Definition

The term ‘anaemia’ refers to a reduction of haemoglobin or red cell concentration in the blood. With the widespread introduction of automated equipment into haematology laboratories the haemoglobin concentration has replaced the haematocrit (or ‘packed cell volume’) as the key measurement. Haemoglobin concentration can be determined accurately and reproducibly and is probably the laboratory value most closely correlated with the pathophysiological consequences of anaemia. Thus, anaemia is simply defined as a haemoglobin concentration below the accepted normal range.

The normal range for haemoglobin concentration varies in men and women and in different age groups (Table 11.1). The definition of normality requires accurate haemoglobin estimation in a carefully selected reference population. Subjects with iron deficiency (up to 30% in some unselected populations) and pregnant women must be excluded or the lower level of normality will be misleadingly low. Normal haemoglobin ranges may vary between ethnic groups and between populations living at different altitudes.

Prevalence

The prevalence of anaemia and the aetiologies vary in different populations. In developed countries where most studies have been performed, anaemia is more common in women than in men. Particularly susceptible groups include pregnant women, children under 5 years and the elderly. The majority of cases in younger people are caused by iron deficiency. Anaemia is surprisingly common in the elderly, affecting roughly 10% of people over 65 years. Up to a third of these cases remain unexplained (see p. 92). In developing countries, factors influencing the prevalence of anaemia include climate, socioeconomic conditions and, most importantly, the incidence of coexistent diseases.

General features

In anaemia the blood’s reduced oxygen-carrying capacity can lead to tissue hypoxia. The clinical manifestations of significant anaemia (see also p. 14) are to a large extent due to the compensatory mechanisms mobilised to counteract this hypoxia. Cardiac overactivity causes palpitations, tachycardia and heart murmurs. The dyspnoea of severe anaemia may be a sign of incipient cardiorespiratory failure. Pallor is due primarily to skin vasoconstriction with redistribution of blood flow to tissues with higher oxygen dependency such as the brain and myocardium.