What is a Leukocyte Differential Count?
A white blood cell (WBC) differential count (or "diff") is a fundamental blood test that dissects your immune system. Instead of a single number, it breaks down your white blood cells into their five primary types to identify infections, leukemia, or immune disorders.
[attachment_0](attachment)⚠️ Clinical Insight: Absolute vs. Relative
Your report shows two values: a percentage (%) and an absolute count (abs). The absolute count is the definitive value.
- A "normal" 60% neutrophil count is meaningless if the total WBC count is dangerously low.
- Takeaway: All clinical decisions must be based on the absolute numbers.
Why is it Ordered?
Diagnosis
To find the cause of fever, fatigue, or unexplained bruising. It differentiates bacterial vs. viral infections.
Monitoring
Tracking response to antibiotics, chemotherapy effects, or progression of blood disorders.
Meet Your Immune Team
White blood cells come from two main lines: Myeloid and Lymphoid.
Neutrophils
The "infantry" of the immune system. Their main job is to kill and digest bacteria and fungi.
Lymphocytes
The "special ops" (T-cells & B-cells). They fight viruses and manage long-term immunity/antibodies.
Monocytes
They handle chronic infections and clear out dead-cell debris.
Eosinophils
Key for fighting parasites and managing allergic reactions (asthma).
Basophils
Rare. They release histamine during reactions. High levels are a red flag for bone marrow cancer.
Interpreting Normal Values
Reference intervals vary by lab. The table below represents typical adult ranges.
| Test Component | Normal Range (Absolute) |
|---|---|
| Total WBC | 4.0 - 11.0 x 10⁹/L |
| Neutrophils | 1.8 - 7.7 x 10⁹/L |
| Lymphocytes | 1.2 - 4.8 x 10⁹/L |
| Monocytes | 0.0 - 1.0 x 10⁹/L |
| Eosinophils | 0.0 - 0.5 x 10⁹/L |
High Counts (Leukocytosis)
We classify high counts by which cell line is elevated.
Neutrophilia
Cause: Bacterial infection, extreme stress, or steroid use.
Lymphocytosis
Cause: Viral infections (Mono/EBV, Pertussis).
Eosinophilia
Rule: "Wheezes, Worms, & Weird Diseases." Allergies, Parasites, or Autoimmune issues.
Basophilia
Warning: Highly specific for Myeloproliferative Neoplasms (CML).
Low Counts (Leukopenia)
A low count indicates a compromised immune system.
🚨 Neutropenia (Medical Emergency)
If neutrophils are critically low, the patient cannot fight bacteria. Common causes:
- Chemotherapy
- Severe Sepsis (cells are "used up")
- Bone Marrow Failure
Lymphopenia
Destruction of T-cells. Common in HIV/AIDS, acute viral infections (Flu/COVID), or steroid therapy.
Clues on the Smear
When a machine flags an issue, a human reviews the blood under a microscope. These manual findings are critical.
"Left Shift" (Bandemia)
An increase in immature neutrophils. The marrow is rushing to release soldiers to fight a bacterial infection.
Toxic Changes
Granulation inside neutrophils. A sign of severe sepsis or inflammation.
Atypical Lymphocytes
"Angry-looking" cells. The classic sign of Mononucleosis (Mono).
Blast Cells
CRITICAL: Baby cells that belong only in the marrow. Their presence in blood is a red alert for Acute Leukemia.
Summary
The differential count is a powerful tool, but it is not a diagnosis in itself.