Manual of basic techniques for a health laboratory - libdoc.who.int

9. Haematology 287Measurement technique1. Place the graduated tubes in the centrifuge and centrifuge for30 minutes at 2300g. If the rotor arm of the centrifuge (measuredfrom the axis of rotation to the base of the bucket holdingthe tube) is 15cm long, 3600rpm will be needed to attainthis force; with a 20-cm arm 3100rpm will be needed.Important: a force of less than about 2300g will give a falseresult.2. Read the level at which the erythrocytes meet the layer ofleukocytes (Fig. 9.39). Make sure that the correct set of graduationsis being used, upwards towards the 100 mark. The figureobtained is a percentage (the “packed cell volume”); divide by100 to obtain the erythrocyte volume fraction.Fig. 9.39 Measuring the packed cell volumeResultsSee page 282.9.5 Estimation of the erythrocyte number concentrationThe number of erythrocytes contained in 1 litre of blood is called the erythrocytenumber concentration. (In traditional units, it is expressed as the number of erythrocytesper cubic millimetre and is called the erythrocyte or red cell “count”.)Accurate methods for counting erythrocytes require an electronic counter system.Unfortunately, such instruments are often not available in peripheral laboratories.A simple but far less accurate method uses a counting chamber in which erythrocytesare counted under the microscope. However, this method is of such low precisionthat it should not be used. It is recommended instead that the erythrocytevolume fraction (see section 9.4) or the haemoglobin concentration (see section9.3) is measured and the erythrocyte number concentration calculated.Reference rangeTable 9.7 shows the reference ranges for different age groups.High valuesPatients who are dehydrated or have polycythaemia will have high erythrocytenumber concentrations.Low valuesPatients with anaemia caused by insufficient production, loss or haemolysis of erythrocyteswill have low erythrocyte number concentrations.Note: Anaemia is a clinical syndrome that has many different underlying causes.The clinical picture is determined by the extent and duration of anaemia. TheTable 9.7 Normal erythrocyte number concentrations, by age groupAge groupErythrocyte number concentrationSI units (per litre) Traditional units (per mm 3 )Newborn infants 5.0–7.0 ¥ 10 12 5.0–7.0 ¥ 10 6Infants (1–6 months) 3.8–5.9 ¥ 10 12 3.8–5.9 ¥ 10 6Children (4 years) 3.8–5.4 ¥ 10 12 3.8–5.4 ¥ 10 6Women 4.0–5.4 ¥ 10 12 4.0–5.4 ¥ 10 6Men 4.5–6.2 ¥ 10 12 4.5–6.2 ¥ 10 6