The lymphocyte count on a blood sample is measured as a percentage of the total number of white blood cells. In addition to lymphocytes, there are several other types of white blood cells, including granulocytes (neutrophils), monocytes, eosinophils, and basophils. Granulocytes are usually the main white cell type in a blood sample. Lymphocytes are next, with a range of 15% to 40%.

Although the blood test report gives the percentage of lymphocytes, the absolute number of lymphocytes per cubic centimeter of blood is more helpful. The absolute count is calculated by multiplying the total white blood cell count times the percentage of lymphocytes.

A high lymphocyte count is defined as an absolute lymphocyte count of 4,000 or greater. Doctors call this lymphocytosis.

First, be sure that you are not just looking at the percentage. Let's say your percentage of lymphocytes is 50% (higher than usual) and your total white blood cell count is 6,000. Fifty percent of the number 6,000 is 3,000. This is a normal absolute lymphocyte count and does not require any specific action or treatment. Your doctor may want you to repeat the blood test later on.

The most common cause of lymphocytosis is infection, usually a viral infection. In a person over the age of 50 who does not have an active infection, chronic lymphocytic leukemia should be considered. In the uncommon situation of a very high lymphocyte count in a younger person, it may be due to acute lymphocytic leukemia.