Full Blood Count

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PatientPlus articles are written by UK doctors and are based on research evidence, UK and European Guidelines. They are designed for health professionals to use, so you may find the language more technical than the condition leaflets.

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 article 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 separate article Peripheral Blood Film. Follow the links provided for more information about the related pathology.

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A sample of peripheral blood destined for FBC analysis should be sent to the laboratory in an EDTA bottle and preferably analysed within four hours of collection. Samples that were difficult to obtain (eg, lengthy venepuncture using a narrow gauge 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 laboratory to laboratory, so refer to your local laboratory's normal values when assessing your results. NB: the values provided in this article 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 (Hb) concentration - 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 macrocytic and microcytic anaemias.
  • 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 Hb levels[4]

  • Anaemia with low MCV (microcytic):
  • 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):

    See also separate articles: Macrocytosis and Macrocytic Anaemia, Anaemia in Pregnancy, Childhood Anaemia and 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 Hb may be found:
      • Where there has been recent travel to high altitude (>3,000 m).
      • In hypoxic respiratory conditions - eg, chronic obstructive pulmonary disease (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 - eg, hepatoma, uterine fibroids and cerebellar haemangioblastoma.
    • In these patients, there must be an additional evaluation of the risk of thrombosis.

    Hct or packed cell volume (PCV)[5]

    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 PCV measures the red cells that have settled to the bottom of a micro-capillary 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 (RCC)[2] 

    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 where it is low.

    Red cell distribution width (RDW)[2] 

    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 beta 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 WCC. 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 - eg, steroids.
      • Transient leukaemoid reaction in Down's syndrome.
      • Mild increase: stress (eg, postoperatively), 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:
    • Chronic idiopathic neutropenia is an often severe neutropenia which usually runs a benign course - this is a diagnosis of exclusion.


    Guideline normal values: 1.3-3.5 x 109/L, comprising 20-45% of WBCs.


    Guideline normal values: 0.04-0.44 x 109/L, comprising 1-6% of WBCs.


    Guideline normal values: 0.20.8 x 109/L. comprising 2-10% of WBCs.


    Guideline normal values: up to 0.01 x 109/L, comprising 0-1% of WBCs.

    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 - eg, granulomata, metastatic tumour, leukaemia.
        • Viral infections.
        • Ionising radiation causing marrow suppression.
        • Chemical toxicity - eg, 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 of >30 x 109/L need no specific therapy.[8] Clearly, aspirin should be avoided.

    Causes of thrombocytosis/thrombocythaemia (increased platelet count)[8]

    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 - eg, TB, rheumatological disorders.
      • Post-splenectomy or splenic hypofunction/hypoperfusion or congenital asplenia.
      • Trauma (including surgery).
      • Acute haemorrhage.
      • Iron-deficiency anaemia.
      • Malignancy (eg, lung and breast cancer).
      • Some leukaemias (particularly CLL or CML).

    Platelet distribution width (PDW)

    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.

Further reading & references

  1. Complete Blood Count, University of Michigan Health System, Department of Pathology, 2012.
  2. Full Blood Count, Lab Tests Online, 2012
  3. Normal ranges, University of Michigan Health System, Department of Pathology, 2012
  4. Merrit B et al; Hemoglobin Concentration (Hb), Medscape, Aug 2012
  5. Hematocrit; Lab Tests Online®, 2012
  6. Naushad H et al; Leukocyte Count, Medscape, Jan 2012
  7. McRae S; Platelet Abnormalities, IMVS Newsletter, 2008
  8. Provan D, Singer CRJ, Baglin T et al; Oxford Handbook of Clinical Haematology, 2nd Edition. Oxford University Press (2004).

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.

Original Author:
Dr Olivia Scott
Current Version:
Peer Reviewer:
Dr Hannah Gronow
Document ID:
9381 (v2)
Last Checked:
Next Review:
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