Also known as: Hct, Crit, Packed cell volume (PCV)
Name: Haematocrit
Related tests: Full blood count
Why is the test done?
When the doctor suspects anaemia (too few red blood cells), polycythaemia (too many red blood cells) or dehydration.
When is it done?
As part of the full blood count (FBC), which may be requested for a wide variety of reasons.
What samples are required?
A blood sample taken from a vein in the arm, by finger-prick (children and adults) or by heel-prick (newborn babies).
What is being analysed?
The haematocrit is a measure of the percentage of the blood made up of cells. The value is expressed as a percentage or fraction of cells in the blood. For example, a value of 40% means that there are 40 mL of cells in 100 mL of blood. The haematocrit rises when the number of red blood cells increases or when the plasma volume falls, as in dehydration. The haematocrit falls below normal, indicating anaemia, when the body produces fewer red blood cells or destroys more of them, or when blood is lost through bleeding. The haematocrit reflects the number of red blood cells and their volume (MCV). If the size of the red blood cells decreases, the haematocrit is affected in the same way, and vice versa.
How is the sample collected for testing?
The test is carried out on a blood sample taken with a needle from a vein in the arm, by finger-prick (children and adults) or by heel-prick (newborn babies).
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?
The haematocrit is an integral part of a proper assessment of health. The test is used to:
• measure the severity of anaemia (too few red blood cells)
• detect polycythaemia (too many red blood cells)
• help decide whether a transfusion is needed in severe anaemia with symptoms
• assess the effectiveness of such transfusions
When is it requested?
The haematocrit is normally requested as part of a full blood count (FBC). It is also repeated at regular intervals in many situations, including:
• diagnosis of anaemia or polycythaemia
• monitoring treatment of anaemia
• treatment of dehydration
• monitoring the severity of bleeding
What does the test result mean?
A low haematocrit indicates anaemia, such as anaemia caused by iron deficiency or other deficiencies. In some cases further tests are needed to find the exact cause of the anaemia.
Other conditions that can occur with a low haematocrit include vitamin and mineral deficiencies, recent bleeding, liver cirrhosis and cancer. The most common cause of a raised haematocrit is dehydration, and with adequate fluid intake the value returns to normal. In some cases it may reflect a condition called polycythaemia vera, in which a person has more red blood cells than normal.
This condition can result from bone marrow disorders or, more commonly, as compensation for respiratory failure (the bone marrow produces more red blood cells so that more oxygen can be carried throughout the body). If the haematocrit is persistently high, the cause should be investigated with a doctor.
As for the need for a transfusion, a haemoglobin of 8 g and a haematocrit of 24% are normally considered the lower limits below which a transfusion becomes necessary.
Is there anything else I should know?
Pregnancy normally lowers the haematocrit, because excess fluid dilutes the blood.
Living at high altitude raises the haematocrit; this is the body’s response to the reduced availability of oxygen at such heights
1. How is anaemia treated?
2. Can the haematocrit be measured at home?
3. Who is at risk of haematocrit abnormalities?
1. How is anaemia treated?
Treatment depends on the type of anaemia and its cause. Folic acid, vitamin replacement, iron and red blood cell (RBC) transfusion are some of the treatments used for anaemia.
2. Can the haematocrit be measured at home?
No. This test must be carried out in specialised laboratories.