High hemoglobin and hematocrit on a blood test mean your blood is carrying more red blood cells than normal, either because your body is producing too many or because the liquid portion of your blood (plasma) has dropped low enough to concentrate the cells you already have. For men, levels above 16.5 g/dL hemoglobin or 49% hematocrit raise concern; for women, the thresholds are 16 g/dL and 48%. The cause can be as simple as dehydration or as serious as a bone marrow disorder, so the next steps depend entirely on why it’s happening.
Dehydration: The Most Common Cause
The single most common reason for a high hematocrit reading is dehydration. When you lose fluid through illness, heat exposure, heavy exercise, or simply not drinking enough water, your plasma volume shrinks. The number of red blood cells hasn’t changed, but they’re now packed into less liquid, so the percentage looks elevated. This is sometimes called “relative” or “spurious” polycythemia because it’s a measurement artifact, not a true increase in red blood cells.
The tip-off is usually context. If you gave blood after fasting, exercising hard, or being sick with vomiting or diarrhea, a mildly elevated result may correct itself once you’re properly hydrated. Your doctor can recheck the levels after you’ve had fluids, and if they normalize, no further workup is needed.
Low Oxygen Levels and Secondary Causes
When your tissues aren’t getting enough oxygen, your kidneys release a hormone called erythropoietin (EPO) that tells the bone marrow to ramp up red blood cell production. This is the body doing exactly what it’s designed to do, but it pushes hemoglobin and hematocrit above normal ranges. The most common triggers include:
- Chronic lung disease such as COPD or emphysema, which limits oxygen exchange
- Smoking, which introduces carbon monoxide and reduces the oxygen your blood can carry, leading to what’s called smoker’s erythrocytosis
- Living at high altitude, where thinner air contains less oxygen
- Obstructive sleep apnea, where repeated breathing pauses overnight cause intermittent drops in oxygen
- Congenital heart defects that mix oxygen-rich and oxygen-poor blood
Sleep apnea deserves special mention because many people don’t realize they have it. A meta-analysis of over 3,600 patients found that about 2% of people with mild to moderate sleep apnea develop elevated red blood cell counts, and that figure rises to roughly 6% in severe cases. If your high levels come alongside daytime fatigue, loud snoring, or morning headaches, a sleep study may be a logical next step.
Less commonly, certain tumors (particularly kidney, liver, or brain tumors) can produce EPO on their own, driving red blood cell production without any actual oxygen shortage. Kidney diseases, including cysts and polycystic kidney disease, can do the same. Anabolic steroid use and testosterone therapy are also known causes.
Polycythemia Vera: A Bone Marrow Problem
Polycythemia vera (PV) is a different situation entirely. Rather than the body responding to low oxygen, the bone marrow itself is overproducing red blood cells due to a genetic mutation. About 98 to 99% of people with PV carry a mutation in a gene called JAK2, which essentially keeps the “make more blood cells” signal stuck in the on position. The marrow often overproduces white blood cells and platelets too, not just red blood cells.
PV is classified as a type of blood cancer, though it typically progresses slowly. The immediate danger isn’t the disease transforming into something aggressive (though that can happen over years) but rather the thickening of the blood itself, which raises the risk of dangerous clots. An enlarged spleen, intense itching after warm showers, and night sweats are characteristic features that help distinguish PV from other causes.
How Doctors Identify the Cause
Once high hemoglobin and hematocrit are confirmed on a repeat test (ruling out dehydration), the diagnostic path typically involves two key blood tests. The first is a serum EPO level. In polycythemia vera, EPO is usually very low because the bone marrow is producing cells on its own, ignoring the normal feedback loop. Over 85% of PV patients have a below-normal EPO level. In secondary causes like lung disease or sleep apnea, EPO tends to be normal or elevated because the kidneys are deliberately calling for more red blood cells.
The second test is for the JAK2 mutation. A positive result, combined with high hemoglobin and low EPO, points strongly toward polycythemia vera. If the JAK2 test is negative and EPO is elevated, the search shifts to identifying what’s starving your tissues of oxygen or what might be producing EPO inappropriately.
Symptoms to Pay Attention To
Mildly elevated levels often cause no symptoms at all, which is why they’re frequently caught on routine blood work. As hemoglobin and hematocrit climb higher, though, the blood becomes thicker and flows less efficiently. This can produce headaches, dizziness, blurred or double vision, and a flushed or reddened face. Some people notice unusual fatigue despite having plenty of red blood cells, because the thickened blood actually delivers oxygen less effectively to small vessels.
Itching after a bath or shower (called aquagenic pruritus) is a hallmark of polycythemia vera specifically and is worth mentioning to your doctor. Unusual bleeding, like frequent nosebleeds or bleeding gums, can also signal that something is off with blood cell production. Shortness of breath when lying down, heavy night sweats, and blind spots in your vision are all reasons to follow up promptly.
Health Risks of Persistently High Levels
The primary concern with chronically elevated hemoglobin and hematocrit is blood clots. Thicker blood moves more slowly through vessels and is more prone to clotting, which can lead to deep vein thrombosis, pulmonary embolism, heart attack, or stroke. A large population-based study found that people in the highest hematocrit group (above 48.6% for men, above 43.2% for women) had a 17% higher incidence of stroke compared to those in the lowest group. The association was strongest for ischemic strokes, the type caused by a clot blocking blood flow to the brain. Higher hematocrit has also been linked independently to ischemic heart disease and venous blood clots.
How High Levels Are Managed
Treatment depends on the underlying cause. For secondary polycythemia driven by low oxygen, the priority is treating whatever is reducing your oxygen levels. That might mean CPAP therapy for sleep apnea, better management of COPD, or quitting smoking. When someone with lung disease has a hematocrit above 56% or develops symptoms of thick blood, therapeutic phlebotomy (essentially a controlled blood draw) can bring the level down to the 50 to 52% range.
For polycythemia vera, the target is stricter. Research has shown that keeping hematocrit below 45% significantly lowers the rate of blood clots. One study found that reducing hematocrit to an average of 45.5% improved blood flow to the brain by 73%. Treatment typically involves periodic phlebotomy sessions where about 450 mL of blood (roughly one pint) is drawn, similar to donating blood. Sessions may happen frequently at first and then taper to every one to two months for maintenance. Low-dose aspirin is also commonly used to reduce clotting risk.
For people whose elevated readings were caused by dehydration or a temporary situation, no ongoing treatment is needed beyond addressing the root cause and rechecking levels to confirm they’ve returned to normal.