Haemoglobin

You do not need to fast before this test. A healthcare professional will take a blood sample from a vein in your arm. In young children and babies, the sample may be taken from a finger-prick or heel-prick. 

Try to be well hydrated before your test. Being dehydrated can make your haemoglobin result appear higher than it actually is. Drink a normal amount of water before your appointment. 

Haemoglobin (Hb) is a protein found inside red blood cells. Its job is to carry oxygen from your lungs to every part of your body, and to carry carbon dioxide back to the lungs to be breathed out. 

The amount of haemoglobin in your blood is measured in grams per litre (g/​L). The result tells your doctor how well your blood is able to transport oxygen. It is one of the most important measurements in the full blood count.

If your haemoglobin is low (anaemia) 

A haemoglobin level that is lower than normal means you have anaemia. This means your blood is carrying less oxygen than it should. You may feel tired, weak, short of breath, dizzy, or faint. Some people notice they look paler than usual or hear a pounding in their ears. 

Anaemia has many possible causes. Your doctor will look at other results from your full blood count, such as the size and shape of your red blood cells, to work out the most likely reason. 

Common causes of a low haemoglobin include: 

• iron deficiency: the most common cause of anaemia worldwide, often due to diet, heavy periods, or blood loss 
• vitamin B12 or folate deficiency which can affect how red blood cells are made 
• bleeding, for example, from the gut, or following injury or surgery 
• kidney disease: as the kidneys produce a hormone that stimulates red blood cell production; when kidney function is reduced, this is affected 
• inflammatory conditions such as rheumatoid arthritis or long-term infections 
• haemolysis where red blood cells are broken down faster than they can be replaced 
• inherited haemoglobin disorders such as thalassaemia or sickle cell anaemia 
• liver cirrhosis 
• bone marrow failure or cancers affecting the bone marrow 

Treatment depends on the cause. Iron, vitamin B12, or folate supplements are used for nutritional deficiency-related anaemias. Blood transfusions may be needed if the bone marrow is not working properly. In kidney disease and some cancers, a hormone called erythropoietin (EPO) may be used to stimulate red blood cell production. 

Women of childbearing age are at greater risk of low haemoglobin because of blood loss during periods. Haemoglobin also falls slightly during normal pregnancy. 

If your haemoglobin is high (polycythaemia) 

A haemoglobin level that is higher than normal usually means you have too many red blood cells. This is called polycythaemia. When severe, it can make your blood thicker than normal, which can increase the risk of blood clots, heart attack, or stroke. 

Common causes of a high haemoglobin include: 

• dehydration which is a temporary cause that resolves once you are well hydrated 
• smoking: heavy smokers have higher haemoglobin levels than non-smokers 
• living at high altitude as your body makes more red blood cells to compensate for lower oxygen levels 
• severe lung disease leads to reduced oxygen supply which triggers more red blood cell production 
• excess production of red blood cells in the bone marrow such as in a condition called polycythaemia vera 

Your doctor will investigate the cause if your result is persistently or significantly raised. Dehydration is common and easily resolved, whereas polycythaemia vera requires specialist assessment and treatment.

If your haemoglobin is low, your doctor will look at the other results in your full blood count to understand the likely cause. They may request further tests such as iron studies, vitamin B12, or folate levels, or a reticulocyte count to check how your bone marrow is responding. 

If a nutritional deficiency is found, your doctor will recommend the appropriate supplements. Most people see their haemoglobin improve over weeks to months with the right treatment. 

If your haemoglobin is high, your doctor will consider whether dehydration or lifestyle factors are responsible. If the cause is not straightforward, they may request further tests to check for an underlying condition such as polycythaemia vera. 

If you need surgery and your haemoglobin is low, your operation may be delayed until your levels have improved, or steps may be taken to manage this safely. 

Several factors can affect your haemoglobin level that are not signs of illness: 

• dehydration makes haemoglobin appear falsely high; drink normally before your test 
• time of day as haemoglobin levels are slightly higher in the morning and lower in the evening 
• age: levels are slightly lower in older adults and in children 
• sex: normal ranges differ between men and women; women have lower normal ranges than men 
• pregnancy: haemoglobin falls slightly during normal pregnancy as blood volume increases 
• menstruation: women may have temporarily lower levels around their period 
• smoking raises haemoglobin levels 
• altitude: living at high altitude raises haemoglobin as an adaptation to lower oxygen levels 

Always tell your doctor about any medicines you are taking, as some can affect haemoglobin levels. Erythropoietin (EPO), which is used to treat anaemia in kidney disease and some cancers, will raise haemoglobin as part of its intended effect. 

Depending on your result, your doctor may also request: 

• a full blood count (FBC) if not already done, to look at red blood cells, white blood cells, and platelets together 
• haematocrit (PCV) which measures the proportion of blood made up of red blood cells, and is used to assess polycythaemia 
• red cell indices (MCV, MCH, MCHC) which measure the size and haemoglobin content of red blood cells, helping to identify the type and cause of anaemia 
• reticulocyte count which checks how many new red blood cells your bone marrow is producing 
• iron studies and ferritin to check for iron deficiency 
• vitamin B12 and folate to check for deficiency-related anaemia 
• blood film to examine the shape and appearance of red blood cells under a microscope 

Anaemia is a condition where you do not have enough healthy red blood cells or haemoglobin to carry oxygen around your body. It is very common and has many causes, including nutritional deficiencies, chronic illness, blood loss, and inherited conditions.