Lymphocytosis (lim-foe-sie-TOE-sis), or a high lymphocyte count, is an increase in white blood cells called lymphocytes. Lymphocytes help fight off diseases, so it's normal to see a temporary increase after an infection.
A count significantly higher than 3,000 lymphocytes in a microliter of blood is generally considered to be lymphocytosis in adults. In children, the threshold for lymphocytosis varies with age. It can be as high as 9,000 lymphocytes per microliter. The exact thresholds for lymphocytosis can vary slightly from one lab to another.
You can have a higher than normal lymphocyte count but have few, if any, symptoms. It usually occurs after an illness and is harmless and temporary.
But it might represent something more serious, such as a blood cancer or a chronic infection. Your doctor might perform other tests to determine if your lymphocyte count is a cause for concern.
If your doctor determines that your lymphocyte count is high, the test result might be evidence of one of the following conditions:
- Infection (bacterial, viral, other)
- Cancer of the blood or lymphatic system
- An autoimmune disorder causing ongoing (chronic) inflammation
Specific causes of lymphocytosis include:
- Acute lymphocytic leukemia
- Chronic lymphocytic leukemia
- Cytomegalovirus (CMV) infection
- Hepatitis A
- Hepatitis B
- Hepatitis C
- Hypothyroidism (underactive thyroid)
- Other viral infections
- Whooping cough
When to see a doctor
A high lymphocyte count is usually found when your doctor has ordered tests for other reasons or to help diagnose another condition you have.
Talk with your doctor about what your test results mean. A high lymphocyte count and results from other tests might indicate the cause of your illness.
Often, follow-up testing over several weeks shows that the lymphocytosis has resolved. Special blood tests can be helpful if lymphocytosis persists. Your doctor might refer you to a doctor who specializes in blood diseases (hematologist) if your condition persists or if the cause isn't apparent.