Mast cell tumors occur commonly in the dog.
There are three basic grades of mast cell tumor (Grade 1, 2 and 3). The Grade of the mast cell tumor is useful because it gives the veterinarian or oncologist an idea of how aggressive the mast cell tumor will be.
Another way of talking about this is how “malignant” the mast cell tumor is expected to behave. The more malignant, the higher the odds of spread to distant sites, and the more difficult the treatment.
If a mast cell tumor is an aggressive grade 2 or a grade 3, often staging will be performed. Staging is the process where the doctor looks for evidence to suggest tumor spread into distant sites.
A particular test was used, and sometimes is still used, to see if there is mast cell tumor spread. This test is called a buffy coat smear, or buffy coat analysis.
A buffy coat analysis is performed by placing a small amount of blood in a very thin tube called a microhematocrit tube. The tube end is plugged and the tube is spun at high speeds in a centrifuge.
The high speed causes cells that are more dense to settle towards the bottom of the tube, while those that are less dense remain higher in the tube.
When examined with the naked eye, a thin layer of white cells will form, just on top of the red cell layer. This is called the buffy coat. The buffy coat contains different kinds of white blood cells that have separated out from the heavier red blood cells in the spun tube.
These white cells can be removed and examined on a microscope slide. This is the buffy coat analysis.
About 15 years ago, a buffy coat smear was a common practice to stage mast cell tumors. However, it has fallen out of favor. The reason why is was done was to locate mast cell tumor cells in the circulation, which would give an indication that the tumor cells have spread throughout the body. This is called mastocytemia (mast cells in the blood).
However, we do not rely on this test much any longer, and spending money on a buffy coat smear is usually not the best allocation of funds in a dog cancer budget for staging. This is true even though the test is sometime still run. (Note that it can still be used to gauge treatment effects.)
The reason why the buffy coat smear is probably not a good use of funds for staging is that there are conditions that might increase the mast cells in the buffy coat when tumor spread is absent (non-cancerous mast cells are normal members of the white blood cell family).
As it turns out, over 95% of dogs who don’t have mast cell tumors will be positive for mast cells in the buffy coat. Some parasites, skin conditions, and other issues may increase mast cell numbers. Since the test will usually be positive, there is normally little point in running a buffy coat smear analysis to detect cancer spread.
Here is the abstract to read more.
For more information on mast cell tumors and other cancers in the dog, see the Dog Cancer Survival Guide.