At a Glance
Why Get Tested?
If your doctor suspects that you have anaemia (too few red blood cells), polycythaemia (too many red blood cells), or dehydration
When to Get Tested?
As part of a full blood count (FBC), which may be requested for a variety of reasons
Sample Required?
A blood sample drawn from a vein in your arm or by a finger-prick (children and adults) or heel-prick (newborns)
Test Preparation Needed?
None
The Test Sample
What is being tested?
Blood is a mixture of cells and plasma. The haematocrit (Hct or PCV) is a measurement of the proportion of blood that is made up of cells. The value is expressed as a fraction of cells in blood. For example, a Hct of 0.40 means that there are 40 millilitres of cells in 100 millilitres of blood.
The haematocrit rises when the number of red blood cells increases or when the blood volume is reduced, as in dehydration. The falls to less than normal, indicating anaemia, when your body decreases its production of red blood cells or increases its destruction of red blood cells.
How is the sample collected for testing?
A sample is obtained by drawing blood through a needle placed in a vein in the arm or by a finger-prick (for children and adults) or a heel-prick (for newborns).
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
The Test
How is it used?
This test is used to evaluate:
- anaemia (decrease of red blood cells)
- polycythaemia (increase in red blood cells)
- dehydration
- blood transfusion decision, and
- the effectiveness of those transfusions.
When is it requested?
The haematocrit is normally requested as a part of the full blood count (FBC). It is also repeated at regular intervals for many conditions, including:
- the diagnosis of anaemia
- the treatment of anaemia
- recovery from dehydration
- monitoring of ongoing bleeding to check its severity, and
- monitoring of polycythaemia.
What does the test result mean?
A decreased haematocrit (PCV) indicates anaemia, such as that caused by iron deficiency. Further testing may be necessary to determine the exact cause of the anaemia.
Other conditions that can result in a low haematocrit include vitamin or mineral deficiencies, recent bleeding, cirrhosis of the liver, and malignancies.
The most common cause of increased haematocrit is dehydration, and with adequate fluid intake, it returns to normal. However, it may reflect a condition called polycythaemia vera — that is, when a person has more than the normal number of red blood cells due to a problem with the bone marrow . More commonly polycythaemia is a compensation for inadequate lung function (the bone marrow manufactures more red blood cells in order to carry enough oxygen throughout your body).
Is there anything else I should know?
Pregnancy usually causes a slightly decreased Hct due to extra fluid in the blood.
Living at high altitudes causes an increased Hct - this is your body's response to the decreased oxygen available at these heights.
Common Questions
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Article Sources
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