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VOL: 99, ISSUE: 04, PAGE NO: 31

Blood is unique in that it is the only fluid tissue in the body. Blood is a specialised type of connective tissue in which living blood cells, the formed elements, are suspended in a non-living fluid matrix called plasma.

- The formed elements of blood are erythrocytes (red blood cells) leucocytes (white blood cells) and platelets.

- If a sample of blood is taken it is treated with an agent to prevent clotting and spun in a centrifuge. The red cells settle to the bottom, the leucocytes and platelets form a buffy coat, and the plasma rises to the top (above right). Erythrocytes transport oxygen and carbon dioxide.

Before a red blood cell (RBC) count can be measured a blood sample is required from the patient.

This is done by venepuncture, defined as the puncture of a vein for any therapeutic purpose.

Information relating to blood taking procedures, blood spillages and needle-stick injuries can be found in the Manual of Clinical Nursing Procedures (see further reading). Local polices should also be available.

- Collect the sample in an EDTA bottle (ethylenediamine tetra-acetic acid prevents blood samples from clotting). Consult local policy.

- Fill the blood bottle and invert gently several times to adequately mix the sample and anticoagulant.

- Enter any drugs the patient is taking on the request form.

- The RBC count helps to assess the blood oxygen capacity and can be useful in diagnosing anaemia, protein deficiency and dehydration.

- The RBC count provides no information about the size, shape or concentration of haemoglobin (Hb) within the cells, but it may be used to calculate two erythrocyte counts. Mean corpuscular volume (MCV) and Mean corpuscular Hb (MCH).

- The normal RBC range for an adult male is 4.5-5.5x1012/l and for an adult female is 4-5x1012/l.

- A raised RBC count may indicate absolute or relative polycythaemia.

- A low RBC may mean anaemia, fluid overload or haemorrhage lasting more than 24 hours.

- The normal MCV range in adults is 80-90 femtolitre (fl) and increases with age in males.

- A raised MCV may indicate vitamin B12 or folate deficiency; liver disease; myxoedema; aplastic anaemia; haemolysis; aplasia; or bone infiltration.

- A lowered MCV may indicate chronic blood loss, occult blood loss, iron deficiency anaemia, thalassaemia or anaemia of chronic diseases.

- The normal MCH range in adults is 27-33pg (picograms/cell). This value should not be interpreted alone but with other red cell values.

- A raised MCH may indicate vitamin B12 or folate deficiency, or myxoedema.

- A lowered MCH may indicate iron deficiency anaemia, thalassaemia, chronic blood loss or megaloblastic anaemia.

Avoid venepuncture:

- in veins that are fibrosed, inflamed or fragile;

- in bruised areas;

- in sites close to infection;

- on the affected side of patients with CVA (cerebrovascular accident) or patients post-mastectomy.

- If the tourniquet has been on for more than two minutes before inserting the needle, release and allow blood to return to the hand before reapplying.

- If a venous valve is entered during the procedure the patient feels acute pain: withdraw immediately.

- If after two attempts you are not successful, seek assistance.

- Observe the patient throughout the procedure for signs of dizziness, fainting or paraesthesia.

- Be aware that the brachial artery and the median nerve are also sited near the sites most commonly used for venepuncture.

If you have access to the NHS Net:

Jamieson, E.M., et al (2002)
Clinical Nursing Practices. Edinburgh: Churchill Livingstone.

Mallett, J., Dougherty, L. (2000)The Royal Marsden Hospital Manual of Clinical Procedures. Oxford: Blackwell Science.

McGee, M. (2000)A Guide to Laboratory Investigations. Abingdon: Radcliffe Press.

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