Understanding blood cells: What can a blood count tell us?

Blood consists of plasma and formed elements. These include red blood cells, white blood cells and platelets. They perform different tasks: transporting oxygen, supporting immune defence and helping to stop bleeding.

A blood count provides an overview of these cell lines. Meaningful interpretation, however, considers the pattern of several values, the laboratory reference range, symptoms, medication and previous results rather than a single number alone.

What does a blood count measure?

A complete blood count, also called a full blood count, measures the number and some of the characteristics of different blood components. It commonly includes:

The total white blood cell count forms part of a standard blood count. A differential blood count additionally reports the different types of white blood cells separately, providing more detail about their composition.

• red blood cells or erythrocytes,
• white blood cells or leukocytes,
• platelets or thrombocytes,
• haemoglobin and haematocrit,
• and indices describing the size and haemoglobin content of red blood cells.

Red blood cells: carrying oxygen to tissues

Red blood cells are medically known as erythrocytes. They contain haemoglobin, an iron-containing protein that binds oxygen in the lungs and carries it to organs and tissues. They also contribute to transporting carbon dioxide back to the lungs.

The red blood cell count alone is not enough to assess the red cell profile. Other values add important context:

Together, these measurements can help characterise conditions such as anaemia. They do not, however, identify the underlying cause on their own.

• Haemoglobin (Hb) describes the amount of oxygen-carrying protein in the blood.
• Haematocrit (Hct) indicates the proportion of blood volume occupied by red blood cells.
• MCV describes their average size.
• MCH and MCHC describe the average haemoglobin content and haemoglobin concentration of red blood cells.

White blood cells: different roles in immune defence

White blood cells, or leukocytes, are part of the immune system. They are produced in the bone marrow and can travel through blood and tissues to where they are needed.

A differential blood count mainly distinguishes five types:

Raised or lowered results can have many causes, including infections, inflammation, medication effects or other medical conditions. Their distribution therefore needs to be considered together with the wider health context.

• Neutrophils form part of the rapid innate immune response and respond to many bacterial infections.
• Lymphocytes include B cells and T cells and play a central role in targeted immune responses.
• Monocytes can remove microorganisms and cellular debris and develop into other immune cells in tissues.
• Eosinophils are involved in allergic reactions and defence against certain parasites.
• Basophils release signalling substances involved in processes such as allergic reactions.

Platelets: small fragments with an important function

Platelets are also called thrombocytes. Strictly speaking, they are not complete cells but small cell fragments produced in the bone marrow.

When a blood vessel is injured, platelets adhere to the damaged area, become activated and collect together. Working with clotting factors, they help form a stable seal and stop the bleeding.

A blood count mainly reports the number of platelets. It does not fully assess how well they function; separate platelet function tests are available for that purpose. An abnormal count may therefore warrant further assessment, but it does not automatically explain the cause or a person's individual risk of bleeding or clotting.

Why reference ranges do not tell the whole story

Laboratory reference ranges describe where results in a comparison population typically fall. They are not universal personal targets and may differ between laboratories. Age, hydration, altitude, acute illness, medication and other factors may also influence results.

Useful questions when looking at an abnormal blood count include:

Comparing results with previous reports can help show whether a value is stable or changing. Whether repeat testing or further investigation is appropriate should be discussed with a qualified healthcare professional.

• Which value has changed, and by how much?
• Are several related values affected?
• Is the change new, or has it been present for some time?
• Are there relevant symptoms, medical conditions or medicines?
• Has the result been confirmed or supplemented by further tests?

The blood count as part of the wider picture

Red blood cells, white blood cells and platelets perform different functions, yet their results are assessed together in a blood count. Changes over time can therefore be more informative than one isolated measurement.