What is the buffy coat?
Blood consists of millions of cells suspended in a liquid called plasma. These cells include red blood cells (which carry oxygen), white blood cells (which fight infection), and platelets (which help with clotting). Examining red blood cells is easy because there are so many of them; thousands of red blood cells can be seen in a single drop of blood. By comparison, white blood cells are present in very small numbers, and examining them is more difficult.
The fastest way to examine large numbers of white blood cells is to look at a buffy coat smear. The buffy coat is simply a concentration of all the white blood cells and platelets in a sample of blood. To prepare a buffy coat, a special machine spins the blood sample at very high speed in a process called centrifugation (much like a spinning ride at an amusement park).
"The fastest way to examine large numbers of white blood cells is to look at a buffy coat smear."
This causes the blood to separate into three parts: a large bottom layer of red blood cells, a large top layer of clear plasma, and a narrow middle band that contains all the white blood cells and platelets. This band is creamy white to buff-colored, and as a result it is called the buffy layer or buffy coat. The buffy coat material is collected and spread on a glass slide where it is examined under the microscope.
What sample is needed?
A simple blood sample is all that is needed for a buffy coat examination.
Why examine the buffy coat?
The main reason to examine a buffy coat is to look for abnormal white blood cells that are circulating in the blood. One of the most important cells to look for in a buffy coat is called a mast cell.
What are mast cells, and what does it mean if mast cells are found in the buffy coat?
Mast cells are a specific type of white blood cell. They play an important role in allergies and related conditions. In healthy pets, mast cells are found almost exclusively in body tissues and only very rarely in the blood. Mast cells can become neoplastic (neoplasia means "new growth" or tumor), and when they do, they form a mass or growth called a mast cell tumor. These growths may develop in the skin or in an internal organ, and they can be benign or malignant (cancerous). (Image via Wikimedia Commons / Joel Mills (CC BY-SA 3.0.)
If a mast cell tumor is highly malignant it may spread through the body, enter the bone marrow, and ultimately appear in the blood stream. This widespread distribution of malignant mast cells through the body is called systemic mastocytosis. The simplest way to detect this condition is to examine a buffy coat smear.
"If a mast cell tumor is highly malignant it may spread through the body, enter the bone marrow,
and ultimately appear in the blood stream."
If a pet has been diagnosed with a mast cell tumor, it is important to do a buffy coat examination before surgically removing the growth. Knowing ahead of time whether mast cells are present in the buffy coat may help your veterinarian predict what will happen after the growth is removed. The chances for a good outcome are much better if the buffy coat is negative, suggesting that the tumor has not spread. The presence of large numbers of mast cells in the buffy coat usually indicates that systemic mastocytosis is present. This is a worrisome finding since the condition is difficult to cure and can be fatal. However, there are medications that can help to control the disease and prolong the life of the pet.
If the buffy coat examination is negative, what then?
A negative buffy coat examination is good news, but it is not a guarantee that the mast cell tumor has not spread. Your veterinarian may want to do further testing and may suggest sampling lymph nodes or bone marrow to see if the tumor has spread. Alternatively, your veterinarian may suggest doing a series of buffy coat examinations at regular intervals to monitor your pet for systemic mastocytosis.