A 47-year-old female consults her doctor complaining of a sore throat of two weeks’ duration, and has been aware of bruising affecting her forearms, thighs and shins, apparently developing spontaneously, for a week. She consults her General Practitioner who performs a full blood count.
What information may be gleaned from this investigation that may aid diagnosis in this case?
The full blood count (FBC) is one of the most common investigations requested by doctors, and an abnormal FBC may be the first indication that a patient may have one of the primary haematological disorders discussed in this book. The test is the cornerstone of haematological diagnosis and the main test performed in haematology laboratories. Between 500 and 700 such tests will be performed daily in an average-sized district general hospital in the UK. The FBC is generated by automated instruments that count and size circulating blood cells by a variety of methods, one of the most common being ‘aperture impedance’ – the number and volume of cells are proportional to the frequency and height of electric pulses generated when cells pass through a small aperture. It is by this method, for example, that the mean cell volume (MCV) of red cells is determined. These machines also measure directly the haemoglobin content of the red cells, by spectrophotometric analysis after the red cells are lysed, and the haematocrit. As well as these directly measured parameters there are other calculated variables including the mean corpuscular haemoglobin (MCH) and the mean corpuscular haemoglobin concentration (MCHC). Furthermore, such analysers can provide additional information about the various categories of white blood cells (WBCs) through complex multiparameter analysis of cell size and content, for example by assessing myeloperoxidase content that is found in neutrophils.
In interpreting a FBC, the most significant parameters to focus on are the haemoglobin – which may indicate the presence of anaemia or polycythaemia; the MCV – which will help in the classification of anaemias (see Chapter 7); the white cell count with differential – which would be important in the diagnosis of lymphoid or myeloid disorders; and a platelet count. The interpretation of any abnormality should be made in light of the knowledge that 5% of the population will display laboratory values outside the so-called normal range, and that well-recognized differences in these variables can be observed in persons of different race and between the sexes. For example, persons of African descent may have a lower white cell count (WCC), especially neutrophils, than Caucasians.1
Red cells – anaemia and polycythaemia
The approach to the anaemic patient is described further in Section 3, and polycythaemia in Section 6, and will not be further discussed here. The rest of this chapter will focus instead on the interpretation of an abnormal white cell or platelet count.