Haemoglobin is a protein containing iron which can be found in all the red blood cells. It givs the blood cells their characteristic red colour. Haemoglobin allows the red blood cells to bond with oxygen in the lungs and carry that oxygen to tissues and organs throughout the body.
It also transports a small amount of carbon dioxide – a byproduct of cell metabolism – from tissues and organs to the lungs, where it is exhaled. Haemoglobin is also what gives red blood cells their round shape, which is needed to allow them to flow through the blood vessels without problems.
Hb is an important part of any health check. Several diseases can affect the red blood cells, and hence the level of haemoglobin in the blood. In general, the haemoglobin level rises or falls in tandem with the number of red blood cells.
The haemoglobin level falls when there is a decline in the production of red blood cells from the bone marrow, an increase in the destruction of red blood cells, or if blood is lost because of haemorrhage. Having less red blood cells than normal can lead to anaemia, a condition in which tissues and organs in the body do not get sufficient oxygen, which in turn causes fatigue and weakness.
It is also possible to overproduce red blood cells, and if far too many red blood cells are produced – a condition also known as polycythaemia – this leads to sluggish blood flow and associated problems.
This test can indicate whether there is a problem with the production of red blood cells and/or the life of the blood cells, but it is unable to determine the underlying cause. As well as checking the number of blood cells, there are other tests that can be carried out at the same time or subsequently in order to establish a cause. This includes blood smears, iron studies, vitamin B12 and folate levels, and – in more serious cases – a bone marrow examination.
High levels are seen in cases of polycythaemia. Polycythaemia means having too many red blood cells in the blood. This increases the risk of a blood clot or haemorrhage.
Low levels are seen in cases of anaemia. A slightly reduced haemoglobin level in whole blood can be seen in a number of conditions where the erythrocyte count is normal but the plasma volume is greater than normal, such as in pregnancy or cases of Waldenström macroglobulinaemia, or following a splenectomy.