Lymphocytopenia

Lymphocytopenia (lymphopenia) means a low number of lymphocytes, the white blood cells that include T cells, B cells and natural killer cells. In adults it usually means fewer than about 1,000 lymphocytes per microliter. Common causes include acute viral infections (including COVID-19 and influenza), physical stress, steroid and other immune-suppressing medicines, chemotherapy and radiation, HIV, autoimmune diseases such as lupus, malnutrition, heavy alcohol use and some cancers. Inherited immune deficiencies are rare causes. Short-lived lymphocytopenia during an illness is common. Long-lasting or severe lymphocytopenia raises the risk of some infections.

Lab tests linked to lymphocytopenia:

• White blood cells: the total white count. It can be normal even when lymphocytes are low, so the absolute lymphocyte count from the differential is the key number.
• Doctors check the absolute lymphocyte count, a repeat CBC, and may add HIV testing, lymphocyte subsets (CD4, CD8, B and NK cells), immunoglobulin levels and other tests based on the history.

How results are read:

• A low absolute lymphocyte count during or soon after an infection often recovers on repeat testing a few weeks later.
• A count that stays low, especially with repeated or unusual infections, needs further assessment of the immune system.
• A low CD4 count can point to HIV or other causes of reduced immunity.

When to see a doctor:

• See a doctor about a low lymphocyte count on more than one test, or about frequent, severe or unusual infections.
• Get urgent care for a high fever or feeling very unwell if you are on immune-suppressing treatment.

Clinical takeaway: lymphocytopenia is defined by the absolute lymphocyte count, and a count that stays low is a reason to look for immune or other causes.