The blood count

Many of the diseases discussed in this book are first suggested by an abnormality in the blood count (often referred to as the full blood count). The test is performed on a small specimen of anticoagulated venous blood; the normal anticoagulant is ethylene diamine tetra-acetic acid (EDTA). A typical report is illustrated in Figure 9.1. As can be seen, it contains a large amount of numerical information pertaining to the three cell lines in the peripheral blood: red cells (and haemoglobin), white cells (with a differential count of each specific cell type) and platelets.

When interpreting the report it is sensible to initially focus on the haemoglobin (Hb) concentration, total white cell count (WBC) and platelet count – most blood abnormalities of clinical significance are associated with a derangement of at least one of these values. Much of the remaining information details the nature of the red cells and their degree of haemoglobinisation, and the precise make-up of the white cell count. The former values are helpful in the diagnosis of anaemia, and the latter in the diagnosis of a variety of diseases of white cells (e.g. leukaemias) and reactions to systemic disease. To understand the role of the automated blood count in clinical practice, and particularly its limitations, it is helpful to understand how the numerical values are generated.