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Haematocrit, viscosity, erythrocyte sedimentation rate: meta-analyses of prospective studies of coronary heart disease.

Various epidemological studies have investigated the possible associations between blood rheology (i.e. haematological characteristics that could influence blood flow) and coronary heart disease rates. Some involved whole blood viscosity (i.e. bulk blood’s intrinsic resistance to flow), but most involved its major determinants: haematocrit, plasma viscosity, and red cell aggregation (as indicated by the erythrocyte sedimentation rate) (Table 1). There is an elevated risk of occlusive vascular disease in patients with poorly controlled polycythaemia (in whom the haematocrit is markedly elevated), and both prospective and retrospective epidemiological studies of the general population have reported some association between determinants of blood rheology and coronary heart disease risk. Prospective epidemiological studies, in which coronary heart disease events are recorded for some years after ‘baseline’ blood collection, should be less prone to bias than retrospective epidemiological studies because they limit the influence of pre-existing disease itself on the factors being investigated. This should still be the case even in long-term prospective studies among patients with previous vascular disease since, in such studies, subsequent ‘cases’ are compared with ‘controls’ with similar disease at baseline selected from within the same study. Hence, to help determine whether there is any real association between coronary heart disease and various measures of blood rheology, we report a