Full Blood Count (FBC) Interpretation PDF

Full blood count (FBC) Interpretation geekymedics.com/fbc-interpretation Dr Euan Haynes July 27, 2021 1/10 2/10 Introduction This guide provides a structured approach to the interpretation of a full blood count (a.k....

Full blood count (FBC) Interpretation geekymedics.com/fbc-interpretation Dr Euan Haynes July 27, 2021 1/10 2/10 Introduction This guide provides a structured approach to the interpretation of a full blood count (a.k.a. FBC, complete blood count, CBC) which you should be able to apply in most circumstances. Note: This guide is only relevant to non-pregnant adults as normal ranges differ significantly in pregnant patients and in the paediatric population. Are you learning to interpret blood tests? Check out our Blood Test Case Bank, containing over 95 cases with step-by-step interpretations and detailed explanations What tests are included in a FBC? The full blood count gives insight into the cellular components of blood including white blood cells, red blood cells, and platelets providing numerical values relating to these cells (e.g. amount of them, size, contents). These results provide vital clues to the presence of underlying pathology. The long list of acronyms and numbers can seem daunting at first, however, by following a structured approach you can make sense of them all! A standard FBC can be broken down into the following red cell, white cell and platelet tests. Red cell tests Red cell tests include: Haemoglobin (Hb): amount of haemoglobin (oxygen-carrying protein) in whole blood Haematocrit (Hct): percentage of the blood sample that is made up of red cells Mean corpuscular volume (MCV): the average size of the red cells present in the blood sample Red cell distribution width (RDW): a range from the largest red cell present to the smallest red cell present Red cell count (RCC): the number of red cells present per unit volume of blood Reticulocyte count: the number of reticulocytes (immature red cells) Mean corpuscular haemoglobin (MCH): the amount of haemoglobin per red blood cell Mean corpuscular haemoglobin concentrate (MCHC): average concentration of haemoglobin in a given volume of blood White cell tests White cell tests include: White blood cell count (WCC): the number of white blood cells White blood cell differential: the breakdown of the white blood cell count into different cells Platelet tests Platelet tests include: Platelet count: the number of platelets Mean platelet volume (MPV): the average size of platelets Platelet distribution width (PDW): measures variability in platelet size Tip: Those tests shown in bold are the most important to understand and those that this article will focus on. However, some laboratories will provide results for all the above and more as part of a standard full blood count. These additional results tend to be used in more specific situations by specialists. Reference ranges We have included key reference ranges below to assist you when interpreting a full blood count. Haemoglobin (Hb): 130 – 180 g/​L 115 – 165 g/​L 3/10 White cell count (WCC): Total: 3.6 – 11.0 x 109/L Neutrophils: 1.8 – 7.5 x 109/L Lymphocytes: 1.0 – 4.0 x 109/L Monocytes: 0.2 – 0.8 x 109/L Eosinophils: 0.1 – 0.4 x 109/L Basophils: 0.02 – 0.10 x 109/L Platelet count: 140 – 400 x109/L Red cell count (RCC): 4.5 – 6.5 x 1012/L 3.8 – 5.8 x 1012​/L Haematocrit: 0.40 – 0.54 L/​L 0.37 – 0.47 L/​L Mean cell volume (MCV): 80 – 100 fL Mean corpuscular haemoglobin (MCH): 27 – 32 pg/cell Reticulocyte count: 0.2 – 2% Note: Reference ranges are lab-specific, therefore always refer to your local reference ranges when interpreting a full blood count. Red cell tests Red cell tests can quantify the amount of haemoglobin/red cells present in a sample: An abnormally low number of haemoglobin/red cells is known as anaemia An abnormally high number of haemoglobin/red cells is known as polycythaemia. Red cell tests can also provide important insights into the underlying cause(s) of anaemia or polycythaemia by looking at the average size of the cells and how much haemoglobin is contained within them. Haemoglobin (Hb) Anaemia Anaemia refers to a decrease in the total amount of haemoglobin in the blood. There are a wide range of causes of anaemia, which can be sub-categorised based on the average red cell size (mean cell volume/MCV): Microcytic anaemia: low haemoglobin associated with a reduced MCV Macrocytic anaemia: low haemoglobin associated with an increased MCV Normocytic anaemia: low haemoglobin associated with a normal MCV When you identify a low haemoglobin, you should look to the MCV to see which sub-type of anaemia is present as this information, alongside a good clinical assessment, can help narrow the differential diagnosis. In general, when you identify anaemia it is sensible to check haematinics (e.g. ferritin, B12/folate) as deficiencies are common and easy to treat. For more information, see the Geeky Medics guides to anaemia and interpretation of haematinics. Polycythaemia Polycythaemia is an abnormally high haemoglobin concentration, this is usually seen alongside an elevated haematocrit (the percentage of blood that is made up by red cells). Polycythemia can be due to an increase in the number of red cells, termed absolute polycythaemia, or it can be caused by a decrease in the amount of blood plasma (the liquid component of blood), which is termed relative polycythaemia. Absolute polycythemia can be caused by an issue in the bone marrow leading to a proliferation of red cell precursors (primary polycythaemia) or conditions that increase the amount of erythropoietin (EPO) circulating in the blood (secondary polycythaemia). EPO is a hormone produced in the kidneys which stimulates the bone marrow to make more red blood cells. As a result, when there is a state of EPO excess, this leads to polycythemia. 4/10 Primary polycythaemia is typically caused by myeloproliferative neoplasms (e.g. polycythaemia rubra vera). Secondary polycythaemia has a wide range of possible causes, some of these create a chronic state of hypoxia while some are due to ectopic EPO production or stimulation. Some examples of causes of secondary polycythaemia include: Chronic obstructive pulmonary disease Smoking Obstructive sleep apnoea Cyanotic heart disease Lung fibrosis Exogenous steroids Excess alcohol intake (can also cause low blood cell counts) Certain malignancies (renal cell carcinoma, cerebellar haemangioma, Wilm’s tumour) EPO abuse (e.g. in athletics) Endogenous steroids (Conn’s syndrome, Cushing’s syndrome) Relative polycythaemia involves a reduction in the volume of blood plasma causing red blood cells to become more concentrated while the actual number of them does not change (like putting less water into cordial would make a more concentrated drink, less plasma in the blood sample will concentrate the red blood cells). This can be caused by low fluid intake or states of excess fluid loss, such as significant burns. Tip: Most polycythaemia is secondary, with the most common causes being smoking and excess alcohol intake. Mean corpuscular volume (MCV) Mean corpuscular volume (MCV) is a measure of the average size of the red cells present in the blood sample. This test is particularly useful in the context of anaemia, where it can be used to aid the differential diagnosis of potential underlying causes and guide which investigations to perform next. The following list of underlying causes are ‘textbook’ but there is often some overlap between them. For instance, don’t assume a patient with a macrocytosis cannot also be iron deficient. Microcytic anaemia Causes of microcytic anaemia include: Iron deficiency anaemia: the commonest cause in the UK (see our article on iron deficiency anaemia) Haemoglobinopathies (e.g. thalassemia syndrome/trait) Anaemia of chronic disease/inflammation (can be microcytic, but often normocytic) Lead poisoning (rare) Sideroblastic anaemia (rare) Hookworm infection (a common cause of microcytic anaemia in low/middle-income countries) Hint: An acronym to remember the causes of microcytic anaemia is TAILS: Thalassemia/haemoglobinopathies, Anaemia of chronic disease/inflammation, Iron deficiency anaemia, Lead poisoning, Sideroblastic anaemia Normocytic anaemia Causes of normocytic anaemia include: Anaemia of chronic disease/inflammation Acute blood loss Increase plasma volume (e.g. pregnancy, fluid overload) Mixed aetiology anaemias Haemoglobinopathies (e.g. thalassaemias) Aplastic anaemia Haemolysis Hypersplenism (leads to increased destruction of red blood cells) Macrocytic anaemia Causes of macrocytic anaemia include: B12/folate deficiency Toxins (e.g. alcohol, chemotherapy) Liver disease Reticulocytosis (reticulocytes are immature red blood cells) Pregnancy Myeloma 5/10  Myelodysplastic syndrome Hypothyroidism Haematocrit (Hct) Haematocrit (Hct) is the percentage of the blood sample that is made up of red cells and tends to closely follow the trend of the haemoglobin level and red cell count. Haematocrit can be affected by either the number of red blood cells or the volume of blood plasma. A raised haematocrit can result in increased blood viscosity due to there being a high number of red cells relative to blood plasma making blood ‘sticky’. If the haematocrit continues to rise, hyperviscosity syndrome can develop, which is associated with the development of thrombi and symptoms such as headaches, blurred vision and chest pain. To prevent hyperviscosity some patients require venesection to reduce their haematocrit to safe levels with specific target levels varying depending on the underlying cause of the polycythemia. Red cell count Red cell count is the number of red cells present per unit volume of blood. This test can be used in combination with the haemoglobin level and the haematocrit to confirm the presence of anaemia or polycythaemia. The causes of a high or low red cell count are the same as those for haemoglobin and haematocrit. Red cell distribution width (RDW) Red cell distribution width (RDW) is useful to take a more in-depth look at the MCV (average of the red cell size). The RDW is a range from the largest red cell present to the smallest red cell present. This is useful information in the context of mixed anaemia (i.e. anaemia involving the presence of both macrocytic red cells and microcytic red cells). Mixed anaemia may develop in conditions where absorption from the gut is impaired, such as coeliac disease, leading to deficiencies in both iron (which typically causes microcytic anaemia) and B12/folate (which typically causes macrocytic anaemia). In these situations, the MCV can be misleadingly normal, however, this large variation in red cell size will be evident with an elevated RDW. The presence of red cells of varying sizes is known as anisocytosis and can be associated with iron deficiency anaemia as well. Reticulocyte count A reticulocyte is an immature red cell that is normally released from the bone marrow into the peripheral blood, fully maturing into a red blood cell within 24 hours. Therefore, the reticulocyte count can be used to assess the response of the bone marrow to anaemia. A raised reticulocyte count in the context of anaemia implies that the bone marrow is effectively functioning to produce lots of new red blood cells to correct the anaemia. This would therefore suggest red blood cells are being destroyed in the peripheral circulation (e.g. haemolysis, bleeding) rather than there being an issue with the production of red blood cells in the bone marrow itself. Whereas, a low reticulocyte count in the context of anaemia implies a problem with the bone marrow not being able to make enough cells. This could be due to nutritional deficiencies (e.g. B12/folate or iron) or a primary bone marrow disorder (e.g. aplastic anaemia, bone marrow infiltration from solid organ malignancies). A raised reticulocyte count in the absence of anaemia may indicate that the body is effectively compensating for blood loss or haemolysis (i.e. the increased production is managing to replenish the number of cells being lost in the peripheral circulation). Alternatively, a raised reticulocyte count in the absence of anaemia may be due to the body adapting to increased oxygen demands. White cell tests Leukocytosis A high white cell is known as leukocytosis and can be caused by a wide range of pathological processes. Important factors in determining the cause of leukocytosis include the rate of the white cell count rise and which type of white cell(s) are raised. Common causes of acute leukocytosis: Reactive: infection, inflammation, post-surgery Steroids: stress response (i.e. endogenous steroids) or medication (i.e. exogenous steroids) Haematological: acute leukaemias Common causes of chronic leukocytosis: Reactive: chronic infection, smoking Haematological: leukaemia, certain subtypes of lymphoma Hyposplenism: typically mild Pregnancy 6/10 Neutrophils and lymphocytes make up the majority of the white cells in the blood, so these two cell types are usually the cause of a raised total white cell count. A comprehensive assessment is always required to help narrow the differential diagnosis of leukocytosis and guide the need for further investigations. Leukopenia A low white cell count is known as leukopenia and can also be caused by a wide range of pathological processes. Important factors in determining the cause of leukopenia include the rate of the white cell count fall and the type of white cell(s) which are low. Neutrophils and lymphocytes account for the majority of white cells in the blood, so these types of white cells are typically reduced in the context of leukopenia. A comprehensive assessment is always required to help narrow the differential diagnosis of leukopenia and guide the need for further investigations. Common causes of leukopenia include: Infection: can be seen as a transient phenomenon in viral illness or as a result of consumption in sepsis Medications: antibiotics, immunosuppressants, anti-epileptics, cytotoxic agents (e.g. chemotherapy) B12/folate deficiency Autoimmune disease Iron deficiency HIV (any cytopenia could be due to HIV) Racial variation: middle eastern and black patients can have lower baseline neutrophil counts which are not pathological Bone marrow failure: often this will be seen alongside low platelets and low haemoglobin White cell differential Neutrophils Neutrophils usually account for 40-70% of the total white blood cells and are key in the acute phase of infection (particularly bacterial infections). Neutrophilia As most of the white cells present in the blood are neutrophils, the causes of a high neutrophil count are the same as those that cause a high total white cell count. Neutropenia As most of the white cells present in the blood are neutrophils, the causes of a low neutrophil count are the same as those that cause a low total white cell count. Hint: When neutrophil counts are low (