Summary

• Haematocrit is a laboratory measurement of the proportion of your blood that is made up of red blood cells (RBCs).
• Red blood cells carry oxygen from your lungs to the rest of your body
• The test indicates the viscosity, or thickness, of your blood and this correlates with the size and number of RBCs in your blood sample.
• The haematocrit rises when the number of RBCs is higher (polycythaemia) and falls when the number of RBCs is lower (anaemia).
  

What is being tested?

Your blood is made up of red blood cells (RBCs), white blood cells (WBCs) and platelets. These float in the fluid part of your blood called plasma.

The haematocrit is the name given to a measurement of what proportion of your blood is made up of red blood cells (RBCs). When the number of RBCs is high the haematocrit result is high and when the number of RBCs is low the haematocrit result is low.

The haematocrit result is expressed as a fraction of red cells in blood. For example, a haematocrit of 0.40 means that there are 40 millilitres of red cells in 100 millilitres of blood while the remaining 60 millilitres of blood is made up of plasma, white blood cells, and platelets.

The haematocrit is a measurement of the viscosity and thickness, of your blood. When there are more red cells in the same volume, they bump into each other and into vessel walls more often. This makes blood stickier and slower to flow. When there are fewer cells, blood flows more freely.

Why get tested?

The haematocrit test is used to helps screen for, diagnose and monitor conditions that affect your blood or bone marrow. It is included in the full blood count (FBC), a routinely used test that assesses all the parts of your blood. It measures the number and the size of RBCs, white blood cells (WBCs), and platelets in your blood.

Haematocrit may also be ordered on its own if you are suspected of having a condition affecting your RBCs as well as monitor treatment if you need it. RBCs play an essential role in carrying oxygen to every cell in your body. For more on this see red blood cells.

Too few RBCs – anaemia

If the number of RBCs in your blood sample is10 per cent under what is considered normal, you are said to be anaemic.

Too many RBCs – polycythaemia (erythrocytosis)

If you have too many RBCs you are said to have polycythaemia. This makes your blood thicker than it should be, and without treatment it could increase your risk of blood clots.

There are two types of polycythaemia. Primary polycythaemia is due to a bone marrow problem (myeloproliferative neoplasm) as in polycythaemia vera (PV). PV is a condition in which the bone marrow produces too many RBCs, along with high numbers of WBCs and platelets. It is caused by a variant or mutation in particular genes, especially the JAK2 gene which is responsible for making a protein that is needed to produce blood cells.

Secondary polycythaemia is more common and occurs when a health condition is preventing oxygen from getting to your tissues. This may be due to problems with lung function, heart function or rarely, due to problems with the kidneys or adrenal glands (small glands on your kidneys). Your body makes up for lower oxygen levels by making more blood cells that can carry more oxygen.

Tests that will be used together with haematocrit

The results of other tests included in the FBC will be used to assess your RBCs.

• Red Blood Cell (RBC) count measures the number of red cells circulating in your blood.
• Haemoglobin This is the iron-containing, oxygen-carrying red protein in RBCs. Measuring it can show if you don’t have enough iron or enough vitamins such as vitamin B12 and folate that are needed to make haemoglobin. Low haemoglobin levels cause anaemia.
  Haemoglobin results are used in making calculations to gain more details about your red blood cells.
  • Mean cell haemoglobin (MCH) This is calculated by dividing the total amount of haemoglobin by the total number of RBC in a blood sample. This gives the average amount of haemoglobin inside each RBC. The MCH is increased in macrocytic anaemias (larger red blood cells) and decreased in microcytic anaemias (small red blood cells).
  • Mean cell haemoglobin concentration (MCHC) is calculated by dividing the total amount of haemoglobin by the amount of space RBCs are taking up in your blood sample (the volume of RBCs). It shows the concentration of haemoglobin inside a red blood cell and takes into account the cell's volume. This can give more detail about your condition.
• Mean Cell Volume (MCV) MCV measures the average size of your RBCs. It is high when your cells are larger than normal (macrocytic) such as in vitamin B12 deficiency, folate deficiency, liver disease or hypothyroidism (when your thyroid gland is not making enough hormones). When the MCV is lower, your RBCs are smaller than normal (microcytic) as in iron deficiency anaemia and the inherited condition, thalassaemia.
• Reticulocyte count (in some circumstances) RBCs that are not fully developed are known as reticulocytes. Most red blood cells are fully mature before they are released but a tiny amount are released into the blood as reticulocytes. Reticulocytes can be distinguished from mature red blood cells because they still contain remnant genetic material (RNA) inside the cells which is not found in mature red blood cells. Circulating reticulocytes generally lose their RNA within one to two days, thus becoming mature red blood cells. A reticulocyte count can help show if your bone marrow is making enough RBCs and give information that will help identify the cause of low or high RBCs.

Possible further tests for anaemia

• Iron studies (Ferritin, transferrin and transferrin saturation/TIBC) This measure iron levels in your blood. Iron is essential for making red blood cells.
• Vitamin B6, vitamin B12 and folate tests Having enough of these is important for making haemoglobin.

Having the test

Sample

Blood.

Any preparation?

None.

Your results

Reading your test report

Your results will be presented along with those of your other tests on the same form. You will see separate columns or lines for each of these tests.

Low haematocrit

A low haematocrit means you may have a low number of RBCs, and that you are anaemic. This could be related to any one of a variety of causes. Further testing may be needed to work out the exact cause of the anaemia.

High haematocrit
High haematocrit levels indicate conditions where there is either too many RBCs being made or a high concentration of RBCs in your body. Also, if the blood volume is reduced, such as in dehydration, haematocrit can increase.

Reference intervals - comparing your results to the healthy population

Your results will be compared to reference intervals (sometimes called a normal range).

• Reference intervals are the range of results expected in healthy people.
• They are used to provide a benchmark for interpreting a patient's test results.
• When compared against them, your results may be flagged high or low if they sit outside this range.
• Some reference intervals are harmonised or standardised, which means all labs in Australia use them.
• Others are not because for these tests, labs are using different instruments and chemical processes to analyse samples.
• Always compare your lab results to the reference interval provided on the same report.

If your results are flagged as high or low this does not necessarily mean that anything is wrong. It depends on your personal situation.

Reference intervals for haematocrit vary between labs so you will need to go through your results with your doctor. Your doctor is the best person to interpret your results.

More to know?

• Pregnancy usually causes a slightly decreased haematocrit due to extra fluid in the blood.
• There may be temporary decreases in haematocrit during menstrual periods.
• Living at high altitudes causes an increased haematocrit. This is your body's response to the decreased oxygen available at these heights.
• People who have a chronic illness (such as rheumatoid arthritis), an inherited blood disorder, or malnutrition may have an abnormal haematocrit.
• Smoking can cause a high haematocrit due to increased red blood cell production and decreased oxygen levels.

Questions to ask your doctor

The choice of tests your doctor makes will be based on your medical history and symptoms. It is important that you tell them everything you think might help.

You play a central role in making sure your test results are accurate. Do everything you can to make sure the information you provide is correct and follow instructions closely.

Talk to your doctor about any medications you are taking. Find out if you need to fast or stop any particular foods or supplements. These may affect your results. Ask:

• Why does this test need to be done?
• Do I need to prepare (such as fast or avoid medications) for the sample collection?
• Will an abnormal result mean I need further tests?
• How could it change the course of my care?
• What will happen next, after the test?

More information

Pathology and diagnostic imaging reports can be added to your My Health Record. You and your healthcare provider can now access your results whenever and wherever needed.

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