike above 7 mg/mL, noticeably increasing plasma thickness and, with it, whole blood viscosity. Any condition that raises the concentration of circulating proteins can have the same effect.
Everyday Causes of Thicker Blood
Dehydration
When you lose fluid through sweat, illness, or simply not drinking enough, plasma volume drops while your red blood cell count stays the same. The result is hemoconcentration: a higher ratio of cells to liquid, which makes blood thicker. A study on firefighters found that after heavy exertion, hematocrit, hemoglobin, and whole blood viscosity all climbed significantly. Rehydrating brought hematocrit and hemoglobin back to baseline, but viscosity remained elevated for a period even after fluids were replaced. This suggests dehydration has a more profound and lingering effect on blood thickness than the simple cell-count change alone would predict.
High Altitude
Living or traveling at high elevations triggers your body to produce more red blood cells to compensate for lower oxygen levels. This response, called erythrocytosis, has been recognized since it was first documented in 1890. Hematocrit typically climbs to a moderately high steady state after a few weeks at altitude and stays there as long as you remain. For most people this is a healthy adaptation, but in some individuals the response overshoots, leading to chronic mountain sickness and genuinely problematic blood thickness.
Smoking and Chronic Low Oxygen
Anything that chronically reduces the oxygen your blood can carry will stimulate red blood cell production. Smoking is one of the most common culprits. Carbon monoxide from cigarette smoke binds to hemoglobin, effectively reducing oxygen delivery, and the body responds by making more red blood cells. Chronic lung diseases have a similar effect.
Medical Conditions That Thicken Blood
Several diseases directly increase blood viscosity, sometimes dangerously so.
Polycythemia vera is a bone marrow disorder in which the body overproduces red blood cells. Hematocrit can climb well above the normal range, and the elevated level is directly linked to vascular occlusion and neurological events like stroke.
Waldenström macroglobulinemia is the single most common cause of hyperviscosity syndrome. It’s a type of blood cancer in which abnormal white blood cells produce excessive amounts of a large antibody protein. That protein floods the plasma, dramatically raising its viscosity. Multiple myeloma works through a similar mechanism, flooding the blood with abnormal immunoglobulin proteins.
Leukemia and thrombocytosis thicken blood through sheer cell numbers. Leukemia floods the bloodstream with white blood cells, while thrombocytosis involves an overproduction of platelets. Both increase the cellular fraction of blood and impair flow.
Conditions that trigger chronic inflammation can also raise viscosity indirectly. Inflammatory states push fibrinogen and other acute phase proteins to high levels, thickening the plasma. Autoimmune diseases, chronic infections, and even obesity can sustain this kind of low-grade inflammatory effect on blood.
How Thick Blood Affects Your Body
When blood flows too slowly through small vessels, tissues don’t get adequate oxygen. The early symptoms tend to be vague: headaches, fatigue, difficulty concentrating, blurred vision, muscle weakness, and tingling in the hands or feet. These happen because microvascular flow stalls, particularly in the brain and extremities.
The more dangerous consequence is clotting. Sluggish blood is more prone to forming clots in both arteries and veins. This can lead to stroke, transient ischemic attacks (mini-strokes), heart attack from coronary artery occlusion, and deep vein thrombosis in the legs. Dehydration on top of already thick blood makes the risk worse, because it further concentrates cells and proteins.
Interestingly, one common treatment for high red blood cell counts, repeated blood removal (phlebotomy), can backfire if done too aggressively. Draining blood depletes iron stores, and iron-deficient red blood cells become smaller and stiffer. Stiff cells don’t deform well as they pass through capillaries, which paradoxically increases viscosity and can raise stroke risk even as the total red blood cell count drops.
How Thick Blood Is Detected
A standard complete blood count (CBC) is usually the first step. It reveals your hematocrit and hemoglobin levels, both of which correlate directly with blood viscosity. If hematocrit is elevated, further testing looks for the reason: bone marrow disorders, oxygen-related triggers, or dehydration.
When protein-driven viscosity is suspected, blood tests measuring total protein, immunoglobulin levels, and fibrinogen concentration help identify the source. Serum viscosity can be measured directly in a lab, though this is typically done only when a condition like Waldenström macroglobulinemia or multiple myeloma is being investigated. Other markers that correlate with viscosity include white blood cell count, albumin, globulin, and cholesterol levels.
What You Can Control
For most people without an underlying blood disorder, the biggest modifiable factor is hydration. Drinking enough fluid throughout the day keeps plasma volume adequate and prevents the hemoconcentration that makes blood sluggish. This matters especially during exercise, hot weather, illness with vomiting or diarrhea, and air travel.
Quitting smoking removes a persistent trigger for excess red blood cell production. Regular physical activity also helps, both by improving cardiovascular fitness and by influencing how flexible your red blood cells are, which is one of the four key determinants of viscosity. Addressing chronic inflammation through weight management, diet, and treatment of underlying inflammatory conditions can help keep fibrinogen and other plasma proteins in a normal range.
If you have persistently elevated hematocrit on routine blood work, or you’re experiencing unexplained headaches, vision changes, and fatigue together, those are signs worth investigating further. The cause could be as straightforward as chronic dehydration or as significant as polycythemia vera, and the distinction matters for how it’s managed.