Leukocytosis

Last updated Nov. 3, 2025, 9:54 a.m. by ivo

Tags: Leukocytosis

The white blood cell count reference range will vary depending on the age, gender and pregnancy status of the patient, as well as the laboratory performing the test. In general, the white blood cell count is considered elevated when it is above 11 x 10⁹/L.

Causes

• The most common causes of leucocytosis include infection, inflammation, allergy and autoimmune disease.
• Drugs (including steroids and lithium) and physical stressors can provoke a leucocytosis.
• Cigarette smoking is also a common cause.
• Obesity
• Leucocytosis is an expected finding in asplenic/hyposplenic states.
• Neutrophilia may be a manifestation of a myeloproliferative neoplasm such as chronic myeloid leukaemia; in these cases, we look for additional abnormalities such as ‘left shift’ with basophilia, thrombocytosis, splenomegaly or constitutional symptoms. If any of these features are present, qualitative PCR testing on peripheral blood for the BCR-ABL1 translocation can be performed to exclude this diagnosis.
• Fluctuation of the white blood cell count, the combination of neutrophilia and monocytosis, or the combination of neutrophilia/monocytosis and lymphocytosis is suggestive of a reactive phenomenon.

Clinical history and examination

• Evidence of infection
• Medications
• Smoking
• Joint pain/swelling
• Recent surgery/trauma
• Chronic inflammatory conditions

Laboratory

• Repeat FBC and blood film
• E/LFTs
• Urine MCS, CXR, viral serology (EBV, pertussis) as clinically indicated
• CRP/ESR
• ANA/ENA/dsDNA as clinically indicated
• BCR-ABL1 PCR on peripheral blood

Follow up

• Secondary/reactive causes should be managed as appropriate
• If there is no reactive/secondary cause identified, and in the absence of concerning features, observation with full blood counts, initially every 3-6 months, is appropriate.

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When To Refer

Symptoms

• Fevers > 38.3°C
• Weight loss
• Drenching night sweats

Physical examination

• Lymphadenopathy
• Hepatosplenomegaly

Laboratory

• Anaemia (<100 g/L)
• Thrombocytopenia (<100x10⁹/L)
• Thrombocytosis (>450x10⁹/L)
• Immature cells on peripheral blood film
• WBCs (>30x10⁹/L)
• Basophilia
• Eosinophilia