Haemoglobin: what it is, what it tells you and why we test it

Haemoglobin is the protein in your red blood cells that carries oxygen around your body. It is the number that shows whether you have anaemia. You find it in the full blood count, the most common blood test.

Why we test it

Every red blood cell is packed with haemoglobin. In your lungs it picks up oxygen and carries it to your muscles, heart and brain.

When it is low, it is called anaemia: less oxygen gets through and you tire sooner. The most common cause is a lack of iron, which can be found and treated.

It matters when it is high too. That often comes from too little water or smoking, but sometimes a doctor needs to look into it.

What it tells you, and what it does not

If it is low, it is called anaemia, but the number alone does not say why. You may be short of iron or vitamin B12, or there may be blood loss or an inherited trait.

If it is normal, that does not have to mean your iron is fine: your iron store drops well before haemoglobin does.

And it changes with how much water is in your body: if you are dehydrated it rises, even without more red cells.

That is why it is read together with other tests:

• Ferritin: How much iron you have in store
• MCV: How big your red cells are: it helps find the cause
• Haematocrit: How much of your blood is red cells
• Red blood cells: How many cells carry oxygen

Normal haemoglobin levels

The lab decides what is normal. It is usually 12 to 16 g/dL for women and 14 to 18 for men. Your own report may print different limits.

It is higher in men, partly because testosterone makes the body produce more red cells. Women lose a little iron every month with their period.

In pregnancy it drops a little, because the blood gets more diluted. Anaemia starts below 11 in the first and third trimester, and a little lower in the second.

In the Rosa report you also see your optimal range. Ferritin, MCV and haematocrit are in the same panel.

What your result means

Find the row for your result, in g/dL. Which values are dangerous? Below 11 should always be seen by a doctor, below 8 without waiting.

• Below 8: Severe anaemia. Your blood carries much less oxygen than normal. Talk to a doctor today, even if you feel fine. If you are short of breath at rest, have chest pain or faint, call 112.
• 8 to under 11: Moderate anaemia. Your blood carries less oxygen, and you often feel tired. Talk to a doctor soon to find the cause.
• 11 up to your report’s limit, Women: usually 11 to under 12, Men: usually 11 to under 14: Low. It can be mild anaemia. Iron is usually behind it, sometimes an inherited trait. Check your ferritin and MCV too, and talk to your doctor to find the cause.
• Within your report’s limits, Women: usually 12 to 16, Men: usually 14 to 18: Normal. Your blood carries enough oxygen. But your iron store can already be low. In the Rosa report you also see whether you are in your optimal range.
• Above your report’s limit, Women: usually above 16, Men: usually above 18: High. Too little water or smoking are often behind it, sometimes testosterone. If your haematocrit is above 56, see a doctor the same day. Otherwise repeat the test well hydrated, and if it stays above 52 in men or 48 in women, talk to a doctor.

If your haemoglobin is high or low

• Low haemoglobin: Usually iron is short. Sometimes an inherited trait is behind it, and then iron usually does not help.
• High haemoglobin: Too little water or smoking are often behind it. It is read with haematocrit.

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 are dehydrated the number rises, even though your red cells are the same. You can drink water even while fasting.
• A recent blood donation: After you give blood, haemoglobin drops and can take a few months to come back up.
• Living in the mountains: At high altitude the air has less oxygen, and your body makes up for it with more haemoglobin.
• Smoking: People who smoke often have slightly higher haemoglobin, and that can hide anaemia.
• Pregnancy: The blood gets more diluted and the number drops a little. That is why the limit for anaemia is lower.
• A lot of endurance sport: Here too the blood gets diluted and the number can look low. Ferritin helps tell whether iron is really short.

When to worry

Talk to a doctor if:

• Your haemoglobin is below your report’s limit, even slightly.
• It is below 8: talk to a doctor today.
• It is high and your haematocrit stays above 52 in men or 48 in women (above 56, see a doctor the same day).
• You see black stools or blood in your stools: call 112 or go to the emergency department.
• You are short of breath at rest, have chest pain or feel faint: call 112 or go to the emergency department.

With Rosa, a haemoglobin below 11 is always flagged to a doctor, below 8 urgently. A haematocrit above 52 in men or 48 in women is always flagged too, above 56 urgently.

Frequently asked questions

Do normal values change with age? In adults the limits on your report usually do not: they differ between women and men, not with age. But anaemia becomes more common with age, and the cause should always be found.

What does HGB or Hb mean on a blood count? It is the abbreviation for haemoglobin. On your report it sits next to red blood cells (RBC), haematocrit (HCT) and MCV, and they are read together.

Are haemoglobin and HbA1c the same thing? No. Haemoglobin shows how much oxygen your blood can carry. HbA1c, or glycated haemoglobin, shows how much sugar has stuck to your haemoglobin over the last few months: it is a blood sugar test.

Do I need to fast for a haemoglobin test? Not for haemoglobin. It is usually done with other tests that do need fasting, though, so it is best to arrive fasted. You can drink water.

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 low and treatment starts, your doctor usually checks it again after a few weeks.

Sources

• World Health Organization, guideline on haemoglobin cutoffs to define anaemia in individuals and populations. Geneva, 2024. PMID 38530913
• British Society of Gastroenterology, guidelines for iron deficiency anaemia in adults. Gut, 2021. PMID 34497146
• British Society for Haematology, guideline for the diagnosis and management of polycythaemia vera. British Journal of Haematology, 2019. PMID 30478826
• Kiss and colleagues, oral iron after blood donation and how long haemoglobin takes to recover. JAMA, 2015. PMID 25668261
• White and colleagues, testosterone therapy and secondary erythrocytosis. International Journal of Impotence Research, 2022. PMID 34987178