Anaemia is present when there is a decrease in the level of haemoglobin in the blood below the reference level for the age and sex of the individual (Table 8.1). Alterations in the level of Hb may occur as a result of changes in the plasma volume. A reduction in the plasma volume will lead to a spuriously high Hb – this is seen with dehydration and in the clinical condition of apparent polycythaemia. A raised plasma volume produces a spurious anaemia, even when combined with a small increase in red cell volume as occurs in pregnancy. After a major bleed, anaemia may not be apparent for several days until the plasma volume returns to normal.

The various types of anaemia, classified in terms of the red cell indices, particularly the MCV. There are three major types:

• hypochromic microcytic with a low MCV
• normochromic normocytic with a normal MCV
• macrocytic with a high MCV.