Haematocrit (also written as hematocrit) is a blood test measurement that expresses the proportion of whole blood made up of red blood cells, stated as a percentage. For example, a haematocrit of 42% means that 42% of the total blood volume consists of red blood cells, with the remainder being plasma and other components.
Normal haematocrit ranges are approximately 36 to 48% in women and 40 to 52% in men, though laboratory reference ranges vary slightly. A low haematocrit indicates anaemia, meaning the blood has a reduced red cell mass and therefore a reduced capacity to carry oxygen to tissues. This can cause fatigue, breathlessness, and in cardiac patients, increased strain on the heart. A high haematocrit (polycythaemia) thickens the blood and can increase the risk of clotting.
In cardiac patients, haematocrit is routinely checked as part of a full blood count. After cardiac arrest and during critical care admission, anaemia is common due to blood loss, haemodilution from fluids, or reduced red cell production in critical illness. Transfusion thresholds and management strategies vary by patient condition. Optimising haematocrit is part of supporting oxygen delivery to the heart muscle and brain during recovery.
Haematocrit is also used to calculate other derived blood measures and is one of the values reported alongside haemoglobin and red cell count in a standard full blood count (FBC).
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