Red cell distribution width (RDW)
Blood count (CBC) ·Also called: RDW, anisocytosis, red cell size variation
What it is
Rather than describing an average, RDW describes spread. The analyser measures thousands of individual cells and reports how widely their sizes scatter around the mean. Pathologists call the same finding anisocytosis when they see it down a microscope.
Why your vet measures it
RDW can rise before MCV moves, because a modest number of large young cells widens the spread without shifting the average much. In a pet being monitored for anaemia, a rising RDW is an encouraging early sign that new cells are entering circulation.
What a high result can mean
- A regenerative response, with young cells of a different size joining the mature population
- Iron deficiency, which produces a mixed population of normal and undersized cells
- A recent blood transfusion, mixing donor and patient cells
- Fragmentation of red cells, seen with some vascular tumours and clotting disorders
What a low result can mean
- Rarely meaningful on its own — a uniform population is the normal state
Questions worth asking your vet
- Does a raised RDW here mean my pet's marrow is starting to respond?
- Should we recheck it in a week or two to watch the trend?
Related terms
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