RDW (red cell distribution width) in cats and dogs
RDW (red cell distribution width) shows how much red blood cells differ from each other in size. It is the analyzer’s version of anisocytosis — the size variation a vet sees on a blood smear. A high RDW means there is a mixture of larger or smaller cells; a low one has no clinical meaning.
What it shows
While counting, the analyzer measures the volume of every red cell and builds a histogram: cell size along one axis, number of cells along the other. MCV is the middle of that curve; RDW describes its width. eClinPath calls RDW the electronic equivalent of anisocytosis judged on the smear.
The classic RDW is a coefficient of variation: the spread of cell volumes (standard deviation) divided by the average volume, in %. eClinPath gives an example for a dog: average 70 fL, spread about 10 fL, RDW about 14%. In a dog with a strong regenerative response, extra large cells push the average to 80 fL and the spread to 20 fL — RDW becomes 25%.
A useful property: a small group of unusual cells can raise RDW before the average size, MCV, changes visibly.
Higher than reference — what it may be a sign of
- Regeneration — after bleeding or hemolysis the marrow releases young red cells, which are larger than mature ones. The mixture widens the curve.
- Iron deficiency — increasing numbers of small cells also widen it.
- Both at once — a population of small cells and a population of large ones together.
RDW does not say which side the extra cells are on. The answer comes from MCV, from the histogram printed by some analyzers and from the blood smear, which shows whether large or small cells dominate.
Lower than reference
An RDW below the reference range is not a clinically relevant finding, according to eClinPath. An RDW inside the range also says little on its own: it does not rule out changes in cell size, so the vet still reads MCV and the smear.
Look at together with
- MCV — whether the extra cells are large or small.
- Reticulocytes — whether a high RDW reflects the marrow’s response.
- MCHC — together with small cells, under-filled cells point to iron deficiency.
- Hemoglobin and hematocrit — whether there is anemia.
- Blood smear — anisocytosis as the eye sees it.
Units and conversion
Forms can show RDW in two ways, and they are not the same number:
- RDW or RDW-CV, in % — the spread divided by the average volume. Because it is a ratio, the same absolute spread gives a smaller % when the cells are larger on average.
- RDW-SD, in fL (femtolitres) — the spread of cell volumes itself, in the same units as MCV, without dividing by the average.
Some analyzers report both. Manufacturers use different statistical methods, so values from different analyzers are not directly comparable. In the app RDW-CV and RDW-SD are stored as two separate indicators and are never converted into each other: there is no factor between % and fL.
Reference range
The reference range is on your lab’s form. It is tied to a specific analyzer and its method, so only results from the same machine line up in a series. A high RDW is a reason to look at MCV, reticulocytes and the smear; a low RDW can be left aside.
Cats and dogs
The sources give no separate feline rules for RDW: it is read the same way as in dogs. In cats a regenerative response can be followed by weeks of larger young red cells in the blood — eClinPath notes they can persist up to 3 weeks after recovery — so the mixture of sizes may outlast the anemia itself.
In dogs breed affects the average size: poodles can have large red cells and Akitas small ones without anemia. In principle a uniform breed shift moves MCV but not necessarily RDW, because the cells are all equally large or small. A 2023 study of 100 canine samples found that smear grading of anisocytosis agreed only moderately with analyzer RDW and varied between observers, so the authors recommend relying on the RDW number; the smear still shows whether the odd cells are large or small.
Add a lab form — Pet Tracker Space recognises the lines, keeps the form’s units and draws each indicator over time with your lab’s reference band.
Sources
- eClinPath (Cornell University) — Red blood cell distribution width (RDW)
- eClinPath (Cornell University) — The concept of the RDW
- eClinPath (Cornell University) — MCV (mean cell volume)
- Jensen AL, Krogh AKH, Nielsen LN. Comparison of visual assessments of anisocytosis in canine blood smears and analyzer-calculated RDW. Frontiers in Veterinary Science, 2023
This page explains what a lab value means. It is not a diagnosis and not a substitute for your veterinarian — only they can interpret results for your pet.