Full Blood Count

Synonyms: FBC, complete blood count (USA), CBC (USA)

There are a number of reasons why you might request a full blood count (FBC). A cursory glance at the FBC report will give you an idea about the presence of anaemia, infection or blood disorders. However, closer scrutiny will reveal a great deal more. This record will give you an overview of the main parameters measured and what they assess.

The FBC should be evaluated along with a blood film report - see our record on the Peripheral Blood Film. Follow the links provided for more information about the related pathology.

A sample of peripheral blood destined for FBC analysis should be sent to the lab in an EDTA bottle and preferably analysed within 4 hours of collection. Samples that were difficult to obtain (e.g. lengthy venepuncture using a narrow gage needle such as a small butterfly) may result in abnormalities due to cell lysis or clotting. In a hospital setting, it as also important to avoid taking a sample from the same site as an infusion in order to avoid haemodilution. There is a variety of techniques that blood analysers use to identify the various components and these may differ from lab to lab, so refer to your local lab's normal values when assessing your results - the values provided in this record are a guide rather than a fixed indicator of limits. It is helpful to group results in terms of:

• Red cell parameters
• White cells
• Platelets

You can then look in more detail at the additional information relating to the red and white blood cells.

Haemoglobin concentration

Haemoglobin concentration (Hb): Guideline normal values: 13.0-18.0 g/dL in adult males and 11.5-16.5 g/dL in adult, non-pregnant females.
This is usually the first parameter on a results form. It defines anaemia when low but may also be high in a number of other conditions. The identification of the type of anaemia is aided by:

• Mean cell volume (MCV) - guideline normal values: 77-95 fL. This is a good starting point for the evaluation of anaemia and usefully classifies anaemia into macro- and microcytic anaemias - see below.
• Mean cell haemoglobin (MCH) - guideline normal values: 27.0-32.0 pg. High values are found in macrocytosis and low values are seen in iron deficiency.
• Mean cell haemoglobin concentration (MCHC) - guideline normal values: 32.0-36.0 g/dL. This is of particular use in the evaluation of microcytic anaemias. High values are seen in severe or prolonged dehydration, hereditary spherocytosis and cold agglutinin disease. MCHC is low in iron deficiency anaemia and thalassaemia.

Abnormal haemoglobin levels²

Low Hb - anaemia
• Anaemia with low MCV (microcytic):
  • Iron deficiency anaemia - look at serum ferritin level
  • Anaemia of chronic disorders
  • alpha/beta thalassaemia
• Anaemia with normal MCV (normocytic):
  • Recent bleeding
  • Anaemia of chronic disease (including renal disease)
  • Combined iron and B12/folate deficiency
  • Most non-haematinic deficiency causes
• Anaemia with high MCV (macrocytic):
  • Folate or B12 deficiency
  • Hypothyroidism
  • Haemolytic anaemia
  • Liver disease
  • Alcohol excess
  • Marrow dysplasia and failure syndromes
  • Secondary to anti-metabolite drug therapy, e.g. hydroxyurea
  • Aplastic anaemia
  • Sideroblastic anaemia (can also be microcytic)
  See also:
  Macrocytic anaemia
  Anaemia of pregnancy
  Childhood anaemia
  Sickle-cell disease and sickle cell anaemia
High Hb
• It is important first to ascertain the validity of this result if it does not tie in with known clinical findings. At this point, exclude dehydration and diuretic therapy which may both increase the haematocrit (Hct).
• Anoxia is the major stimulus to red blood cell production and therefore an elevated haemoglobin may be found:
  • Where there has been recent travel to high altitude (> 3,000 m)
  • In hypoxic respiratory conditions, e.g. COPD
  • Heavy cigarette smoking (as a result of increased carboxyHb levels)
  • Ventilatory impairment secondary to gross obesity and alveolar hypotension
  • Secondary causes such as:
    • Spurious polycythaemia (pseudopolycythaemia or Gaisbock's syndrome) - hypertensive, obese, cigarette smokers who drink to excess.
    • Primary proliferative polycythaemia (polycythaemia rubra vera) - plethoric facies with a history of pruritus after change of environmental temperature/bathing and splenomegaly.
    • Inappropriate erythropoietin excess - this occurs in a variety of benign and malignant renal disorders. May also be a rare complication of some tumours, e.g. hepatoma, uterine fibroids and cerebellar haemangioblastoma.
• In these patients, there must be an additional evaluation of the risk of thrombosis.

Haematocrit or Packed Cell Volume

Guideline normal values (Hct): 0.40-0.52 in adult males and 0.36-0.47 in adult females.
These terms are sometimes used interchangeably. Essentially, the packed cell volume (PCV) measures the red cells that have settled to the bottom of a microcapillary tube after this has been centrifuged. The Hct is similar but derived using automated blood counters. These values are high in polycythaemia of any cause and low in anaemia of any cause.

Red cell count

Red cell count (RCC): Guideline normal values: 4.5-6.5 x 10¹²/L in adult males and 3.8-5.8 x 10¹²/L in adult females.
This is useful in the diagnosis of polycythaemic disorders and thalassaemias where the RCC is high and of hypoproliferative anaemias and aplasias when it is low.

Red cell distribution width

Red cell distribution width (RDW) measures the range of cell size in a sample of blood. The term anisocytosis refers to how great this range is. It may be of value in some anaemias. For example, a microcytic anaemia with a normal RDW suggests a β thalassaemia trait whereas the same anaemia with a high RDW points towards iron deficiency. Interpretation of this measurement tends to be more the preserve of haematology staff.

The FBC provides a total white cell count (WCC)/white blood cell count (WBC) and an automated differential white cell count. Typically, this includes information about:

• Neutrophils
• Lymphocytes
• Monocytes
• Eosinophils
• Basophils

The FBC report often shows the % of each type of white cell but, unless the absolute WCC is known (as x 10⁹), it may be of limited value.

Neutrophils (polymorphs or polymorphonucleocytes)

• Guideline normal values: 2–7.5 x 10⁹/L, comprising 40–75% of WBCs.
• Raised in:
  • Bacterial infections
  • Trauma
  • Surgery
  • Burns
  • Haemorrhage
  • Inflammation
  • Infarction
  • Polymyalgia rheumatica
  • Polyarteritis nodosa
  • Myeloproliferative disorders
  • Certain drugs, e.g. steroids
  • Transient leukaemoid reaction in Down's syndrome
  • Mild increase: stress (e.g. post-operatively), exercise
  • Moderate increase: heat strokes, patients with solid tumours
  • Large increase in numbers may be seen in leukaemias, disseminated malignancy and severe childhood infections
• Decreased in:
  • Viral infections
  • Certain drugs, e.g. carbimazole, sulphonamides, methotrexate
  • Severe sepsis (consumption by attempt to combat infection)
  • Hypersplenism
  • Systemic lupus erythematosus
  • Rheumatoid arthritis (destroyed by chronic inflammatory process) - with splenomegaly: Felty's syndrome
  • Vitamin B12 or folate deficiency
  • Chronic benign neutropenia of infancy/childhood - usually resolves by age 4
  • Bone marrow failure (impaired production)
  • Brucellosis
  • Typhoid
  • Kala-azar
  • TB
• Chronic idiopathic neutropenia is an often severe neutropenia which usually runs a benign course - this is a diagnosis of exclusion.

Lymphocytes

• Guideline normal values: 1.3–3.5 x 10⁹/L, comprising 20–45% of WBCs.
• Raised in:
  • Viral infections, e.g. EBV, CMV, rubella
  • Characteristic of infectious mononucleosis
  • Toxoplasmosis
  • Whooping cough
  • Brucellosis
  • Tuberculosis
  • Syphilis
  • Chronic lymphocytic leukaemia
  • Large numbers of abnormal/atypical lymphocytes are characteristically seen in EBV infection (these are T-lymphocytes reacting against EBV-infected B-lymphocytes)
  Normal infants and children < 5 have a higher proportion and concentration than adults.
• Decreased in:
  • Steroid therapy
  • Systemic lupus erythematosus
  • Uraemia
  • Legionnaire's disease
  • AIDS
  • Marrow infiltration
  • Post-chemotherapy/radiotherapy

Eosinophils

• Guideline normal values: 0.04–0.44 x 10⁹/L, comprising 1–6% of WBCs.
• Raised in:
  • Asthma/allergy
  • Parasitic infestations (especially invasive helminths)
  • Polyarteritis nodosa
  • Skin disease such as eczema, pemphigus, urticaria
  • Malignant diseases (including eosinophilic leukaemia)
  • Following irradiation
  • Löffler's syndrome
  • During the convalescent phase of infections
  • As part of the hypereosinophilic syndrome
  • Eosinophilia-myalgia syndrome

Monocytes

• Guideline normal values: 0.2–0.8 x 10⁹/L. comprising 2–10% of WBCs.
• Raised in:
  • Acute and chronic infections (especially TB, brucellosis, protozoan disease)
  • Malignant disease (especially M4 & M5 acute myeloid leukaemia and Hodgkin's disease)
  • Myelodysplasia

Basophils

• Guideline normal values: up to 0.01 x 10⁹/L, comprising 0–1% of WBCs.
• Raised in:
  • Viral infections
  • Urticaria
  • Hypothyroidism
  • Post-splenectomy
  • Chronic myeloid leukaemia
  • Ulcerative colitis
  • Malignancy
  • Systemic mastocytosis (or urticaria pigmentosa)
  • Haemolysis
  • Polycythaemia rubra vera

The normal platelet count is 150–400 x 10⁹/L. Below is a list of the common or important causes of raised or decreased platelet counts, which is by no means exhaustive.

Causes of thrombocytopenia (decreased platelet count)

• Decreased platelet production:
  • Hypoplasia of megakaryocytes:
    • Aplastic anaemias
    • Leukaemias
    • Myelofibrosis
    • Marrow invasion, e.g. granulomata, metastatic tumour, leukaemia
    • Viral infections
    • Ionising radiation causing marrow suppression
    • Chemical toxicity, e.g. chemotherapy, toxins, medication-induced, alcohol excess
    • HIV
  • Ineffective thrombopoiesis:
    • Vitamin B12 deficiency
    • Folic acid deficiency
• Increased platelet destruction:
  • Immune-mediated platelet destruction:
    • Drug-induced immune thrombocytopenia
    • Alloimmune thrombocytopenia, e.g. neonatal, post-transfusion
    • Autoimmune thrombocytopenia, e.g. idiopathic immune thrombocytopenia, secondary immune thrombocytopenia due to infections, rheumatological diseases and lymphoproliferative disorders
  • Non-immune mediated platelet destruction:
    • Disseminated intravascular coagulation
    • Prosthetic intravascular devices
    • Thrombotic thrombocytopaenic purpura
    • Massive haemorrhage and destruction
    • Extracorporeal circulation devices
• Increased splenic sequestration:
  • Splenomegaly
  • Portal hypertension

Although the underlying cause needs to be addressed, it is worth noting that most patients with a platelet count > 30 x 10⁹/L need no specific therapy.² Clearly, aspirin should be avoided.

Causes of thrombocytosis/thrombocythaemia (increased platelet count)²

This is a platelet count of > 450 x 10⁹/L. It may be due to a primary myeloproliferative disorder or to a secondary reactive feature.

• Essential or primary thrombocytosis:
  • This is defined as a non-reactive chronic myeloproliferative disorder that causes chronic elevation of platelet count
  • These patients are at risk of a haemorrhage (the platelets are dysfunctional) or thrombosis or both
  • Disorders include:
    • Primary thrombocythaemia
    • Polycythaemia rubra vera
    • Chronic granulitic leukaemia
    • Idiopathic myelofibrosis
• Reactive or secondary thrombocytosis:
  • Acute infective or inflammatory disorders
  • Chronic inflammatory disorders, e.g. tuberculosis, rheumatological disorders
  • Post-splenectomy or splenic hypofunction/hypoperfusion or congenital asplenia
  • Trauma (including surgery)
  • Acute haemorrhage
  • Iron-deficiency anaemia
  • Malignancy (e.g. lung and breast cancer)
  • Some leukaemias (particularly CLL or CML)

Platelet distribution width

Platelet distribution width (PDW) measures the range of platelet size in a sample of blood. This gives an idea of the amount of active platelet release. Interpretation of this is generally the remit of haematology staff.