Haematocrit (Hct) and PCV: what a high or low result means

Blood is a mix of liquid plasma and cells. Haematocrit tells you how much of the total volume is red cells. A result of, for instance, a few tenths means that a few tenths of the blood volume is red cells and the rest is plasma, white cells and platelets. In the full blood count, Lab Tests Online UK describes haematocrit as the proportion of blood that is made up of red blood cells, expressed as a percentage, and notes that it mirrors the red cell count and haemoglobin.

Modern analysers usually calculate the figure from the red cell count and the average red cell size rather than spinning the blood in a tube, but the idea is the same. Because the three numbers (red cell count, haemoglobin and haematocrit) are closely linked, a laboratory scientist can spot a mistake or an unusual sample when they fail to line up.

Lab Tests Online UK says the PCV is used to diagnose or evaluate anaemia (a decrease in red cells), polycythaemia (an increase) and dehydration, which can show up as a relative rise. It is usually requested as part of a full blood count for symptoms such as headaches, blurred vision, tiredness, itchy skin, difficulty breathing or dizziness.

It is also repeated at regular intervals to monitor treatment of anaemia, polycythaemia or dehydration, to judge the need for or effect of blood transfusion, and to check the severity of ongoing bleeding.

No preparation is needed and blood is taken from a vein in the arm or by finger or heel prick in children. Fluid status matters: if you are dehydrated the plasma volume shrinks and the cell proportion looks higher, whereas receiving intravenous fluids can dilute the blood and lower it. Pregnancy lowers PCV through the extra fluid in the blood, and recent transfusion can make the result misleading. Living at high altitude raises it as the body adapts to lower oxygen.

Smokers also tend to carry higher red cell numbers. Women of childbearing age may have temporary decreases around menstrual periods and pregnancy, according to Lab Tests Online UK.

A low PCV points to anaemia or to haemodilution. Lab Tests Online UK lists bleeding, deficiencies of iron, vitamin B12, folate or other minerals, inflammatory conditions such as rheumatoid arthritis, kidney disease, cirrhosis of the liver, haemolysis (premature destruction of red cells), bone marrow disorders and some medicines including chemotherapy.

The NHS notes that iron deficiency anaemia is the most common type of anaemia, often caused by blood loss or pregnancy, and that heavy periods are a very common cause. Symptoms of low red cell levels include tiredness, breathlessness, palpitations, pale skin and headaches. A low result usually triggers further tests such as iron studies, vitamin B12 and folate, and sometimes a reticulocyte count.

Dehydration is the commonest reason, and the figure falls back once fluids are replaced. If it stays raised, doctors consider polycythaemia. In primary polycythaemia (polycythaemia vera) the bone marrow makes too many red cells by itself. Secondary causes listed by Lab Tests Online UK include some lung or heart diseases, excessive alcohol, smoking, liver or kidney disease, obesity, drugs such as testosterone, growth hormone, EPO and diuretics, some tumours that make erythropoietin, and rare inherited haemoglobins.

When PCV is persistently high, an ultrasound of the kidneys, an erythropoietin level and a JAK2 gene test may be arranged. Treatment aims to prevent complications and treat the underlying cause; it may involve removing blood or medicines, and lifestyle steps such as stopping smoking and reducing alcohol.

A borderline result in someone who feels well is often repeated. If anaemia is likely the clinician will look at the red cell indices together and arrange the tests that fit the pattern. For a high result, the first step is usually to check hydration and smoking status and to repeat the test after a period of normal fluid intake.

Always take your whole blood count report to the appointment, because the haematocrit makes most sense next to haemoglobin, the red cell count and the red cell size.