Haematocrit

Haematocrit (also written HCT or PCV — Packed Cell Volume) is the percentage of whole blood volume made up of red blood cells. It is reported automatically as part of every Full Blood Count (FBC) and is one of the three core red cell parameters alongside haemoglobin and red blood cell count.

Haematocrit helps detect anaemia (low) and polycythaemia (high). In Australia, it is closely monitored in men on TRT (testosterone replacement therapy) because testosterone can drive haematocrit above 0.52, which is the threshold the Endocrine Society and Australian Lifeblood use to recommend therapeutic venesection (blood donation). Dehydration can also produce a transient rise.

A high haematocrit means red cells make up too much of the blood, thickening it. The most common cause is dehydration, which produces a temporary rise that corrects with fluids. Sustained elevation can come from chronic low oxygen (smoking, sleep apnoea, lung disease, living at altitude), testosterone replacement therapy or performance-enhancing drug use, and the bone marrow disorder polycythaemia vera. In Australia, men on TRT are watched closely, because a haematocrit above 0.52 is the usual threshold for recommending therapeutic venesection.

A low haematocrit indicates anaemia and parallels a low haemoglobin. The causes are those of anaemia generally: iron deficiency (the most common, especially in menstruating women), vitamin B12 or folate deficiency, blood loss, chronic disease, kidney disease, and inherited conditions such as thalassaemia. It can also appear falsely low when blood is diluted by intravenous fluids or in pregnancy.

Haematocrit is reported in every full blood count, so it is measured at routine check-ups and whenever anaemia or polycythaemia is investigated. It is monitored regularly — typically every few months and then 6-12 monthly once stable — in men on testosterone replacement therapy.

Haematocrit moves together with haemoglobin and the red blood cell count, and is interpreted with the red cell indices (MCV, MCHC, RDW). When low, iron studies, B12 and folate identify the cause; when high, it is considered with the clinical picture and sometimes erythropoietin.