Packed cell volume; PCV; Hct
If your doctor suspects that you have anaemia (too few red blood cells), polycythaemia (too many red blood cells), or dehydration
As part of a full blood count (FBC), which may be requested for a variety of reasons
A blood sample drawn from a vein in your arm or by a finger-prick (children and adults) or heel-prick (newborns)
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.
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).
No test preparation is needed.
This test is used to evaluate:
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:
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 an increased haematocrit is dehydration, and with adequate fluid intake, it returns to normal. However, if it persists when a patient is not dehydrated, it is suggestive of a condition called polycythaemia — that is, when a person has more than the normal number of red blood cells. Polycythaemia is called primary polycythaemia (Polycythaemia vera) when it is due to a bone marrow problem (myeloproliferative neoplasm). More commonly polycythaemia is a compensation for inadequate delivery of oxygen to the body's tissues, which may be due to problems with lung function, heart or rarely due to abnormalities of the kidneys or adrenal glands.
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.
Full blood count
RCPA Manual: Packed cell volume (PCV), haematocrit (Hct)