Haemoglobin blood test: what it measures and what can change it
Haemoglobin is the protein in red cells that carries oxygen. It is the main number in a full blood count.
General information about the test, from Hea's guide and the NHS · Last reviewed 5 November 2026
What it measures, and why GPs check it
Haemoglobin is the protein in red cells that carries oxygen. It is the main number in a full blood count.
A low level is anaemia; the cause matters more than the number itself.
Levels in UK guidance
Ranges differ between labs — your report shows your lab's range.
- WHO and UK guidance use below 130 g/L for men and below 120 g/L for women as anaemia. WHO guideline on haemoglobin cutoffs to define anaemia (2024); BSG guidelines for the management of iron deficiency anaemia in adults (2021; PMID 34497146)
- Laboratories phone a doctor the same day below 70 g/L (below 50 g/L with small red cells) and above 190 g/L. RCPath G158 v2 (2026), Table B1
- Anaemia is worked up with the red cell size (MCV) and with ferritin, B12 and folate; the pattern points to the cause. Lab Tests Online UK: Full blood count (FBC); BSH guidelines for cobalamin and folate disorders (2014; PMID 24942828)
What can shift the number
Now or in the last 3 months
- Pregnant. In pregnancy the usual ranges for many tests are different: show these results to your midwife or GP. NICE NG201 Antenatal care (2021); NICE NG3 Diabetes in pregnancy (2015, updated 2020); RCOG Green-top Guideline 76: thyroid disorders in pregnancy (2025; PMID 40240075); NICE NG239 (2024) rec 1.3.2; BSG guidelines on the management of abnormal liver blood tests (2018); Lab Tests Online UK: Lipid profile; C-reactive protein (CRP); ESR
- Gave blood, lost blood or had a transfusion. Bleeding, giving blood or a transfusion changes blood count and iron results: each blood donation removes iron, and platelets rise after bleeding. BSG guidelines for the management of iron deficiency anaemia in adults (2021; PMID 34497146), History and examination; NHS Blood and Transplant: Haemoglobin and iron; Lab Tests Online UK: Platelet count
Long-term conditions
- Coeliac disease, Crohn's or colitis, a stomach ulcer, or stomach or bowel surgery. Coeliac disease, bowel inflammation and stomach surgery reduce iron absorption or cause blood loss; coeliac disease is found in 3–5% of iron deficiency anaemia. BSG guidelines for the management of iron deficiency anaemia in adults (2021; PMID 34497146), statements 7 and 8, table 1
Some medicines and supplements can change this result too. When you add your results, Hea asks which ones you take.
What a GP reads it with
Read together with: Ferritin, MCV (mean cell volume), Vitamin B12, Folate. Lab Tests Online UK: Full blood count (FBC); BSG guidelines for the management of iron deficiency anaemia in adults (2021; PMID 34497146)
A GP usually repeats the count with iron studies, asks about periods, diet and bowel symptoms, and investigates further in men and in women after the menopause.
Questions to ask your GP
- Which tests are needed to find the cause?
- How does this compare with my previous count?
Safety first
If your lab or GP has flagged your result as urgent, follow their advice first.
Not for emergencies: call 999 or NHS 111.
Questions people ask
What are the symptoms of anaemia?
With anaemia you may feel tired, weak, short of breath, dizzy or faint. Some people look paler than usual or hear a pounding in their ears. If you have symptoms like these, your GP may check your haemoglobin as part of a full blood count.
Lab Tests Online UK: Haemoglobin, Page updated 7 October 2026What can cause low haemoglobin?
A low haemoglobin is called anaemia, and it has many causes. Common ones are low iron (from diet, heavy periods or blood loss), low vitamin B12 or folate, bleeding, kidney disease and long-term inflammation or infection. Inherited conditions such as thalassaemia or sickle cell disease can also cause it. Your doctor looks at the size of your red cells and other tests to find the cause.
Lab Tests Online UK: Haemoglobin, Page updated 7 October 2026What happens after a low haemoglobin result?
Your GP asks about your lifestyle and medical history. The blood test, a full blood count, also checks for other types of anaemia, such as anaemia from low vitamin B12 or folate. If the reason is not clear, the GP may arrange more tests, such as one for blood in a sample of your poo, or refer you to a specialist.
NHS: Iron deficiency anaemia, Page last reviewed 26 January 2024How can I raise my haemoglobin?
By treating the cause, which your doctor looks for first, often with iron, vitamin B12 and folate tests. If one of these is low, your doctor will recommend a supplement; take it only as your GP or pharmacist advises. Other causes, such as bleeding or kidney disease, are treated in other ways.
Lab Tests Online UK: Haemoglobin, Page updated 7 October 2026What can affect a haemoglobin result?
Being dehydrated can make haemoglobin read higher, so drink as usual before the test; you do not need to fast. Levels are slightly higher in the morning than in the evening. They can be a little lower around a period and in pregnancy, and smoking and living at high altitude raise them.
Lab Tests Online UK: Haemoglobin, Page updated 7 October 2026Got your result? Start with this question in your AI.
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Sources
Each line about the test above names the guideline it comes from, as Hea's guide cites it. The answers to the questions come from:
- Lab Tests Online UK: Haemoglobin, Page updated 7 October 2026
- NHS: Iron deficiency anaemia, Page last reviewed 26 January 2024