Lymphocytosis is an increase in the number or proportion of lymphocytes in the blood. In absolute lymphocytosis, the total lymphocyte count is elevated. In adults, absolute lymphocytosis is present when the absolute lymphocyte count is greater than 4000 per microliter, in older children greater than 7000 per microliter and in infants greater than 9000 per microliter. Lymphocytes normally represent 20 to 40% of circulating white blood cells. Relative lymphocytosis occurs when there is a higher proportion (greater than 40%) of lymphocytes among the white blood cells, while the absolute lymphocyte count (ALC) is normal (less than 4000 per microliter). Relative lymphocytosis is normal in children under age 2.

Lymphocytosis is usually detected when a complete blood count is routinely obtained. The absolute lymphocyte count (ALC) can be directly measured by flow cytometry, or calculated by multiplying the total white blood cell (WBC) count by the percentage of lymphocytes found in the differential count.

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Lymphocytosis - Causes
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