2.1. RED BLOOD CELL INDICES
Principle: Red blood cell (RBC) indices determination is the most common laboratory test and is usually carried out by automated electronic cell counters. These counters produce many parameters of which only a few, such as the mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH) and haemoglobin (Hb) concentration, are strictly relevant and useful for haemoglobinopathies screening. The principles on which the different cell counters are based are beyond the aims of this book and are available from the manufacturers. As a general rule the preference should be for counters that directly measure the mean corpuscular volume. The counter needs to be calibrated daily with appropriate material to obtain accurate results.
Interpretation of the results: MCV and MCH are variably reduced in thalassaemia carriers. MCH is more reliable than MCV, since the MCV does not remain stable due to a tendency for the red cells to increase in size over time. The MCH is derived from the calculation: (MCH = Hb/RBC number).
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The value at which the normal range for a parameter changes to an abnormal range is called the cut-off value. The most widely used cut-off values of MCV and MCH for indicating thalassaemia are 79 fl and 27 pg, respectively. Values below these may indicate α- or β-thalassaemia or iron deficiency and other methods are needed to make a precise diagnosis. For β-thalassaemia, the degree of microcytosis and hypochromia as reflected by the lower than normal MCV and MCH is related to the severity of the underlying mutation which determines the production of β-globin chains. In general mild β-thalassaemia mutations cause less microcytosis and hypochromia than β° and severe β+ mutations, although there is some overlap (1,2). Carriers of silent β-thalassaemia mutations have normal or only slightly reduced MCV and MCH values. However carriers for both β and α-thalassaemia may have normal MCV and MCH values because of less globin chain imbalance (3). α-Thalassaemia carriers have lower than normal MCV and MCH values. α+-Thalassaemia (-α/αα) carriers may have normal or reduced MCV and MCH values, while in -α/-α and -/αα carriers the MCV and MCH are always reduced (4). δβ-Thalassaemia carriers have slightly reduced MCV and MCH values. Note that an individual’s iron status should always be taken into account when evaluating MCH and MCV values for thalassaemia screening.
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