What is a full blood count - and what can it tell us?

There is a wide range of 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 us about your health.

Common tests

Common blood tests may include a 'full blood count' used by the doctor to establish the number, size etc 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 test's title can be misleading to patients, who may assume that this is a thorough 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. "You could take an armful of blood and you couldn't do that."

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, the GP may then request more tests to confirm a diagnosis.

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. 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 tourniquet around the top of the arm, and may ask you to clench and release your fist.

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

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How to prepare for a blood test

A blood test is one of the most common medical tests and is a good way to get a picture of your overall health, which helps medical professionals assess health and disease. Most people will likely need a blood test at some point in their lives so it's important to know how to prepare.

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PSA test

The PSA test (prostate specific antigen) is a blood test to see if you might have prostate cancer and to monitor treatment for prostate cancer. The PSA blood level is also increased in other conditions. So having an increased PSA test result does not mean that you have prostate cancer. Experts disagree on how useful the PSA test is. There is a lot of ongoing research about PSA. At the moment there is no national screening programme for prostate cancer in the UK.

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Test limitations

Some patients 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 GP requests a blood test after a consultation.

It's important to remember that blood tests will only give a snapshot of a person's 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 GP 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 GP will initially talk with the patient, to find out about their symptoms and lifestyle," explains Dr Cooper. "Then they will examine them for additional clues as to what might be causing their 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 a patient's 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 GP who will be able to provide support and guidance.