Haematocrit: what it is, what it tells you and what a low level means

Haematocrit shows how much of your blood is made of red cells, the cells that carry oxygen. If it is low, haemoglobin is usually low too: that is anaemia, often from too little iron. That is why it is always read together with haemoglobin.

Why we test it

Red cells carry oxygen all around your body. Haematocrit shows how much room they take up in your blood, as a percentage. The rest is the liquid part, plasma.

If it is low, it can be a sign of anaemia. If it is very high and stays there, the blood gets thicker and should be seen by a doctor.

It is part of the full blood count, the most common blood test. On your report it appears as HCT or Ht.

What it tells you, and what it does not

If it is low, haemoglobin is usually low too: that is called anaemia. Iron deficiency is often behind it.

If it is high, the reason is often simple: too little water before the draw. Of the whole Rosa panel, it is the number most affected by water. More rarely smoking, testosterone or a condition to look into with your doctor is behind it.

On its own it adds little to haemoglobin. It mostly works as a check: if the two do not agree, the first suspect is a problem with the sample.

That is why it is read together with other tests:

• Haemoglobin: Whether there is anaemia
• Red blood cells: How many there are, not just how much room they take
• Ferritin: Whether iron is short, a frequent cause of a low level
• MCV: How big your red cells are

Normal haematocrit levels

The lab decides what is normal. It is usually 36 to 46% for women and 40 to 50% for men. Your own report may print different limits.

Men have a higher level: testosterone pushes the body to make more red cells.

It also moves with water: it rises if you drank too little or sweated a lot, with no change in your red cells. In pregnancy it is normally lower.

In the Rosa report you also see your optimal range, read together with your haemoglobin.

What your result means

Find the row for your result, in %. Which values are dangerous? Above 48 for women and above 52 for men should be seen by a doctor, above 56 urgently. If it is low, haemoglobin is what counts: below 11 g/dL it should be seen by a doctor, below 8 the same day.

• Below your report’s limit, Women: usually below 36, Men: usually below 40: Low. It usually goes with a low haemoglobin. Sometimes the blood is just more diluted, as in pregnancy. Check your haemoglobin and ferritin and look for the cause with your doctor: today if your haemoglobin is below 8 g/dL.
• 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.

If your haematocrit is low or high

• Low haematocrit: It usually goes with a low haemoglobin. The causes and symptoms are below.
• High haematocrit: Often it just means too little water before the draw. If it stays high, the cause needs finding.

Low haematocrit: the causes

They are usually the same as for low haemoglobin. More rarely the kidneys are involved.

• Too little iron: A common cause. Without enough iron your body makes fewer red cells, and smaller ones.
• Heavy periods: In women before menopause, a very common cause of low iron.
• Blood loss: Small hidden losses, for example from the gut, or a recent blood donation. In men and after menopause, hidden losses are the first thing doctors look for.
• Too little B12 or folate: B12 and folate are also needed to make red cells. B12 runs short more often in people who eat no animal foods or have taken certain medicines for years.
• Inherited traits: For example thalassaemia trait (Mediterranean anaemia), which is not rare in Italy. It usually causes no problems.
• More diluted blood: In pregnancy, or with a lot of endurance sport, the liquid part of the blood grows and the number drops. In pregnancy this is usually normal; in athletes, ferritin helps tell whether iron is short too.

Low haematocrit: the symptoms

If the drop is small, often none. When there is anaemia, these can appear:

• Tiredness that does not go away
• Shortness of breath on stairs or during sport
• Pale skin
• A racing heart
• Dizziness

These symptoms can have other causes too: haemoglobin and ferritin help tell whether anaemia is involved.

Low haematocrit in pregnancy

A small drop is usually normal. In pregnancy your blood volume grows, and the liquid part grows more than the red cells. So haematocrit drops, especially from the second trimester on.

But pregnancy also needs a lot more iron, and a shortage is common. That is why your obstetrician checks your haemoglobin and, if needed, your ferritin, and decides whether you need a supplement.

What can skew the result

A few things change the number for a while. It helps to know them before you read it.

• Too little water: If you drank little or sweated a lot, the number rises. You can drink water even while fasting.
• Altitude: People who live high up, or spend weeks there, make more red cells, and the number rises.
• Smoking: Smoke brings a gas into your blood that takes the place of oxygen. Your body makes up for it with more red cells, and the number rises.
• The tourniquet: If the band on your arm stays tight for a minute or more before the draw, the blood gets more concentrated and the number reads a little higher.

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 low together with your haemoglobin, especially if you are a man or a woman after menopause.
• It changes a lot from one test to the next, for no clear reason.
• You are short of breath at rest, have chest pain or feel faint: call 112 or go to the emergency department.

With Rosa, a haematocrit above 52 in men or above 48 in women is always flagged to a doctor, above 56 urgently. For a low level, haemoglobin is what counts: below 11 g/dL it is always flagged to a doctor, below 8 urgently.

Frequently asked questions

What is haematocrit, in plain words? It is the share of your blood made of red cells, as a percentage. The rest is the liquid part, plasma. On your report it appears as HCT or Ht, in the blood count.

What are normal haematocrit levels? Usually 36 to 46% in women and 40 to 50% in men, but what counts is the range your own lab prints. In pregnancy it is normally lower.

Low haematocrit: when should I worry? When your haemoglobin is low too, or you are tired all the time. Anaemia should always be looked into with a doctor, and in men and after menopause blood loss is looked for first. With Rosa, a haemoglobin below 11 g/dL is always flagged to a doctor, below 8 urgently.

Slightly low haematocrit: should I worry? Usually not, if your haemoglobin is normal: haemoglobin is what shows anaemia. Check it again at your next test.

Low haematocrit and haemoglobin: what does it mean? It usually means anaemia: your blood carries less oxygen. If your red blood cells are low too, the picture is the same. Iron deficiency is often behind it, but the cause needs finding with your doctor, with ferritin and MCV.

How often should it be checked? For people who feel well there is no fixed rule: Rosa measures it twice a year, with the blood count. If it is out of range, it is usually checked again after about three months, or sooner if your doctor says so.

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