High haematocrit: what it means and what to do

A high haematocrit often just means you drank too little before the blood test. Many lab reports call it high above 46 for women and above 50 for men. If it stays high when you are well hydrated, your doctor looks for the reason. It can be smoking, testosterone, sleep apnoea or, more rarely, a bone marrow condition.

What high haematocrit means

Haematocrit is the share of your blood made of red cells. It rises when red cells increase, but also when the liquid part shrinks, for example if you drank too little.

That is why the first step is to repeat the test well hydrated. If it stays high, haemoglobin and red blood cells are usually high too, and your doctor looks for the cause.

When it is very high and stays there, the blood gets thicker and flows less easily. That can raise the risk of clots: it is why it should be seen by a doctor.

What causes high haematocrit

Start with the simplest one to rule out.

• Too little water: If you drank little or sweated a lot, the blood gets more concentrated and the number rises. It comes back down once you drink normally again.
• Smoking: People who smoke often have a higher haematocrit. Stopping usually brings it down.
• Testosterone or anabolic steroids: They push the bone marrow to make more red cells. It is one of the most frequent side effects of testosterone therapy.
• Altitude: People who live high up, or spend weeks there, make more red cells to make up for the thinner air.
• Too little oxygen: It happens with sleep apnoea or some lung and heart diseases: the body makes up for it with more red cells. Loud snoring and waking up tired can be a sign.
• A bone marrow condition: Rarer: the bone marrow makes too many red cells on its own. It is called polycythaemia vera, and a haematologist looks after it.

Symptoms

Often none: it is usually found by chance, on a blood test.

When it has been high for a while, headaches, dizziness, tiredness and a redder face than usual can appear. These complaints have many other causes too, though.

High haematocrit and testosterone

People who take testosterone, as a treatment or for the gym, often have a higher haematocrit. The risk is higher with some injections than with gels or patches.

If you take it, tell the doctor who prescribes it, or your own doctor: the doctor decides whether to change anything. Do not change anything yourself. And if you use anabolic steroids without a prescription, say so anyway: it helps read the number right.

Which tests to read alongside it

These tests help show whether the rise is real and where it comes from.

What your result means

The upper end of the scale, in %.

• Within your report’s limits, Women: usually 36 to 46, Men: usually 40 to 50: Normal. Usually fine. Always read it together with your haemoglobin. In the Rosa report you also see whether you are in your optimal range.
• Above your report’s limit, Women: usually above 46, up to 48, Men: usually above 50, up to 52: High. Often it just means too little water before the draw. Sometimes smoking or testosterone is behind it. Repeat the test well hydrated. If it stays high, talk to your doctor.
• Women: above 48, up to 56, Men: above 52, up to 56: Very high. It can still be too little water. If it stays there, the cause needs finding. Talk to a doctor: they will usually ask you to repeat the test.
• Above 56: Extremely high. The blood is much thicker, which can raise the risk of clots. See a doctor the same day, even if you feel well.

How to lower your haematocrit

It depends on the cause. Where to start:

• Repeat the test well hydrated: drink normally the day before and a glass of water in the morning, even while fasting.
• If you smoke, try to stop: the number usually comes down.
• If you take testosterone or anabolic steroids, tell your doctor, who decides whether to change anything.
• If you snore loudly or wake up tired, talk to your doctor: sleep apnoea can be treated.
• What to eat: no food lowers it. What counts is drinking enough, especially in the heat or when you do sport.
• Do not give blood on your own to lower it: the cause comes first. If blood needs to be removed, your doctor decides.

When to worry

Talk to a doctor if:

• Your haematocrit is above 48 if you are a woman, or above 52 if you are a man.
• It is above 56: see a doctor the same day, even if you feel well.
• It stays high when the test is repeated well hydrated.
• It rises from one test to the next.
• You often have headaches or feel dizzy.
• You have pain and swelling in one leg, sudden shortness of breath or chest pain: call 112 or go to the emergency department straight away.

With Rosa, a haematocrit above 52 in men or above 48 in women is always flagged to a doctor, above 56 urgently.

Frequently asked questions

Haematocrit at 53: is it serious? It is usually not an emergency, but it is above the line where a doctor should look at it, for women and men alike. The doctor often has the test repeated well hydrated and, if it stays high, looks for the cause. With Rosa, this value is always flagged to a doctor.

High haematocrit: when should I worry? When it is above 48 in women or above 52 in men, especially if it stays high on a second test done well hydrated. Above 56, see a doctor the same day.

How do you lower a high haematocrit? By removing the cause: drinking enough, stopping smoking, reviewing any testosterone therapy with your doctor. No food lowers it, and only your doctor decides whether blood should be removed.

Can dehydration raise haematocrit? Yes, and it is the first thing to rule out. With too little water the liquid part of the blood shrinks, so red cells take up more room even though there are no more of them. Drink normally and repeat the test.

Does a high haematocrit mean thick blood? Not necessarily. Often the number is high only because you drank too little. If it really is high and stays there, the blood is thicker and the risk of clots can rise: that is why a doctor should see it.

Sources

• British Society for Haematology, guideline for the diagnosis and management of polycythaemia vera. British Journal of Haematology, 2019. PMID 30478826
• British Society for Haematology, guideline for specific situations in polycythaemia vera and for raised red cells from other causes. British Journal of Haematology, 2019. PMID 30426472
• Ohlander and colleagues, raised red cells after testosterone therapy. Sexual Medicine Reviews, 2018. PMID 28526632
• Lippi and colleagues, a tourniquet kept tight during the draw and routine blood count tests. Clinical and Laboratory Haematology, 2006. PMID 16999725
• British Society of Gastroenterology, guidelines for iron deficiency anaemia in adults. Gut, 2021. PMID 34497146