Hematocrit is the percentage of blood that is made up of red blood cells. Hematocrit has been established by the Framingham study to be a risk factor for cardiovascular disease [1]. Blood donation reduces hematocrit, however the effect of blood donation on hematocrit lasts between a few days to a maximum of a few weeks. While the reduction in hematocrit caused by blood donation is relatively short-lived, the effect that blood donation has on blood viscosity lasts much longer. This is because blood donation replaces a part of old red blood cells with new ones.

Red blood cells live for about 120 days from the time they are formed in the bone marrow to the time they are removed by the spleen. During those 4 months, red blood cells grow progressively stiffer and more rigid. This increases the viscosity of blood. Red blood cell stiffness is the second most influential factor that determines a person’s blood viscosity [2]. Blood donation removes a portion of old and stiff red blood cells, replacing them with softer, younger cells.

A Finnish study of 2,682 men aged 42-60 years reported that men who were blood donors had 88% lower risk of heart attacks than non-donors (p<0.001) [3]. The findings of this study were questioned for possible selection bias [4], but a follow-up report by the investigators confirmed the association between blood donation and dramatically reduced risk of heart attack blood donation in middle-aged men (relative hazard = 0.12, 95% CI 0.02–0.86; p=0.035) [5].

1. Gagnon, D. R., et al., Hematocrit and the risk of cardiovascular disease—The Framingham study:A 34-year follow-up. Am Heart J, 1994. 127(3): p. 674–82.

2. Chien, S., et al., Blood viscosity: Influence of erythrocyte deformation. Science, 1967. 157(3790): p. 827–9.

3. Tuomainen, T. P., et al., Cohort study of relation between donating blood and risk of myocardial infarction in 2682 men in eastern Finland. BMJ, 1997. 314(7083): p. 793–4.

4. Hemila, H. and M. Paunio, Blood donation, body iron stores, and risk of myocardial infarction. Confidence intervals and possible selection bias call study results into question. BMJ, 1997. 314(7097): p. 1830–1.

5. Salonen, J. T., et al., Donation of blood is associated with reduced risk of myocardial infarction. The Kuopio Ischaemic Heart Disease Risk Factor Study. Am J Epidemiol, 1998. 148(5): p. 445–51

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