What Does a Low Haematocrit Mean?

What's the difference between haematocrit and haemoglobin?

They are two ways of measuring the same underlying thing. Haemoglobin weighs the oxygen-carrying protein; haematocrit measures the volume the red cells occupy. They rise and fall together in almost all everyday situations, so doctors usually read them as a pair and focus on haemoglobin. A low HCT with a normal haemoglobin is unusual and often just a dilution effect — for example from over-hydration or pregnancy's natural plasma increase.

What causes a low haematocrit in women?

The most common cause by far is iron deficiency from menstrual blood loss, followed by pregnancy (where a modest fall is physiological and expected). Diet, coeliac disease and other absorption problems make up much of the rest. In men and post-menopausal women, iron-deficiency anaemia is never assumed to be dietary until blood loss — particularly from the gut — has been considered.

What else lowers the haematocrit?

• B12 or folate deficiency
• Chronic illness or inflammation — kidneys, long-term infection or inflammatory disease
• Recent bleeding or blood donation
• Overhydration or fluid shifts — a dilution effect rather than true anaemia

When is a low haematocrit serious?

Context and speed matter more than the number alone. A slightly low, stable HCT in a menstruating woman is common and usually straightforward to fix. A fast fall, or a low HCT with chest pain, breathlessness at rest, dizziness, or black/bloody stools, needs urgent medical attention.

Written by Dr Ibrar Ahmed — MBChB MRCP MRCGP PhD PG Dip (Cardiology)
Specialist interest: Cardiovascular Medicine · OneMedicine, CQC-registered clinic