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Whys, whats and whens of blood tests

By Pamela Mason

Most pharmacists will be familiar with the “pin-prick” used to obtain a few drops of blood from a finger tip or ear lobe to measure blood-glucose levels. For other blood tests, blood is usually drawn from the median vein in the antecubital fossa (area in front of the elbow), after applying a tourniquet to the upper arm. Typically, a 10 or 20ml syringe is used, but the amount of blood collected will depend on the tests requested and the number of sample tubes to be filled. For blood gases, arterial blood (drawn at the wrist) is required and the procedure is usually more painful than venous blood collection.

Full blood count

The most commonly requested blood test is the full blood count (FBC), which includes an assessment of the number, size and appearance of the three cellular elements in blood: erythrocytes (red cells), leucocytes (white cells) and thrombocytes (platelets). It can, therefore, be used to detect anaemia, leukaemia, blood clotting problems and many other conditions. Typical parameters with their reference ranges are shown in Panel 1 (p420) and are described below. It should be noted that reference ranges can vary, depending on the laboratory used.

Red blood cell tests

Tests relating to erythrocytes are used primarily to diagnose and investigate the possible causes of anaemia.

Red cell count

The red cell count (RCC) is decreased in anaemia and raised in dehydration. A high RCC also indicates polycythaemia (abnormal increase in erythrocyte production by the bone marrow).

Packed cell volume and mean cell volume

The packed cell volume (PCV) or “haematocrit” is the percentage of the total blood volume occupied by red cells. Traditionally, this has been determined by centrifuging blood in a microhaematocrit tube and measuring the height of the red cell column, but now automated analysers are used to calculate PCV. Centrifuging causes the components of the blood to separate. Red cells move to the bottom of the tube and plasma to the top, with a “buffy coat” (which contains leucocytes and platelets) in the middle.The appearance of the centrifuged sample can also provide subjective information about the patient’s health. For example, the colour and clarity of the plasma can suggest jaundice (dark plasma) or lipaemia (turbid plasma).

PCV can be used as a screening tool for anaemia and to indicate the degree of fluid loss in dehydration. A drop in PCV can also indicate internal haemorrhage before any other symptoms become apparent. PCV is raised in polycythaemia and in chronic hypoxia associated with pulmonary disease and congenital heart disease.

Download the attached PDF to read the full article.

Citation: The Pharmaceutical Journal URI: 10997011

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