Leukocytosis

Leukocytosis is a medical condition characterised by an elevated white cell (leukocyte) count in the blood, which typically signals an inflammatory response. It is often the result of infections but can also occur due to parasitic infections, bone tumours, or leukaemia. Additionally, leukocytosis may be observed after strenuous exercise, convulsions, emotional stress, pregnancy and labour, anaesthesia, and as a side effect of certain medications such as lithium and epinephrine.

Classification

Leukocytosis can be categorised based on the type of white blood cell that is elevated:

1. Neutrophilia: Increase in neutrophils, the most common form.
2. Lymphocytosis: Increase in lymphocytes.
3. Monocytosis: Increase in monocytes.
4. Eosinophilia: Increase in eosinophils.
5. Basophilia: Increase in basophils, which is rare.

This increase in leukocytes, particularly neutrophils, is often accompanied by a "left upper shift" in the ratio of immature to mature neutrophils and macrophages. This shift indicates a proliferation and inhibition of granulocyte and monocyte precursors in the bone marrow, stimulated by inflammatory products like C3a and G-CSF. Conversely, a "right shift" suggests a reduced count or absence of young neutrophils, which can be a haematological sign of pernicious anaemia and radiation sickness.

Causes

Leukocytosis has various causes depending on the type of leukocytosis:

• Neutrophilic Leukocytosis (Neutrophilia): Often caused by acute bacterial infections, sterile inflammation due to tissue necrosis (e.g., myocardial infarction, burns).
• Eosinophilic Leukocytosis (Eosinophilia): Commonly linked to allergic disorders (e.g., asthma, hay fever, drug allergies), parasitic infections, certain malignancies (e.g., Hodgkin's lymphoma), systemic autoimmune diseases (e.g., SLE), some forms of vasculitis, and transiently by cholesterol embolism.
• Basophilic Leukocytosis (Basophilia): Rare, typically associated with myeloproliferative diseases such as chronic myelogenous leukaemia.
• Monocytosis: Seen in chronic infections (e.g., tuberculosis, bacterial endocarditis), systemic autoimmune diseases (e.g., SLE), inflammatory bowel diseases (e.g., ulcerative colitis), and some malignancies like chronic myelomonocytic leukaemia.
• Lymphocytosis: Associated with chronic infections (e.g., tuberculosis, brucellosis), viral infections (e.g., hepatitis, cytomegalovirus infection, infectious mononucleosis), and some malignancies such as lymphocytic leukemias.

Leukocytosis is common in acutely ill patients, occurring in response to various conditions including infections, cancer, haemorrhage, and exposure to certain chemicals or medications like steroids.

Diagnosis

Leukocyte Counts

The diagnosis of leukocytosis is based on leukocyte counts, which are compared against standard blood reference ranges. A leukocyte count exceeding the 97.5 percentile indicates leukocytosis. The reference ranges for various types of leukocytes/WBCs are depicted below.

Treatment

In many cases, leukocytosis does not require treatment. However, hyperleukocytosis, where the WBC count exceeds 50 or 100 × 10⁹/L, particularly in leukaemic patients, must be treated to prevent leukostasis. Leukostasis occurs when WBC counts are so high that they obstruct blood flow, leading to ischaemic problems such as transient ischaemic attacks and stroke.

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Self-assessment MCQs (single best answer)

Which of the following is NOT a typical cause of leukocytosis?

What type of leukocytosis is characterised by an increase in neutrophils?

Which condition is most commonly associated with eosinophilic leukocytosis?

Which of the following is NOT typically associated with basophilic leukocytosis?

What does a "left upper shift" in the ratio of immature to mature neutrophils indicate?

Which type of leukocytosis is often seen in chronic infections like tuberculosis?

In the diagnosis of leukocytosis, what is a leukocyte count exceeding the 97.5 percentile indicative of?

What condition requires treatment to prevent leukostasis when WBC counts exceed 50 or 100 × 10^9/L?

Which of the following is a common cause of lymphocytosis?

Which type of leukocytosis is associated with systemic autoimmune diseases such as systemic lupus erythematosus (SLE)?