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Full Blood Count

This PatientPlus article is written for healthcare professionals so the language may be more technical than the condition leaflets. You may find the abbreviations list helpful.

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

Background

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.

Preliminaries

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.

Red cell parameters1

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

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.

White cells1

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)

Lymphocytes

Eosinophils

  • Guideline normal values: 0.04–0.44 x 109/L, comprising 1–6% of WBCs.
  • Raised in:

Monocytes

Basophils

  • Guideline normal values: up to 0.01 x 109/L, comprising 0–1% of WBCs.
  • Raised in:

The platelet count3

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:
  • Increased splenic sequestration:

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:
  • 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.

Document references

  1. Provan, D (Ed) Oxford Handbook of Clinical and Laboratory Investigation, 2nd Edition. Oxford University Press; Oxford (2005).
  2. Provan D, Singer CRJ, Baglin T et al. Oxford Handbook of Clinical Haematology, 2nd Edition. Oxford University (Press 2004).
  3. Kumar P; Clarke M; Clinical Medicine, 6th Ed, (2005). WB Saunders: London.

Acknowledgements

EMIS is grateful to Dr Olivia Scott for writing this article. The final copy has passed scrutiny by the independent Mentor GP reviewing team. ©EMIS 2009.
Document ID: 9381
Document Version: 1
Document Reference: bgp26187
Last Updated: 16 Jun 2009
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