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Hematocrit is classically performed by centrifugation of blood in a capillary tube to determine the percentage of the volume of whole blood that is red blood cells. Hematocrit results reported by automated hematology analyzers are calculated from measured red cell counts and mean cell volumes (MCV). Automated hematocrits may be slightly lower than manual hematocrits. The source of the blood sample may affect the results; fingerstick hematocrits are often higher than venous hematocrits. Hemoglobin is a more accurate parameter than hematocrit.

The relationship between hematocrit and cardiovascular disease was evaluated in a recently published 34-year follow-up of 5, 209 men and women in the Framingham study, aged 30-62 years at entry. Subjects with hematocrits in the highest quintile (49-70% for men and 46-65% for women) had an increased risk of death from all causes, and the highest morbidity and mortality secondary to cardiovascular disease. There was a significant relationship between hematocrit and congestive heart failure, coronary heart disease and myocardial infarction in younger men and women. The authors conclude that hematocrit is an important risk factor for some cardiovascular diseases, and postulate that the mechanism may be related to an effect of hematocrit on viscosity, possibly promoting atherogenesis, or that the hematocrit may reflect other coexistent risk factors.

Similar conclusions were drawn from another long-term study of 488 apparently healthy men, aged 40-59 years, followed prospectively for 10-16 years. Hematocrit was an independent risk factor for cardiovascular disease mortality, with increased risk associated with hematocrit levels greater than 44% and the greatest risk associated with a baseline hematocrit greater than 50%.

High hematocrit was also shown to be a risk factor for stroke in a prospective study of 7,346 men, aged 40-59 years, followed for 9.5 years. The risk of stroke was not significantly affected by hematocrits up to 50%, but was 1.7 times higher at hematocrits of 50-52%, and 3.3 times higher at hematocrits higher than 52%. A higher relative risk of stroke for elevated hematocrit was still present after adjustment for other risk factors, and the risk was compounded by the presence of hypertension (the relative risk was 9.3 times for those with elevated hematocrit and hypertension). The authors postulate that the increase in the risk of stroke at higher hematocrits may be mediated by increased viscosity. Measurement of hematocrit may become an integral part of laboratory screening to predict cardiovascular and possibly cerebrovascular disease.

Reference range is 40 - 50% in males and 36 - 45% in females. Hematocrit values less than 15% and greater than 60% are considered critical values. Values >70% are considered critical in neonates.

Specimen requirement is one 5 mL lavender top (EDTA) tube of blood.

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