Haemoglobin (Hb) Test

What is haemoglobin?

Haemoglobin (Hb or Hgb) is the iron-containing protein inside red blood cells that binds to oxygen in the lungs and delivers it throughout the body. It also carries carbon dioxide back to the lungs. The haemoglobin test is part of a Complete Blood Count (CBC) and is the primary test for diagnosing anaemia.

Haemoglobin normal range

Grades of anaemia

Questions to ask your doctor

• What type of anaemia do I have?
• Do I need iron, B12 or folate supplements?
• Should I have ferritin and iron studies?
• Do I need a colonoscopy to check for GI bleeding?
• When should I retest?

Frequently Asked Questions

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

1. Anemia. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK499994

References

Sources cited on this page. PubMed links open the original abstract.

1. World Health Organization. Haemoglobin concentrations for the diagnosis of anaemia and assessment of severity. WHO/NMH/NHD/MNM/11.1. WHO Press. 2011;:1–6. PMID 21371683

Haemoglobin structure and oxygen delivery

Haemoglobin (Hb) is a protein made of four globin chains (two alpha and two beta in adult HbA), each carrying a haem group that can bind one oxygen molecule. Each red blood cell contains approximately 270 million haemoglobin molecules – enabling each cell to carry approximately 1 billion oxygen molecules. This extraordinary oxygen-carrying capacity is what makes blood so much more efficient at delivering oxygen than plasma alone.

Oxygen binds cooperatively: binding of the first oxygen molecule changes haemoglobin's shape, making subsequent binding easier (the "sigmoidal oxygen-dissociation curve"). This property means haemoglobin efficiently loads oxygen in the lungs (high pO₂) and efficiently unloads it in tissues (low pO₂, lower pH, higher CO₂ – the Bohr effect), where oxygen demand is greatest.

Interpreting haemoglobin – what the number tells and doesn't tell you

Haemoglobin is measured in grams per litre (g/L) or grams per decilitre (g/dL). Normal ranges:

• Adult men: 130–170 g/L (13–17 g/dL)
• Adult women: 120–155 g/L (12–15.5 g/dL)
• Pregnant women: Below 110 g/L is anaemia (haemodilution physiologically lowers Hb in pregnancy)
• Newborns: 140–200 g/L (fetal haemoglobin – HbF – is higher affinity for oxygen than adult HbA, appropriate for placental extraction of oxygen)

Haemoglobin level alone does not fully characterise anaemia – the MCV, MCH, and blood film provide essential additional information about the type of anaemia (see the FBC section). Important caveats:

• Haemoglobin can be falsely elevated in dehydration (haemoconcentration) or in polycythaemia vera
• Haemoglobin can be falsely low in overhydration or due to laboratory dilution artefact
• A "normal" haemoglobin does not exclude iron deficiency – iron stores can be depleted for months before haemoglobin falls

Haemoglobin variants – HbS, HbC, and thalassaemia

Mutations in the globin genes produce variant haemoglobins with different clinical consequences:

• HbS (sickle cell haemoglobin): A point mutation (glutamate → valine at position 6 of the beta chain) causes HbS to polymerise under low oxygen conditions, distorting red cells into a sickle shape. Sickle cells block small blood vessels, causing vaso-occlusive pain crises, stroke, acute chest syndrome, and organ damage. HbSS (sickle cell disease) is a severe chronic condition; HbAS (sickle cell trait) is generally asymptomatic but can be identified by newborn screening (Guthrie test) or haemoglobin electrophoresis.
• HbC: Another beta-chain variant; milder sickling in HbSC compound heterozygosity.
• Alpha and beta thalassaemia: Reduced production (not structurally abnormal) of alpha or beta chains. Alpha thalassaemia trait causes mild microcytic anaemia; beta thalassaemia major (two copies of beta-zero mutation) requires lifelong blood transfusion or bone marrow transplant.