Blood tests are valuable investigations to support clinical understanding of patients’ health stories. Dr Phil Deakin demystifies the full blood count, offering guidance to its interpretation.
What is a full blood count?
A full blood count (FBC) is one of the most commonly performed blood tests. Why should that be? Why might we consider doing it? What can it tell us? …and why does it all look so horribly complicated?
A full blood count report can certainly appear to be a particularly intimidating document — filled with cryptic abbreviations and obscure units of measure. But, fear not! It’s simply a clinical jigsaw puzzle that becomes much clearer once you put it all together, stand back a bit, and take a look at it as a whole.
Unlike the majority of blood tests which focus on the chemistry of the blood, a FBC highlights what is happening to the very cells that make up our bloodstream by looking at them under a microscope. In some ways our blood is an organ in its own right which can be easily and conveniently biopsied and studied.
Welcome to the world of the haematologists.
There are essentially three groups of things in a full blood count; red cells, white cells, and some little sub-cellular bits called platelets. Each one can be measured in different ways to reveal a surprising amount of information about health, illness and wellbeing.
Here are some typical components of a full blood count and some perspectives on what they might signify.
Red blood cells (erythrocytes)
Erythrocytes are full of an iron-containing red substance called haemoglobin. They are made and matured in the bone marrow then released into the bloodstream to do their work carrying oxygen around the body. Mature erythrocytes are the only cells in the body without nuclei, and lacking the onboard machinery to renew or repair themselves, their lifespan is limited to 100 to 120 days.
Making erythrocytes is a complex process. As well as raw materials in the form of iron, amino acids and lipids, there are also a staggering array of hormones, cofactors and facilitators needed to assemble and activate them properly. They need to be fully developed, be the right size and the right shape and produced in the right quantity. Not too much and not too little of anything. Results falling outside this ‘Goldilocks zone’ of the reference range can tell us many stories of potential mischief elsewhere in the body.
Haemoglobin
Although strictly not part of a full blood count, this test is usually done routinely on the same blood sample and measures the total amount of haemoglobin present in a standard volume of blood. If there is too little haemogloblin (anaemia), then some sort of deficiency or blood loss is likely, whereas too much (polycythaemia) can point to metabolic imbalances or deregulation of the red cell production process. Looking at other parts of the FBC can help to clarify what is going on.
Haemoglobin levels are usually expressed using the international standard of grams per decilitre (g/dL), but some sources and laboratories continue to use the old measurement of grams per litre (g/L), that’s a power of ten difference, so it’s always best to check their reference range before interpreting the result!
Red blood cell count (RBC)
This simply counts the number of red cells in a given volume of blood. If there are too few red cells (anaemia) this could suggest either reduced cell production or increased loss of red cells for some reason.
Red cell production may be reduced in iron deficiency, hypothyroidism, chronic kidney disease, alcohol abuse, and deficiencies of cofactors for red cell production such as vitamin B12 or folic acid. Red cell levels may also drop if the bone marrow is damaged by radiation, leukaemias and lymphomas, medications or other toxins. Circulating red cells may also be lost as a result of bleeding, increased erythrocyte breakdown (haemolysis), an enlarged spleen, sickle cell disease and some autoimmune conditions.
Conversely, a raised red blood cell count is called polycythaemia and is usually secondary to other factors. Red cell production may sometimes be increased to compensate for a lack of available oxygen, for example in smoking, chronic lung disease, heart failure, congenital heart disease, high altitudes, exposure to carbon monoxide and sleep apnoea. Medications such as testosterone may also stimulate red cell production. Diuretics often reduce the volume of fluid in the blood, making it more concentrated and so raising the RBC. Poor fluid intake or increased fluid loss leading to reduced hydration will have a similar effect.
Increased red cell production without a secondary cause is called polycythaemia (rubra) vera. The causes of this primary polycythaemia are unclear and haemoglobin production is also found to be high.
Whatever the cause, too many red cells will tend to make the blood thicker and may increase the risk of stroke and other forms of blood vessel damage.
Packed cell volume (PCV)
This is a measurement of the percentage of the whole blood volume taken up by just the red blood cells. It will often mirror the RBC, but is also influenced by the size of the cells. Sometimes called the haematocrit, it is useful as a guide but is not as precise as looking at cell numbers and cell sizes separately.
Mean cell haemoglobin (MCH)
Not incredibly helpful on its own, this test represents the weight of haemoglobin in an average red blood cell. It is calculated indirectly by multiplying haemoglobin by ten and dividing by the red blood cell count.
Mean cell haemoglobin concentration (MCHC)
How pink the cells are — this is much more useful! This test tells us the concentration of haemoglobin in an average red blood cell and is derived by dividing the haemoglobin by the packed cell volume. Low MCHC is called hypochromia and is a strong indicator of iron deficiency. Causes may include dietary deficiencies, malabsorption syndromes, inflammatory bowel disease, chronic low-level blood loss, cancer, and parasites.
A raised MCHC (hyperchromia) may present in hereditary spherocytosis, autoimmune haemolytic anaemias, and in severe burns.
Making sense of anaemia (low haemoglobin)
- Microcytic, hypochromic anaemias are almost always related to some form of iron deficiency, or may also be due to chronic disease states.
- Normocytic normochromic anaemias (low haemoglobin but each red cell appears to be normal) may be due to bleeding, haemolysis, medications, or chronic disease.
- Macrocytic anaemias are often linked to B12 or folate deficiency, liver disease, alcohol, or medications.
Mean cell volume (MCV)
This measures the average size of the red blood cells. Smaller red blood cells (microcytosis) again point to iron deficiency. Other causes may be the result of a variety of metabolic changes including thalassaemia, sideroblastosis, lead poisoning, and anaemias related to chronic disease or cancer.
Immature red blood cells in the bone marrow are called macrocytes. Larger than erythrocytes, they may be released into the blood stream a bit earlier than usual (macrocytosis) when cofactors for maturity are deficient or the bone marrow is under stress. This can happen with liver disease, B12 or folate deficiency, hypothyroidism, myeloproliferative and myelodysplastic diseases, alcohol abuse, and a variety of medications including azathioprine, methotrexate, and trimethoprim.
Platelets (PT)
Also called thrombocytes, platelets are tiny but complex fragmentary cellular bodies produced in the bone marrow from cells called megakaryocytes. They are the first line of defence when blood vessels are damaged, becoming activated then sticking together to plug the gap and triggering cascades of other clotting and repair mechanisms. The platelet count measures the number of platelets present in a given volume of blood.
A low platelet count (thrombocytopaenia) may represent decreased platelet production in the bone marrow due to a variety of congenital syndromes, viral infections, aplastic anaemias, or bone marrow infiltration by lymphomas or metastatic cancers. Many medications including quinine, aspirin, heparin, digoxin, gold and cytotoxic drugs may also reduce platelet production.
Sometimes platelets are produced and released into the bloodstream but don’t survive there for very long. This can happen with conditions such as idiopathic thrombocytopaenic purpura, systemic lupus erythematosus, haemolytic uraemic syndrome, and hypersplenism. Massive internal bleeding may cause platelets to be rapidly consumed and make their levels fall dramatically.
If taking the blood sample has been difficult or traumatic, tissue damage may trigger platelets to clump together in the sample bottle thereby giving a falsely lowered reading. Immediately following intravenous fluids or blood transfusion dilution of the blood may also produce a low platelet count.
A high platelet count (thrombocytosis) can be a fairly non-specific finding. Production of platelets may increase due to many physiological processes and illnesses such as inflammation, malignancy, bleeding, mild iron deficiency, and after splenectomy.
Essential thrombocythaemia occurs when no clear cause for a high platelet count is apparent, and the platelets may often be abnormal in appearance.
Total white cell count (WCC)
White cells are made in the bone marrow and may be stored in lymph tissues. WCC is a very nonspecific measurement of the total number of all white cells in a given volume of blood. It is often useful as a first glance at someone’s infection and immune status.
A raised WCC (leucocytosis) may point to bacterial or viral infections, bone marrow disease, immunological dysfunction or medications.
Low readings can suggest viral infections, some cancers, autoimmune disorders, sarcoidosis, severe infections that consume white cells and some medications, including antibiotics.
A differential white cell count tells us more about each type of white cells present providing a much better picture of what is going on.
Neutrophils
Neutrophils are primarily involved in combating acute bacterial infections but their count can also be raised (neutrophil leucocytosis) by other things including trauma, inflammation, leukaemias and myeloproliferative disorders, surgery, haemorrhage and burns, infarction, polymyalgia, polyarteritis, and medications, including steroids.
A low neutrophil count (neutropaenia) may be due to viral infections, damage to bone marrow by cancer or aplasias, megaloblastic anaemia, tuberculosis, typhoid, brucellosis, and medications including carbimazole, sulphasalazine and many cytotoxic drugs.
Neutrophils are the mayflies of the haematology world as they have a lifespan of less than a day and can therefore disappear from the circulation very quickly if something goes wrong. Severe neutropaenia is a life threatening condition that may lead to sepsis.
Lymphocytes
Lymphocytes are produced to help to combat viral infections and may also be raised (lymphocytosis) in chronic lymphocytic leukaemia, Epstein-Barr virus, cytomegalovirus, rubella, whooping cough, brucellosis, and toxoplasmosis
Low lymphocyte levels (lymphocytopaenia) can be found in uraemia (elevated levels of urea in the blood), bone marrow infiltration, systemic lupus erythematosus, legionnaire’s disease, acquired immune deficiency syndrome, and after chemotherapy or radiotherapy.
Monocytes
Monocytes act as scavenger cells, mopping up debris after infection, inflammation or cell damage. Monocyte levels may be raised (monocytosis) in myelodysplasia, tuberculosis, malaria, Hodgkin’s lymphoma, brucellosis, infective endocarditis, and rickettsial infections.
Levels may be low (monocytopaenia) where production drops with bone marrow infiltration or following chemotherapy or radiotherapy. Severe infections may rapidly consume monocytes also leading to low readings.
Eosinophils
Eosinophils play a major role in our allergic responses and in combating parasitic infections. As you might expect, they are often raised (eosinophilia) in many people with allergies including asthma, urticaria, hayfever, eczema, scarlet fever, and angioneurotic oedema. Some autoimmune conditions may also increase eosinophil production including polyarteritis nodosa, pemphigus, erythema multiforme, and dermatitis herpetiformis. Raised levels are also found in Hodgkin’s disease, Addison’s disease, cancer, radiation treatment, chronic myeloid leukaemia, and many parasitic infections. Medications such as penicillin and streptomycin are also potential causes of eosinophilia.
We aren’t really sure of the significance of low levels of eosinophils.
Basophils
Not of great diagnostic value, basophils are also involved in the allergic response and in mediating inflammation. Raised levels (basophilia) have been reported in viral infections, myxoedema, malignancies, urticaria, ulcerative colitis, haemolysis, myelofibrosis, polycythaemia (rubra) vera, systemic mastocytosis, chronic myeloid leukaemia, and after splenectomy
As the lower limit of the standard reference range for basophils is zero, there is no such thing as a reduced level as this would probably involve minus numbers. Haematologists tend not to do minus numbers.
Blood count anomalies
These days most full blood counts and white cell differentials are done automatically with computerised microscopy. Sometimes anomalies are thrown up where an experienced human eye may be needed to figure out what is going on.
Rouleaux
The disc-shaped erythrocytes sometimes stick together in a stack — rather like a pack of chocolate biscuits that got left in the sun. This often happens in the sample bottle but may also happen in the circulation. Rouleaux formation is more likely when acute-phase proteins produced by inflammation are present, such as fibrinogen and globulins. Reports of rouleaux formation may therefore be a non-specific indicator of inflammation or disease and are associated with many inflammatory, infective and connective tissue disorders, cancers, myelomas, and also in diabetes. Rouleaux are unable to navigate the fine capillaries of the body and may cause damage by blocking them up. In the laboratory the rouleaux form heavy clumps that settle on the bottom of the test tube more quickly than individual erythrocytes. This forms the basis for an inflammatory marker test called an erythrocyte sedimentation rate (ESR). The more inflammation there is, the bigger the clumps, the faster they fall and the higher the ESR reading.
Parasites
Although viruses, bacteria, and fungi are able to enter the bloodstream they are rarely seen on routine microscopy. Some larger blood parasites can be seen, and studying a blood film may be useful if they are suspected. Malaria is an important example of a blood-borne parasite, and others may include filariasis, sleeping sickness, and schistosomiasis. It’s always worth considering blood film microscopy where a patient has become unwell after returning from overseas travel to a remote rural or exotic location.
Blood group
Despite what many patients believe, a full blood count does not include blood group testing!
Reference ranges and individual context
Why are no reference ranges quoted here? Well, for a start endless lists of numbers are really boring to read! And sometimes they can be a bit misleading, as each laboratory will have its own reference depending on the type of sample provided, the calibration of the test machinery, and local and national guidelines and standards.
More importantly, the reference range for a particular test might need to be adjusted for the individual patient. Blood values could vary with age, gender, nutritional status, behavioural considerations, lifestyle factors, medications, cultural background, geographical location, time of day, time of month, and season of the year.
Context is all! Wherever possible try to use the specific reference range guidance for the laboratory that processed the sample, and interpret the result carefully using your own knowledge and understanding of the patient and what has been happening to them.
Complete the puzzle and see the whole picture.
Sources and resources
- Deakin P. Blood tests and some other stuff: An introductory guide for herbalists. Digital Artery; 2019.
- Erythrocytes. Kenhub. https://www.kenhub.com/en/library/anatomy/erythrocytes. Published November 3, 2023. Accessed January 12, 2025.
- Full blood count – understand the test. LabTestsOnlineUK. https://labtestsonline.org.uk/tests/full-blood-count-fbc. Published January 21, 2022. Accessed January 12, 2025.
- Provan D. Oxford Handbook of Clinical and Laboratory Investigation. 4th ed. Oxford University Press; 2018.
