HaematocritHematocrit · Packed cell volume

Haematocrit is the proportion of blood volume made up of red blood cells, expressed as a percentage or a fraction. It moves closely with haemoglobin and reports much the same thing from a different angle.

It is affected by two variables, which is the key to interpreting it: the number of red cells and the volume of plasma they are suspended in. Change either and the percentage changes, even if the other is unchanged.

That is why dehydration raises it. Losing plasma volume concentrates the cells, producing a high reading in someone whose red cell mass is entirely normal, which is one of the most common causes of a mildly raised result.

Endurance training produces the opposite artefact. Plasma volume expands more than red cell mass, lowering the percentage while total oxygen carrying capacity is unchanged or better, which is the dilutional pseudoanaemia seen in trained athletes and should not be treated with iron.

Genuinely high values have a differential worth knowing about: chronic hypoxia from lung disease or sleep apnoea, living at altitude, smoking, testosterone therapy, and the blood disorder polycythaemia vera. Testosterone in particular raises it reliably, which is why it is monitored during treatment.

Very high haematocrit thickens blood and raises clotting risk, which is the reason it is watched rather than an abstract concern, and it is why athletic doping with erythropoietin has caused deaths.

In practice, interpret it alongside haemoglobin and hydration state, and treat a persistently raised value, particularly on testosterone therapy or with heavy snoring, as something for a doctor.