Dlc
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Differential lucocyte count
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Here is a comprehensive overview of the Differential Leukocyte Count (DLC):
Differential Leukocyte Count (DLC)
Definition
The DLC (also called the differential WBC count) is the percentage distribution of the different types of leukocytes in peripheral blood. It is performed on a Romanowsky-stained peripheral blood smear, scanning a defined counting area and classifying each leukocyte encountered until 100–1000 cells have been counted. The absolute concentration of each cell type = percentage × total leukocyte count.
"An increase in absolute concentration is an absolute increase; an increase in percentage only is a relative increase. Reference intervals are more useful if given as absolute concentrations rather than percentages." — Henry's Clinical Diagnosis and Management by Laboratory Methods
The Five Cell Types & Normal Values
| Cell Type | % of WBCs | Absolute Count (×10⁹/L) |
|---|---|---|
| Neutrophils (segmented + bands) | 50–70% | 1.8–7.7 |
| — Segmented | 40–60% | — |
| — Bands | 0–5% | — |
| Lymphocytes | 20–40% (avg ~34%) | 1.5–4.0 |
| Monocytes | 2–8% (avg ~4%) | 0–0.8 |
| Eosinophils | 1–5% | 0–0.45 |
| Basophils | 0–1% (avg ~0.5%) | 0–0.2 |
Individual Cell Morphology
1. Neutrophil (PMN — Polymorphonuclear Leukocyte)
Figure: Mature neutrophil (multi-lobed nucleus with distinct filaments) alongside a band form (horseshoe-shaped nucleus). 1000×.
- Size: ~12 μm
- Nucleus: 2–5 lobes connected by thin filaments; coarse chromatin
- Lobe distribution: 10–30% have 2 lobes, 40–50% have 3 lobes, 10–20% have 4 lobes; ≤5% have 5 lobes
- Granules: Fine, pinkish-lilac (primary = azurophilic, secondary = specific)
- Shift to the left: ↑ bands and immature forms → suggests bacterial infection
- Hypersegmentation (>5 lobes): suggests megaloblastic anemia
2. Lymphocyte
Figure: Reactive lymphocyte with abundant pale-gray cytoplasm and distinct chromatin/parachromatin separation. 1000×.
- Size: 6–15 μm (small to large)
- Nucleus: Round, heavily clumped dark-blue chromatin; may be indented
- Cytoplasm: Scant, pale blue; a clear perinuclear zone
- ~⅓ of large lymphocytes have red-purple azurophilic granules
- Atypical/reactive lymphocytes indicate antigenic stimulation (e.g., EBV)
- Plasma cells (not normally in blood): abundant blue cytoplasm, eccentric nucleus, clock-face chromatin
3. Monocyte
Figure: Two monocytes — the largest normal blood cells — with characteristic kidney/horseshoe-shaped nuclei and fine granular cytoplasm.
- Largest normal blood cell (14–20 μm; 2–3× diameter of RBC)
- Nucleus: Horseshoe/kidney-shaped or partially lobulated; fine parallel chromatin strands; stains less densely than other leukocytes
- Cytoplasm: Abundant, blue-gray, ground-glass appearance; fine red-purple granules; may have vacuoles
- Transforms into tissue macrophage
4. Eosinophil
- Size: Similar to neutrophil or slightly larger
- Nucleus: Usually bilobed
- Granules: Large, bright red-orange (eosinophilic) refractile granules filling the cytoplasm
-
2% in synovial fluid = eosinophilia (seen in Lyme disease, RA, parasites, allergic reactions)
- Peripheral blood eosinophilia >10% may suggest EGPA (Churg-Strauss)
5. Basophil
- Least numerous leukocyte (~0.5%; 95% reference: 0–0.2 × 10⁹/L)
- Nucleus: Less segmented than neutrophil (indented or partially lobulated), often obscured by granules
- Granules: Large, deep purple, water-soluble (may wash out leaving vacuoles); contain histamine and heparin
- May resemble Histoplasma capsulatum if granules are unevenly stained
Method of Performing DLC
- Prepare smear: Air-dried wedge smear or cytocentrifuge preparation, Romanowsky stain (Wright-Giemsa)
- Scan at 10×: Estimate WBC count; scan lateral and feather edges for monocytes and large cells
- Count at high power (100×): Use crenellation technique — move the field side-to-side across the counting area
- Count 100 cells routinely (200–1000 for greater precision)
- Record abnormal cells: NRBCs, blasts, immature granulocytes, reactive lymphocytes must be noted separately
- Automated differential: Hematology analyzers provide 5-part differential using impedance/light scatter; unreliable for abnormal/immature cells — flagged samples require manual review
Clinical Significance of Abnormal Findings
| Finding | Cause |
|---|---|
| Neutrophilia (↑ neutrophils) | Bacterial infection, inflammation, steroids, CML |
| Neutropenia (↓ neutrophils) | Viral infection, aplastic anemia, chemotherapy, drugs |
| Left shift (↑ bands/immature forms) | Acute bacterial infection, sepsis |
| Hypersegmentation | Megaloblastic anemia (B12/folate deficiency) |
| Lymphocytosis | Viral infections (EBV, CMV), CLL, pertussis |
| Lymphopenia | HIV, immunosuppression, steroids |
| Monocytosis | TB, subacute bacterial endocarditis, SLE |
| Eosinophilia | Allergy, asthma, parasites, EGPA, drug reactions |
| Basophilia | CML, allergic reactions, hypothyroidism |
| Blasts | Acute leukemia |
Absolute vs. Relative Count
The absolute count (% × total WBC) is clinically more meaningful than the percentage alone. For example, a patient with lymphopenia (total WBC 2.0 × 10⁹/L, 60% lymphocytes) has a relative lymphocytosis but a normal absolute lymphocyte count.
Source: Henry's Clinical Diagnosis and Management by Laboratory Methods (9780323673204)
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