Times change and my website needed to change too. To see the 2020 update of this page click this link
To see what normal blood and urine values are, go here
For an explanation of causes of most abnormal blood and urine tests, go here
To see how tests are grouped, go here
Your pet’s red blood cells (erythrocytes) should be very uniform in size. Red blood cells or RBCs make up ~25% of all the cells in the body. I have seen it written that the average dog produces and destroys about 800,000 RBCs every second; but I cannot confirm that that is true. Both dogs and cats have an enormous number of RBCs (adult humans have ~ 20–30 trillion).The average life span of a human RBC is 100-120 days. (ref)
Each red blood cell must be extremely small and uniform in size to pass through the small capillaries of your pet’s body without hindrance (getting stuck). One factor in determining RBC size is the number of cell divisions that occur between the red cell's formation from stem cells in your dog or cat 's bone marrow and the time they leave the marrow mature (a protein, [cyclin D3] , is involved in that regulation).
In certain health issues, red blood cells may not be as uniform in size as they should be. Veterinarians use a mathematically calculated value, the RDW, to know when that is occurring. The actual measurement is of the volume or size of your pet’s RBC os its MCV. A high RDW means that there is too much variation in the size of your pet’s RBCs.
Knowing that is occurring gives your veterinarian hints as to the possible cause of a confirmed anemia. The same problem will also be picked up as anisocytosis being noted in your pets CBC/WBC lab results.
The RDW is not a stand-alone test. I do not put much faith in its ability to differentiate one cause of anemia from another. But it does add a bit of evidence - if the rest of your pet’s blood work agrees with what is generally accepted as the causes of abnormal RDW:MCV relationships.
When your pet has a vitamin B12 deficiency, as might occur with chronic intestinal problems like IBD, Its Red Blood Cell Distribution Width (RDW) will increase. The same thing can occur with a folic acid (folate) deficiency. In both cases the MCV tends to be elevated as well. In those cases, anisocytosis is usually also noted in the pet’s CBC/WBC results.
High RDW can also occur with iron deficiency anemia. In those cases, the MCV is often low.
Recent hemorrhage or blood loss will often elevate RDW, but MCV usually remains normal.
Fragmentation (turbulence) anemia or any anemia in which the RBCs are destroyed while in the pet's circulation (a hemolytic anemia) can cause RDW to be high and MCV to be low.
In humans, high RDW is sometimes associated with liver disease. We do not know if that if the case in dogs and cats.
A combination of several causes can also be responsible for an elevated RDW in your dog or cat.