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Updated 25 July 2012

The full blood count

The full blood count (FBC), also known as the complete blood count is a routine investigation requested by a doctor to assess the three cell lines in the blood.

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The full blood count (FBC), also known as the complete blood count is a routine investigation requested by a doctor to assess the three cell lines in the blood.

It generally gives a doctor an idea of the health status of a patient.

Procedure

A doctor/sister/phlebotomist draws venous blood from a peripheral vein, most commonly from the armpit. The amount needed is usually 5ml in an adult patient and 2ml in a paediatric patient. This goes into a test tube that contains an anticoagulant (to prevent blood from clotting), usually EDTA (ethylene diamine tetra-acetic acid) or trisodium citrate.

Apart from minor discomfort from the venepuncture, there are usually no complications.

The blood is then sent to the laboratory for analysis.

Laboratory

The blood is analysed using an automated analyser. In the past, prior to the advent of automation, blood samples were analysed manually by viewing a slide prepared from the blood.

Automated haemotology counters

Automated machines have at least two channels for cell counting. In one channel, red blood cells and platelets are analysed, and in the other, white blood cells are analysed. Extra channels are used for the differential cell counting and reticulocyte counting.

There are basically two methods for cell counting and sizing which are: electrical impedence and light scattering.

Although automated instruments are sophisticated, they cannot recognise all the significant abnormalities that can be recognised by the human observer. Therefore they are designed to produce accurate and precise blood counts on specimens which are either normal, or show only numerical abnormalities, and to alert the operator when the specimen has unusual characteristics, which could either lead to an inaccurate measurement or which would require a blood film review.

This is called "flagging"- the instrument is designed to flag abnormal results.

Results

A FBC result will include the following:

Red cell parameters:

  • Red cell count (RCC): this is an estimation of the number of red blood cells per litre of blood.
  • Haemoglobin (Hb): this is an iron-containing compound found in red blood cells that transports oxygen in the blood. The instrument is able to measure the amount of haemoglobin expressed in grams per decilitre.
  • Haematocrit (Hct) or packed cell volume (PCV): this is a measure of the percentage of red blood cells to the total blood volume.
  • Mean cell volume (MCV): this is an estimate of the volume of red blood cells.
  • Mean cell haemoglobin (MCH):this is the haemoglobin content of the average red blood cell.
  • Mean cell haemoglobin concentration (MCHC): this is the average haemoglobin content in a given volume of packed cells.

White cell parameters

  • White cell (leucocyte) count (WCC): this is an estimation of the total number of white cells per litre of blood.
  • Leucocyte differential count: this provides an estimation of the 5 types of leucocytes (white blood cells) that are found in blood. These are neutrophils, monocytes, lymphocytes, eosninophils and basophils.

Platelets

  • Platelet count: This is an estimation of the number of platelets per litre of blood.

Please note that the laboratories usually indicate the normal reference ranges.

Common diseases associated with abnormal findings in the FBC include:

  • Anaemia: indicated by a low haemoglobin content. There are numerous causes for this condition and the doctor then needs to investigate the cause for the anaemia. The MCV, MCH, MCHC are helpful in classifying the type of anaemia.
  • Erythrocytosis: an increase in the RCC is commonly seen in dehydration, smoking, polycythemia vera.
  • Leucocytosis: an increase in the WCC is seen in a number of conditions including infection, inflammation, leukaemias.
  • Leucopenia: a decrease in the WCC is seen in liver or spleen disorders, severe infection, and marrow failure.
  • Thrombocytosis: an increase in the platelet count may be reactive in response to an infection, bleeding, iron deficiency or due to essential thrombocythemia.
  • Thrombocytopenia: a decrease in the platelet count may be seen in marrow failure, immune thrombocytopenic purpura.

Conclusion

A FBC is a very useful screening tool and helps the doctor monitor patients' health in most cases.

(Dr Ravnit Grewal, Haematopathology registrar, Tygerberg Hospital)

 
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