Leukocytosis

Leukocytosis means a white blood cell count above the normal range, usually more than about 11,000 cells per microliter in adults. It is usually the body's normal response to something else. Common causes include infections, inflammation, injury or surgery, physical or emotional stress, smoking, pregnancy, and medicines such as steroids. Less often, it is caused by a blood cancer such as leukemia or another bone marrow disorder. Leukocytosis usually causes no symptoms itself, and symptoms generally come from the underlying condition. Rarely, extremely high counts in leukemia can interfere with blood flow and cause serious symptoms.

Lab tests linked to leukocytosis:

• White blood cells: the total white cell count. It shows that leukocytosis is present, but not why.
• Doctors look at the differential, which shows which type of white cell is raised (neutrophils, lymphocytes, eosinophils, monocytes or basophils), and a blood smear. They may add CRP, a repeat CBC and, if a bone marrow cause is suspected, further tests.

How results are read:

• Raised neutrophils usually point to bacterial infection, inflammation, stress, smoking or steroids. Raised lymphocytes are common with viral infections.
• A mildly raised count that settles on repeat testing is usually a reaction to a short-term cause.
• Very high counts, immature cells on the smear, or abnormal red cells or platelets are reasons for prompt specialist assessment.

When to see a doctor:

• See a doctor about a white count that stays raised without a clear cause, or comes with weight loss, night sweats or easy bruising.
• Get urgent care for fever with chills, or if you feel very unwell.

Clinical takeaway: leukocytosis is defined by a raised white cell count, and the differential and blood smear help tell a reactive cause from a bone marrow disorder, sometimes with further testing.