Haematocrit

A low haematocrit means anaemia — too few red cells for the blood volume. The key follow-up question is why, and it splits three ways:

• Blood loss — bleeding, internal or external.
• Destruction — red cells being broken down faster than normal (haemolysis), sometimes immune-mediated; often accompanied by a rise in bilirubin.
• Reduced production — the marrow not making enough, as in chronic kidney disease, marrow disorders, or the "anaemia of chronic disease".

The single most useful next step is the reticulocyte count (young red cells). A high reticulocyte count means the marrow is responding — the anaemia is regenerative, fitting loss or destruction; a low one means the marrow is not keeping up — a non-regenerative anaemia, pointing towards a production problem. That distinction shapes the entire work-up. How ill a pet feels also depends on how fast the anaemia developed: a slow drop is tolerated far better than a sudden one.

A high haematocrit most often means dehydration — the red cells are concentrated because the plasma (water) they float in has been lost. It corrects with rehydration. Less commonly, a genuinely raised red-cell mass (polycythaemia) can result from the body over-producing red cells, which needs specific investigation. Distinguishing a "relative" high (dehydration) from a "true" one is the first task, usually helped by hydration status and total protein.

Haematocrit is a valuable number to track in chronic disease. A slowly falling haematocrit is a common feature of chronic kidney disease (the kidneys make the hormone that drives red-cell production) and of chronic inflammatory conditions, and following it over time shows whether an anaemia is stable, worsening, or recovering with treatment. A recovering haematocrit with a healthy reticulocyte response is reassuring evidence the marrow is rebuilding.

Haematocrit is interpreted the same way in both species, with cats normally running lower than dogs. Cats are also prone to specific causes of anaemia (certain infections and marrow conditions), and their reticulocyte response is read using feline-specific rules. In both, the value is always paired with haemoglobin and the red-cell indices to characterise the blood fully.

Haematocrit can be measured directly by spinning a tube (the PCV) or calculated by the analyser as part of the CBC; the two are essentially the same, reported as a percentage (or L/L). It is read alongside haemoglobin, the red cell count, and the reticulocyte count. Read the value against your own report's range.

A low haematocrit is a prompt to establish the type of anaemia — the reticulocyte count to judge whether it is regenerative, plus a look for blood loss, destruction or a production problem as the picture directs. A high haematocrit is usually addressed by correcting dehydration and rechecking. In chronic disease, the trend over time is the most useful reading.