What Causes High Haematocrit?

High haematocrit means your blood has a higher proportion of red blood cells than normal. The main causes fall into three groups: dehydration, conditions that lower your blood plasma volume, and conditions that make your body produce too many red blood cells. Smoking, living at high altitude, and certain medical conditions like sleep apnoea or polycythaemia vera are common drivers. Understanding which category your result falls into matters because the treatment and the health risks differ completely.

What Is Haematocrit and What Do the Numbers Mean?

Haematocrit is the percentage of your blood volume made up of red blood cells. If your haematocrit is 45%, then 45% of your blood is red blood cells and the rest is mostly plasma, the liquid part of blood.

Normal ranges vary by laboratory, but generally they sit around 38% to 50% for men and 34% to 44% for women. Women typically run lower because of menstrual blood loss and lower testosterone levels, which stimulates red blood cell production.

When your haematocrit climbs above the upper limit, your blood becomes thicker. Thicker blood flows less easily through small vessels, which can raise your risk of blood clots, stroke, or heart attack. Not everyone with a high reading needs treatment, but it always deserves an explanation.

What Causes High Haematocrit in Dehydration and Low Plasma Volume?

Dehydration is the most common and most reversible cause of a high haematocrit reading. When you lose fluid, your plasma volume drops. The number of red blood cells stays the same, but they become more concentrated in a smaller amount of liquid.

This is called relative polycythaemia. The haematocrit looks high, but your body is not actually making extra red blood cells. It just has less fluid to suspend them in.

Common causes of dehydration include:

• Not drinking enough water, especially in hot weather or during exercise
• Vomiting or diarrhoea
• Fever
• Diuretic medications, including some blood pressure drugs
• Excessive alcohol intake

Severe burns can also cause this pattern because large amounts of plasma are lost from the damaged skin surface.

The key clue that dehydration is the cause is that your haematocrit returns toward normal once you rehydrate. If your doctor suspects dehydration, they may simply ask you to drink fluids and repeat the blood test. No further investigation is needed if the repeat result is normal.

How Does Smoking and Lung Disease Raise Haematocrit?

Smoking raises haematocrit through a completely different mechanism. Carbon monoxide from cigarette smoke binds to haemoglobin, the oxygen-carrying protein inside red blood cells. This binding blocks oxygen from attaching normally.

Your kidneys detect that oxygen delivery to the tissues is low. They respond by releasing a hormone called erythropoietin, which tells your bone marrow to produce more red blood cells. This is an appropriate physiological response to a problem, but the problem is caused by smoking, not by a genuine need for more cells.

Chronic lung diseases such as COPD can cause the same effect. Damaged lungs transfer oxygen less efficiently into the blood. The kidneys again sense low oxygen and stimulate red blood cell production. This is called secondary polycythaemia because the high red cell count is secondary to another condition.

Living at high altitude produces the same adaptive response. The air at altitude contains less oxygen, so your body compensates by making more red blood cells to carry what oxygen is available.

What Is Polycythaemia Vera and How Is It Different?

Polycythaemia vera is a bone marrow disorder where your body makes too many red blood cells regardless of oxygen levels. It is a type of myeloproliferative neoplasm, meaning the bone marrow produces excess cells due to a genetic mutation.

Most people with polycythaemia vera have a mutation in a gene called JAK2. This mutation causes the bone marrow to be overactive. The result is a genuinely high red blood cell mass, not just concentrated blood.

Unlike dehydration-related high haematocrit, polycythaemia vera does not correct itself with fluid intake. Unlike smoking-related high haematocrit, it does not improve when oxygen delivery improves. The bone marrow simply keeps producing cells.

People with polycythaemia vera often have other blood cell lines elevated too, including white blood cells and platelets. They may experience itching after a warm shower, headaches, dizziness, or a reddened complexion. Some people have no symptoms at all and are only diagnosed after a routine blood test.

Polycythaemia vera is rare. Most people with a high haematocrit do not have it. But it is important to identify because it carries a higher risk of blood clots and requires ongoing management, often including regular blood removal called venesection.

What Role Do Sleep Apnoea and Other Conditions Play?

Obstructive sleep apnoea is an under-recognised cause of high haematocrit. During sleep, the airway repeatedly collapses, stopping breathing for short periods. Blood oxygen levels drop repeatedly through the night.

Your kidneys respond to these repeated oxygen dips by producing more erythropoietin. Over time, this can push your haematocrit above normal. Some research suggests that treating sleep apnoea with CPAP therapy can lower haematocrit levels, although the response varies between individuals.

Other causes of chronic low oxygen include:

• Congenital heart disease where blood bypasses the lungs
• Pulmonary fibrosis
• Severe obesity, which can impair breathing

Certain tumours, particularly kidney cancers and some liver tumours, can produce erythropoietin themselves. This is called inappropriate erythropoietin secretion. The tumour behaves like a rogue hormone factory, driving red blood cell production even when oxygen levels are normal.

Anabolic steroid use is another recognised cause. Testosterone and related compounds stimulate red blood cell production. This is why athletes who misuse these substances often have elevated haematocrit readings.

When Should High Haematocrit Worry You?

A single mildly elevated haematocrit reading is rarely an emergency. If you were dehydrated at the time of the test, the result may simply reflect that. Your doctor will likely repeat the test under better conditions before pursuing further investigation.

Persistent elevation above the normal range deserves attention. The main concern is blood viscosity. Thicker blood moves more slowly and is more likely to clot inside veins or arteries. This raises the risk of deep vein thrombosis, pulmonary embolism, stroke, and heart attack.

Symptoms that warrant urgent medical attention include:

• Chest pain or shortness of breath
• Sudden weakness or numbness on one side of the body
• Difficulty speaking
• Swelling or pain in one leg
• Sudden vision changes

These symptoms can indicate a blood clot and require immediate evaluation. Do not wait to see if they pass.

If your haematocrit is very high, such as above 60%, the risk of complications increases substantially. At this level, blood flow through small vessels becomes noticeably impaired, and you may experience headaches, dizziness, or visual disturbances.

How Is High Haematocrit Treated?

Treatment depends entirely on the underlying cause. There is no single approach that works for everyone.

For dehydration-related elevation, the treatment is fluid replacement. Drinking water or receiving intravenous fluids restores plasma volume and brings the haematocrit back to normal. No further treatment is needed once hydration is corrected.

For smoking-related elevation, stopping smoking is the most effective intervention. Haematocrit levels typically fall over weeks to months after cessation as the body clears carbon monoxide and reduces red blood cell production. This is one of many health reasons to quit.

For high-altitude residents, moving to lower altitude lowers haematocrit, but this is not practical for most people. Their bodies have adapted to their environment, and the elevated haematocrit is a normal response to that environment.

For polycythaemia vera, treatment focuses on reducing red blood cell mass to lower clot risk. Regular venesection removes blood to bring haematocrit down. Some people also need medications to suppress bone marrow activity. Treatment is lifelong and monitored by a haematologist.

For sleep apnoea-related elevation, CPAP therapy treats the underlying oxygen drops. Some people see their haematocrit normalise after consistent CPAP use, though this can take months.

What Tests Follow a High Haematocrit Result?

If your haematocrit remains elevated after correcting dehydration, your doctor will typically order further tests to find the cause.

A full blood count looks at whether other cell lines are also elevated. If white blood cells and platelets are high alongside red cells, polycythaemia vera becomes more likely. If only red cells are high, secondary causes become more likely.

An erythropoietin level can help distinguish between causes. In secondary polycythaemia, erythropoietin levels are typically normal or high because the body is appropriately responding to low oxygen. In polycythaemia vera, erythropoietin levels are usually low because the bone marrow does not need hormonal stimulation to produce excess cells.

Oxygen saturation testing checks whether your blood is carrying enough oxygen. A low reading points toward lung disease, sleep apnoea, or heart conditions that cause secondary polycythaemia.

Testing for the JAK2 mutation confirms polycythaemia vera in most cases. If this mutation is present alongside a high haematocrit and low erythropoietin, the diagnosis is usually clear.