Hematocrit, the percentage of your blood volume made up of red blood cells, rises through two basic paths: your body either produces more red blood cells or loses plasma fluid so the existing cells become more concentrated. Normal hematocrit ranges from about 38% to 49% in men and 36% to 45% in women. Everything from dehydration to chronic lung disease to hormone therapy can push those numbers higher.
Dehydration: The Most Common Cause
The fastest way hematocrit rises is when your blood loses water. You don’t gain any new red blood cells; the liquid portion of your blood simply shrinks, making the cells take up a bigger share of total blood volume. This is sometimes called relative polycythemia because the red blood cell count itself hasn’t changed.
Research on athletes shows how quickly this happens. During intense exercise in a dehydrated state, hematocrit climbed from a resting median of about 48% to nearly 52% at peak effort. Once exercise stopped, levels drifted back to baseline within 30 minutes, even without drinking fluids, as the body redistributed water back into the bloodstream. Anything that depletes body water, including vomiting, diarrhea, heavy sweating, or simply not drinking enough, can produce the same temporary bump.
High Altitude
When you travel to elevations above roughly 2,500 meters (8,000 feet), the air contains less oxygen. Your kidneys detect the drop and release erythropoietin (EPO), a hormone that tells bone marrow to ramp up red blood cell production. Over a few weeks, hematocrit climbs to a new, higher steady state and stays there as long as you remain at altitude.
How high it goes depends on genetics and geography. Andean highlanders living around 4,000 meters typically carry hemoglobin levels of 17 to 19 g/dL, noticeably above sea-level norms. Tibetans at similar elevations average only 14 to 16 g/dL, a sign that their population has adapted over thousands of years to avoid excessive red blood cell production. When elevation triggers too many red blood cells, a condition called chronic mountain sickness, blood viscosity rises enough to impair blood flow to the brain and heart.
Smoking
Cigarette smoke delivers carbon monoxide directly into the lungs. Carbon monoxide binds to hemoglobin far more tightly than oxygen does, creating an inactive form called carboxyhemoglobin that can’t carry oxygen. It also shifts how hemoglobin releases oxygen to tissues, making each remaining functional molecule less efficient. The body reads this as an oxygen shortage and compensates by producing more red blood cells, gradually raising hematocrit.
Population studies consistently show that current smokers have higher hemoglobin and hematocrit than nonsmokers. The effect is dose-dependent: heavier smokers tend to see larger increases. Quitting allows carboxyhemoglobin to clear within days, and over time, red blood cell production normalizes.
Lung Disease and Sleep Apnea
Any condition that chronically lowers blood oxygen can trigger the same EPO-driven response seen at high altitude. In advanced COPD, the lungs can no longer deliver enough oxygen, and the body compensates by making extra red blood cells. About 6% of stable COPD outpatients develop true polycythemia, and roughly 8% of those with severe disease on supplemental oxygen have hematocrit above 55%. The relatively low numbers reflect how effectively long-term oxygen therapy blunts the stimulus for red blood cell overproduction.
Obstructive sleep apnea works through a different timing pattern but the same underlying mechanism. Repeated airway collapse during sleep causes intermittent drops in oxygen saturation, which stabilize a protein called hypoxia-inducible factor. That protein promotes EPO secretion and red blood cell proliferation. Studies of untreated sleep apnea patients show polycythemia rates between about 2% and 10%, with prevalence climbing alongside disease severity. The amount of time a person’s oxygen saturation spends below 90% during sleep is one of the strongest predictors of elevated hematocrit in this group.
Testosterone Therapy
Testosterone directly stimulates red blood cell production, making elevated hematocrit one of the most common side effects of testosterone replacement therapy. Hematocrit begins rising within the first month of treatment and continues climbing for at least three months in a dose-dependent pattern. Older men are especially susceptible: in one study, 75% of men aged 60 to 75 on a moderate dose reached peak hematocrit within 12 weeks, compared to 42% of younger men on the same dose.
Clinical guidelines flag a hematocrit above 54% as the threshold where testosterone should be stopped and blood removal (phlebotomy) considered. Men on testosterone therapy typically have their hematocrit monitored every few months, particularly in the first year.
Polycythemia Vera
Polycythemia vera is a slow-growing blood cancer in which the bone marrow overproduces red blood cells independent of any oxygen signal. It is driven by a mutation in a gene called JAK2, present in nearly all cases. Current diagnostic criteria define the hematocrit thresholds as above 49% in men and above 48% in women, combined with the JAK2 mutation and often an enlarged spleen or abnormal bone marrow biopsy.
Unlike the other causes on this list, polycythemia vera doesn’t resolve by fixing an external trigger. It requires ongoing management to keep hematocrit below 45%, a target established by a major trial showing that patients maintained below that level had significantly fewer blood clots than those allowed to drift between 45% and 50%.
Kidney Tumors
The kidneys are the body’s primary source of EPO, so it makes sense that certain kidney cancers can hijack that system. Clear cell renal cell carcinoma, the most common subtype of kidney cancer, frequently overproduces EPO due to mutations in a tumor suppressor gene called VHL. These mutations stabilize the same hypoxia-sensing pathway that altitude and lung disease activate, but they do so continuously and without any actual oxygen shortage.
Despite the frequent EPO overproduction in these tumors, only 1% to 5% of kidney cancer patients actually develop polycythemia. The rest may produce EPO that gets consumed by the tumor itself or that doesn’t reach high enough blood levels to meaningfully boost red blood cell counts. Still, unexplained polycythemia in an otherwise healthy adult sometimes leads to the discovery of a kidney tumor.
Why High Hematocrit Matters
Red blood cells make blood thicker. As hematocrit climbs, viscosity increases, and at a certain point the heart has to work harder to push blood through smaller vessels. Cerebral blood flow measurably drops once hematocrit rises high enough in people with normal blood volume. A large study of over 7,300 men found that the stroke risk from elevated hematocrit was concentrated in those who also had high blood pressure, rather than being a universal risk. In people with polycythemia vera, keeping hematocrit below 45% through treatment cut the rate of blood clots compared to a more relaxed target of 45% to 50%.
A mildly elevated reading on a single blood test often reflects something as simple as not drinking enough water before the draw. Persistent elevation, especially above 50%, points toward one of the conditions above and typically prompts further testing to figure out whether the body is making too many red blood cells or simply concentrating the ones it has.