White Blood Cell Count
WBC· also: Leukocyte Count, White Cell Count
Clinical Overview
The white blood cell (WBC) count measures the total number of leukocytes in the blood, including neutrophils, lymphocytes, monocytes, eosinophils, and basophils. WBCs are the immune system's primary defenders against infection, inflammation, and foreign substances.
Why This Test Matters
The WBC count is a critical first-line test for detecting infection, inflammation, immune disorders, and blood cancers. Leukocytosis (high WBC) typically indicates infection or inflammation; leukopenia (low WBC) increases infection risk and may signal bone marrow suppression. The WBC differential — the breakdown of each cell type — is essential for narrowing the diagnosis.
Reference RangesWHO/IFCC standards
| Age Group | Reference Range | Unit | Notes |
|---|---|---|---|
| Adults (18–64) | 4 – 11 | ×10³/µL | — |
| Children / Pediatric | 5 – 15 | ×10³/µL | Varies by age; higher in infancy |
Also reported in: ×10⁹/L.
Critical (Panic) Values
Critical Low: < 2 ×10³/µL. Critical High: > 30 ×10³/µL. Values outside these limits require immediate clinical attention.
What Causes Abnormal Results?
High WBC Causes
- Bacterial infection (most common cause)
- Viral infection (can also cause lymphocytosis)
- Inflammation (e.g., rheumatoid arthritis, IBD)
- Leukemia (especially chronic myeloid leukemia)
- Physical or emotional stress
- Corticosteroid use
- Smoking
- Post-splenectomy state
Low WBC Causes
- Viral infections (HIV, hepatitis, influenza)
- Bone marrow suppression from chemotherapy or radiation
- Aplastic anemia
- Autoimmune neutropenia (lupus, rheumatoid arthritis)
- Severe bacterial sepsis (WBC can paradoxically drop)
- Vitamin B12 or folate deficiency
- Certain medications (clozapine, carbimazole, methotrexate)
- Hypersplenism (spleen trapping blood cells)
Signs & Symptoms to Watch For
How to Prepare for This Test
No special preparation is needed. A routine blood draw is sufficient. Note that stress, exercise, and some medications can temporarily alter WBC count.
Factors That Can Affect Results
- Vigorous exercise (transiently raises WBC)
- Emotional stress (raises WBC)
- Corticosteroids (raise neutrophils, lower lymphocytes)
- Time of day (slight diurnal variation)
- Smoking (chronically elevates WBC)
- Pregnancy (physiological leukocytosis is normal)
Related Topics
Frequently Asked Questions
What is a dangerously high WBC count?
A WBC count above 30,000/µL is considered a critical value and may indicate severe infection, a leukemic reaction, or leukemia. Counts above 100,000/µL in leukemia can cause leukostasis — a medical emergency where white cells block blood flow in small vessels, affecting the brain and lungs.
Can a high WBC count mean cancer?
A significantly elevated WBC can indicate leukemia or lymphoma, but the vast majority of elevated counts are caused by infection, inflammation, or medication. Blood cancers are suspected when the WBC is very high, persists without an obvious cause, or when the differential shows immature cells (blasts). A blood smear and bone marrow biopsy are used for definitive diagnosis.
What WBC count indicates a serious infection?
A WBC above 12,000/µL with a left shift (increased band neutrophils) strongly suggests a bacterial infection. However, severe infections or sepsis can sometimes cause a paradoxically low WBC (below 4,000/µL), which actually carries a worse prognosis. CRP, procalcitonin, and blood cultures are needed alongside WBC to assess infection severity.
What does it mean if my WBC is low after chemotherapy?
Chemotherapy destroys rapidly dividing cells including bone marrow stem cells, temporarily reducing WBC production. This chemotherapy-induced neutropenia typically peaks 7–14 days after treatment and increases the risk of serious bacterial infections. G-CSF (filgrastim) injections may be used to speed neutrophil recovery.