Biomarker · Blood cells

Haematocrit

Also known as: Hct, hematocrit, packed cell volume, PCV

The fraction of blood volume made up of red cells, reported as a percentage or a ratio. It tracks haemoglobin closely and is part of every full blood count.

Open in the 3D explorerSpleen anatomy
3D model of the Spleen in the Anatomy Nexus anatomy atlas
Interactive 3D model: Spleen. Orbit, zoom and click the structures.Load the 3D model
Also known as
Hct, hematocrit, packed cell volume, PCV
Units
% or a ratio (L/L); 0.40 L/L = 40%
Measured by
Full blood count

What it is and what it does

Spin a tube of blood and the red cells settle into a packed column; the height of that column as a share of the whole is the haematocrit. Automated analysers calculate it from the red-cell count and their average size. Because red cells carry the haemoglobin, haematocrit and haemoglobin rise and fall together, and the ratio between them (the MCHC) is one of the red-cell indices. Haematocrit also sets blood viscosity: very high values make blood thicker and harder to pump.

Why it may be higher

  • Dehydration and burns (loss of plasma water)
  • Chronic hypoxia: lung disease, smoking, altitude, cyanotic heart disease
  • Polycythaemia vera
  • Erythropoietin or testosterone use

Why it may be lower

  • Any cause of anaemia: iron, B12 or folate deficiency, blood loss, haemolysis, marrow failure, chronic disease
  • Over-hydration or large volumes of intravenous fluid
  • Pregnancy (dilution)

A high or low value can be associated with several different situations. The result is interpreted with other results, symptoms, history and clinical findings.

What can affect the result

  • Fluid intake and dehydration
  • Posture and tourniquet time during sampling
  • Pregnancy
  • Altitude
Reference ranges. Reference intervals for this marker depend on the laboratory, the assay, the units, age, sex and pregnancy. Interpret a result against the interval printed on the laboratory report, not against a figure from the internet.
Educational content. A biomarker page explains what a substance is and why its level can change. It cannot determine the cause of your result: interpret a result using the range supplied by the laboratory that performed the test, with the clinician who ordered it.