What does a full blood count show us?

There are many reasons why you may need to have a blood test. But it's worth remembering that there are limits to what a blood test can tell you about your health.

What is a full blood count?

Common blood tests may include something called a full blood count, sometimes referred to as a complete blood count (CBC). This is used by your doctor to establish the number, size, and other details of red blood cells, white blood cells, and platelets - which play a major part in blood clotting - you have circulating in your system.

However, the tests' title can be misleading to people, who may assume that this is a thorough, extensive indication of your state of health.

"There is no test that can tell you that nothing is wrong with a person," explains Dr Carol Cooper, GP, media doctor and medical journalist. "You could take an armful of blood and you still couldn't do that."

What is tested in a full blood count?

Instead, if your full blood count indicates that a certain blood cell is abnormally high or low, this may indicate infection, anaemia, or other more serious diseases. Depending on the results, your doctor may then request more tests to confirm a diagnosis.

What is a normal blood count?

The normal ranges for a full blood count can vary, depending on factors like your age, sex, and whether you have other health conditions. There are also many things the test measures, but in general, it includes results for these normal ranges:

• White blood cell count (WBC) - 4,000 to 10,000 cells per mcL (4.0 to 10 k/mcL).

• Red blood cell count (RBC) - 4.0 to 5.4 million cells per mcL for women or 4.5 to 6.1 million cells per mcL for men.

• Average size of your red blood cells (MCV) - 80 to 100 fL (femtoliters).

• Platelet count - 150,000 to 400,000 cells per mcL (150 to 400 k/mcL).

• Average platelet size (MPV) - 7.0 fL to 9.0 fL.

Can a full blood count detect cancer?

Results of a full blood count can help toward a cancer diagnosis:

• Having fewer red blood cells than normal - can be a sign of anaemia. This is usually not caused by cancer, but it's a possible sign of bowel or stomach cancer.

• Too many white blood cells (leucocytosis) - very rarely can be a sign of leukaemia, a type of blood cancer.

• Too many platelets (thrombocythaemia) - this is linked with an increased likelihood of there being a cancer somewhere in the body, although it does not confirm it.

Other common tests

Other regularly ordered tests include measuring kidney or liver function, blood glucose (sugar levels), or hormone testing, most commonly thyroid hormone. Raised readings on other tests, called ESR and CRP, indicate that there's inflammation going on somewhere - which could be due to anything from a mild viral infection to an autoimmune condition such as rheumatoid arthritis or even, more rarely, to cancer.

These, however, are separate from a full blood count. Doctors also often test for levels of vitamin D in the blood, as many people become deficient in this so-called sunshine vitamin in the winter months. This is only available in some parts of the country. Cholesterol tests to measure the levels in the blood are also commonly used and can provide a useful insight into a patient’s risk of heart attack or stroke.

Taking blood

If you've been referred for a blood test for the first time, you may feel anxious. Rest assured it is a very straightforward procedure, which often takes under a minute to perform.

Blood is usually taken in a GP practice or at your local hospital. Normally a nurse or phlebotomist will draw the blood from the inner arm, using a needle. For most, the procedure is not painful, and only lasts a short time.

To help the flow of blood, the nurse will often tie a band around the top of the arm, and may ask you to clench and release your fist.

It's important to ask your doctor when they order the test whether it's a fasting test. If this is the case, you should not eat or drink anything other than water for 8-12 hours beforehand as this may affect the result of the test. Check the timing with your doctor.

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Test limitations

Some people may welcome the idea of a blood test and the information or potential reassurance it may appear to offer. Others may find themselves feeling anxious or apprehensive about the potential results if a doctor requests a blood test after a consultation.

It's important to remember that blood tests will only give a snapshot of your levels depending on the test requested, and cannot provide full reassurance of general health.

In addition, while some blood tests may flag up significant problems, blood tests are routine and being prescribed a test does not indicate that your doctor suspects anything serious.

Building a clearer picture

In fact, when you are referred for a test, you will already have gone through two stages of diagnosis that are equally as important as measuring levels in the blood.

"The doctor will initially talk with you, to find out about your symptoms and lifestyle," explains Dr Cooper. "Then they will examine you for additional clues as to what might be causing your symptoms.

"Requesting a blood test is simply part of the investigation process - but tests are simply tests, they don't give the whole picture."

If you are worried about why a test has been ordered or want more information about your state of health, it's important that you are open and thorough when talking with the GP during your appointment. To help, it's worth writing a list of symptoms and concerns before an appointment takes place.

Health anxiety

We all worry about our health from time to time. But for some people, these concerns become overwhelming. It is not uncommon for patients to request blood tests for reassurance, or to worry about symptoms despite having a GP examination.

It's important to remember that a blood test cannot provide complete reassurance about your state of health, and to trust your doctor to make a decision as to whether a test is required.

If you find yourself becoming overly anxious about your health, or about a blood test you have been prescribed, talk with your doctor. They will be able to provide support and guidance.