An FBC is requested for a wide variety of reasons.

An FBC is also commonly used to:

Because the test provides such a broad overview of blood cell health, it is often one of the first investigations performed when symptoms are non-specific.

Red blood cells, also called erythrocytes, are responsible for transporting oxygen from your lungs to every tissue and organ in your body.

The body continuously produces red blood cells in the bone marrow. On average, each red blood cell survives for around 120 days before being removed and recycled by the spleen.

An FBC measures both the number of red blood cells and several characteristics that help doctors understand how well they are functioning.

Abnormal red blood cell measurements may indicate:

These patterns provide important diagnostic clues.

When the MCV is lower than the normal range, the red blood cells are smaller than expected.

Iron deficiency is by far the most common cause of a low MCV in UK general practice.

If the MCV is low, your GP will often request additional blood tests, particularly a ferritin level, to assess your iron stores.

Internal Link: Ferritin Blood Test Explained

Further blood tests are usually required to identify the underlying cause.

When the MCV is above the normal range, red blood cells are larger than expected.

Macrocytosis does not always indicate anaemia. Some people have an elevated MCV while maintaining a normal haemoglobin level.

Doctors therefore consider the MCV alongside the haemoglobin, MCH, MCHC and clinical history.

A reduced MCH usually indicates that each red blood cell contains less haemoglobin than normal.

This commonly occurs in:

Low MCH is frequently seen alongside a low MCV.

Although MCH provides useful information, it is rarely interpreted on its own. Instead, doctors assess it together with MCV and MCHC.

A low MCHC indicates that the red blood cells contain a lower concentration of haemoglobin.

This is commonly seen in:

A raised MCHC is relatively uncommon.

Because high MCHC is uncommon, your doctor will usually review the blood film and other blood indices if it is significantly elevated.

Red Cell Distribution Width (RDW) measures the variation in the size of your red blood cells.

Common causes include:

Their main role is to identify, attack and remove harmful microorganisms such as bacteria, viruses, fungi and parasites.

Unlike red blood cells, which all perform broadly similar functions, there are several different types of white blood cells, each with a specialised role.

An FBC measures:

A mildly low lymphocyte count is common and often has little clinical significance, particularly if the individual is otherwise well.

Monocytes are the largest type of white blood cell.

After leaving the bloodstream, they enter tissues and develop into macrophages, specialised cells that remove dead cells, bacteria and other debris.

Monocytes also help coordinate the immune response by communicating with other immune cells.

A reduced monocyte count is less common and is usually of limited significance on its own.

It may occur in:

A raised eosinophil count is commonly associated with:

If eosinophilia is identified, your GP will consider your symptoms, travel history, medication use and any history of allergic disease before deciding whether further investigation is required.

A low eosinophil count is generally not clinically significant.

It may occur:

In most cases, an isolated low eosinophil count does not require further investigation.

Basophils are the least common type of white blood cell, usually accounting for less than 1% of circulating white blood cells.

They are involved in allergic and inflammatory responses by releasing substances such as histamine and heparin.

Although changes in basophil counts are relatively uncommon, they can occasionally provide useful diagnostic information.

Because basophilia is uncommon, your doctor will interpret it alongside the rest of your blood count and your clinical history.

A low basophil count is usually of little clinical significance.

It may be seen in:

Isolated low basophils rarely require further investigation.

Their primary role is to help stop bleeding by forming blood clots when a blood vessel is damaged.

When you cut yourself, platelets rapidly gather at the site of injury, stick together and form a temporary plug. They also activate the clotting system, helping to prevent excessive blood loss.

An abnormal platelet count does not necessarily mean that you will develop abnormal bleeding or blood clots, but it may indicate that further assessment is required.

Possible symptoms include:

Deficiencies of key nutrients can affect blood cell production.

Examples include:

Internal Link: Folate Blood Test Explained

Both bacterial and viral infections commonly affect blood cell counts.

For example:

Long-term inflammatory conditions may alter blood cell production.

Examples include:

Common sources include:

Your GP may recommend additional investigations depending on your age, symptoms and risk factors.

Examples include:

Fortunately, these conditions are much less common than nutritional deficiencies or infections.

Several medicines may influence blood cell production.

Examples include:

Your doctor will consider your medication history when interpreting your results.

A Full Blood Count cannot diagnose every condition by itself, but it often provides the first indication that further investigation is needed.

Common conditions identified or suspected using an FBC include:

To understand the cause of an abnormal result, doctors frequently combine it with additional blood tests.

Ferritin assesses your body’s iron stores.

For example:

Providing this information helps ensure your results are interpreted accurately.

It is common to feel anxious if you receive your blood test results before speaking to your GP, particularly if one or more values are highlighted as being outside the laboratory reference range.

However, an abnormal result should always be interpreted in context.

Doctors do not diagnose conditions based on one blood test alone. Instead, they consider:

Minor abnormalities are common and often do not indicate serious disease.

For example:

Your GP may recommend repeating the blood test after a few weeks before arranging further investigations.

For example:

This pattern recognition is one reason why self-interpreting blood results online can be misleading. The same abnormal result may have different causes depending on the rest of the blood count and the clinical situation.

Arrange a routine appointment with your GP if you have:

You should also see your GP if you have been informed that your FBC is abnormal but have not yet discussed the results with a healthcare professional.

Most abnormal Full Blood Count results are not emergencies. However, certain symptoms require urgent medical assessment.

A Full Blood Count is a routine blood test that measures your red blood cells, white blood cells and platelets. It helps doctors assess your general health and investigate a wide range of symptoms and medical conditions.

Yes. In the UK, the test is usually called a Full Blood Count (FBC), while in some other countries, including the United States, it is referred to as a Complete Blood Count (CBC).

Yes. Changes in white blood cell counts can provide important clues about infection.

For example: