By The Pawline veterinary teamVeterinary-reviewed by Alastair Greenway, BVM&S MRCVS · 25 July 2026
Reference range · %
1217
Reference
12–17
Direction
Higher values are the concern.
This is the conventional reference interval. Your pet’s own report may print a slightly different range — always read the value against that. Sign in to see your pet’s value on this strip.
RDW measures how variable the red cells are in size. A rising value is an early clue that the marrow is releasing a new population of differently-sized cells — as in a regenerative response — so it can flag change before the average indices move.
RDW (red cell distribution width) captures something the average indices miss: not the typical red cell size, but how much the cells vary in size. A population of uniform cells has a low RDW; a mixture of small and large cells has a high one. This makes RDW a sensitive early marker of change, because a new population of cells entering the bloodstream widens the spread before it necessarily shifts the average MCV.
RDW measures how variable red cells are in size. A rising value is an early clue the marrow is releasing a new, differently-sized population of cells.
Reading a high result
A raised RDW (anisocytosis — cells of unequal size) most commonly reflects a regenerative response: the marrow releasing larger young cells (reticulocytes) alongside the existing mature ones, widening the size range. In that setting a rising RDW, read with a rising reticulocyte count, is a reassuring sign the marrow is responding to an anaemia. A high RDW can also appear in the transition of iron-deficiency anaemia, as small iron-poor cells mix with normal ones, and in early recovery from various anaemias. Its strength is timing — it can move early, flagging that a red-cell population is shifting before the average indices catch up.
Reading a normal result
A normal RDW indicates a uniform red-cell population and is the usual, unremarkable state. In an anaemic patient, a normal RDW with a low reticulocyte count fits a non-regenerative anaemia — the marrow not producing a fresh, differently-sized population — which is itself a useful clue pointing towards a production problem.
In chronic disease
Because it responds early to changes in the red-cell population, RDW is a helpful trend marker: a rising RDW can be the first hint that the marrow is beginning to regenerate, and a normalising RDW tracks the settling of a recovered red-cell population. It is interpreted alongside MCV and the reticulocyte count.
Cat vs dog
RDW is interpreted the same way in both species, with species-specific reference ranges. The rising-RDW-signals-regeneration logic applies to both dogs and cats, always read alongside the reticulocyte response.
How it's measured
RDW is calculated by the analyser from the spread of red-cell sizes it measures, reported as a percentage (RDW-CV). It is read together with MCV and the reticulocyte count. Read the value against your own report's range.
What to do next
A rising RDW, especially with a rising reticulocyte count, points to an active regenerative response — a reassuring sign in an anaemic patient. A normal RDW with a low reticulocyte count fits a non-regenerative anaemia worth investigating for a production problem. As a sensitive, early-moving index, RDW is followed as a trend alongside the rest of the red-cell picture.
Reference ranges reflect conventional laboratory intervals. Always interpret against your own lab's printed range and your veterinary surgeon's judgement. A brighter optimal band is shown only where a published guideline defines one, and it is cited.
This page is educational and is not a diagnosis. Lab results are interpreted in the full clinical context by your veterinary surgeon; a single value rarely tells the whole story.
