Differential leukocyte count

Principle

• Leucocytes on a stained blood film are identified and counted to give the percentage of each type, then converted to absolute counts.

Specimen

• Well-made Romanowsky-stained EDTA blood film.

Procedure

• Scan at low power for film quality, distribution, clumps and abnormal cells at edges and tail.
• Under oil immersion, count in the monolayer area using a battlement (meander) track to avoid counting cells twice.
• Classify at least 100 consecutive WBCs (200 improves precision, especially with abnormal counts).
• Record NRBCs per 100 WBC separately; note RBC and platelet morphology.

Results

• Absolute count = percentage × total WBC count; absolute values are more meaningful than percentages.
• Typical adult values (local ranges apply): neutrophils 40–75% (about 2.0–7.0 × 10^9/L); lymphocytes 20–40% (1.0–3.0 × 10^9/L); monocytes 2–10% (0.2–1.0 × 10^9/L); eosinophils 1–6% (up to about 0.5 × 10^9/L); basophils under 1–2% (up to about 0.1 × 10^9/L).
• Children have relatively more lymphocytes than adults.

Interpretation

• Neutrophilia: bacterial infection, inflammation, steroids, CML. Neutropenia: viral infection, drugs, marrow failure, benign ethnic neutropenia.
• Lymphocytosis: viral infections, pertussis, CLL. Eosinophilia: allergy, parasites, drugs, some neoplasms. Monocytosis: chronic infection such as TB, CMML. Basophilia: myeloproliferative neoplasms, especially CML.

Quality control and pitfalls

• Counting at the feathered edge overestimates large cells; the thick area makes identification difficult.
• Smudge cells (common in CLL) are recorded separately.
• Delayed films show degenerative changes.
• Statistical error is large with 100 cells; rare cells need a larger count.

Clinical use

• Evaluation of infection, allergy, leukaemia and response to therapy.