Also known as: EPO
Name: Erythropoietin
Related tests: Full blood count, Haematocrit , Haemoglobin, RBC, Reticulocyte count
Why is the test done?
To help distinguish between some types of anaemia and, in particular, to determine whether an anaemia may be caused by erythropoietin deficiency.
When is the test done?
When the doctor suspects that an anaemia may be caused by insufficient red blood cell production.
What samples are required?
A blood sample from a vein
What is being tested?
Erythropoietin is a hormone produced mainly by the kidneys and released into the blood in response to low oxygen levels. Erythropoietin travels through the bloodstream to the bone marrow, where it stimulates the production and maturation of red blood cells, which carry oxygen around the body thanks to the haemoglobin they contain. The body has a feedback system that regulates the number of circulating red blood cells: when the number of red blood cells falls, because of reduced production or increased loss (mainly through bleeding or excessive haemolysis), or because of changes in their shape and size, the capacity to carry haemoglobin, and therefore oxygen, can change, causing anaemia. Red blood cell production depends on the bone marrow’s capacity to produce cells, on the availability of iron and of some nutrients such as vitamin B12 and folate, both involved in nucleic acid metabolism, and on an appropriate concentration of erythropoietin and adequate sensitivity to this hormone.
The amount of erythropoietin released depends on the severity of the lack of oxygen and on the kidneys’ ability to make it. The hormone has a short half-life and is eliminated by the kidneys. Erythropoietin continues to be released until the partial pressure of oxygen in the blood returns to normal, and then falls back to baseline levels. Anaemia of variable severity can develop with kidney failure or when the blood-forming tissue is resistant to erythropoietin.
Overproduction of erythropoietin, although quite rare, can be found in some kidney tumours and causes polycythaemia, followed by increased blood volume and viscosity and high blood pressure.
How is it used?
When is it requested?
What does the test result mean?
Is there anything else I should know?
How is it used?
Erythropoietin is not a first-line test and is requested only after abnormalities have been found in the full blood count (particularly in the red blood cell count, haematocrit and haemoglobin) that suggest anaemia; erythropoietin can be useful for distinguishing between types of anaemia and, in particular, for determining whether an anaemia may be caused by erythropoietin deficiency or by a lack of response to the hormone. However, erythropoietin cannot be used to monitor anaemia; tests such as the full blood count, particularly the red blood cell and reticulocyte counts, haemoglobin concentration and haematocrit, are more appropriate for this.
There are also rare cases in which excess red blood cells are caused by overproduction of erythropoietin.
In patients with chronic kidney failure, erythropoietin may be requested periodically to assess the impact of kidney failure on red blood cell production.
When is it requested?
The erythropoietin test is requested when anaemia is found whose cause cannot be attributed to iron deficiency, vitamin B12 or folate deficiency, haemolysis or bleeding (for example from the gut); in these cases there is generally a fall in the red blood cell count, haemoglobin and haematocrit, with normal or reduced reticulocyte levels (the latter showing that the bone marrow has not responded by producing more red blood cells). Erythropoietin can be useful to distinguish between failure of red blood cell production in the bone marrow and a lack of erythropoietin.
In patients with chronic kidney failure, the erythropoietin test may be requested when the doctor suspects that kidney dysfunction may be interfering with production of the hormone.
In patients with a particularly high number of red blood cells, the doctor may request the erythropoietin test to assess whether this abnormality may be caused by overproduction of the hormone.
What does the test result mean?
If the erythropoietin level is raised in an anaemic patient with too few new red blood cells, the anaemia may be due to a failure of blood cell production by the bone marrow. If the patient is anaemic but has erythropoietin levels within or below the reference range, the problem may lie in production of the hormone by the kidneys, and therefore in kidney function.
If a patient has an increased number of red blood cells and a raised erythropoietin level, overproduction of the hormone, by the kidneys or other tissues, may be suspected; if the increase in red blood cells is not accompanied by an increase in erythropoietin, the polycythaemia has causes unrelated to erythropoietin production.
Is there anything else I should know?
If a patient has anaemia due to iron, vitamin B12 or folate deficiency, the anaemia persists even if erythropoietin production is adequate; in these forms of anaemia the red blood cells may have abnormal volume, shape and/or haemoglobin content. In addition, when abnormal forms of haemoglobin are produced, as in thalassaemia, or in some disorders of the blood-forming tissue, the rise in erythropoietin is not enough to correct the anaemia.
A synthetic form of erythropoietin has been produced and is given to patients with chronic kidney failure or other anaemias related to failure of the blood-forming tissue (such as after radiotherapy and chemotherapy). This treatment, given by injection into a vein or under the skin, is expensive and its stimulating action on the bone marrow lasts only a few hours. Use of the synthetic hormone is promising, as it can reduce the need for transfusions and improve many patients’ quality of life.
However, the same synthetic erythropoietin is used by some athletes as a form of doping, as it improves performance by increasing circulating red blood cells, haemoglobin mass and therefore aerobic muscle capacity. This use of erythropoietin can be very dangerous, as it can cause high blood pressure, increased blood viscosity and thrombosis.
1. Can the kidneys go back to producing enough erythropoietin?
2. Why is erythropoietin not measured to follow treatment with this hormone?
1. Can the kidneys go back to producing enough erythropoietin?
Not directly. If kidney failure is completely resolved, erythropoietin production should also return to levels adequate for the body’s needs. In many cases, however, the lack of erythropoietin production is caused by chronic kidney failure and will not resolve. When this deficiency is known, the doctor and the patient can try to minimise the effects of the anaemia by treating the patient with erythropoietin.
2. Why is erythropoietin not measured to monitor treatment with this hormone?
Because the clinically important result of treatment is the effect of erythropoietin on the bone marrow (an increase in red blood cells, reticulocytes and haemoglobin) rather than its simple presence in the blood, so it makes more sense to carry out tests that monitor the anaemia than to measure plasma erythropoietin. The treatment dose also depends on several factors, such as the patient’s condition and the bone marrow’s response