PREPARING BLOOD FILMS AND DIFFERENTIAL WHITE BLOOD CELL COUNTING: A COMPREHENSIVE GUIDE

The microscopic examination of a peripheral blood film is a cornerstone of hematology, offering invaluable insights into a patient’s health status. It allows for the qualitative and quantitative assessment of red blood cells (RBCs), white blood cells (WBCs), and platelets, aiding in the diagnosis and monitoring of numerous conditions, including infections, inflammatory disorders, anemias, and hematological malignancies.

Preparing a Peripheral Blood Film

A well-prepared blood film is crucial for accurate microscopic examination. Poor film quality can lead to misinterpretation and diagnostic errors.

1. Materials Required:

• Anticoagulated Blood Sample: Fresh whole blood collected in an EDTA (ethylenediaminetetraacetic acid) tube. EDTA prevents clotting while preserving cellular morphology.
• Clean Glass Slides: High-quality, clean, grease-free microscopic slides (75 x 25 mm). New slides are preferred; reusable slides must be thoroughly cleaned.
• Spreader Slide: A second glass slide with a smooth, unchipped edge, used to spread the blood drop.
• Pasteur Pipette or Micro-pipette: For dispensing a small, consistent drop of blood.
• Staining Rack: A horizontal rack to hold slides during the staining process.
• Methanol (Absolute): For fixing the blood film.
• Romanowsky Stain: Commonly used stains include Wright’s stain, Leishman’s stain, or Giemsa stain. These are polychromatic stains containing both basic (methylene blue) and acidic (eosin) dyes.
• Buffer Solution: Phosphate buffer (pH 6.8 is common for Wright’s/Leishman’s, pH 7.2 for Giemsa) for optimal staining.
• Distilled Water: For washing slides.
• Timer: For accurate staining times.
• Personal Protective Equipment (PPE): Gloves, lab coat, safety glasses.

2. Specimen Collection and Handling:

Venous blood is typically collected into an EDTA vacutainer tube. It is imperative that the sample is gently inverted several times immediately after collection to ensure thorough mixing with the anticoagulant and prevent microclot formation. Blood films should ideally be prepared within two hours of collection to maintain optimal cell morphology, although satisfactory films can often be made up to six hours post-collection if refrigerated. If delayed, the sample should be re-mixed gently before use.

3. Procedure for Blood Film Preparation:

• Step 1: Prepare the Workstation: Ensure a clean, dry, and level surface. Arrange all materials within easy reach. Don gloves and other PPE.
• Step 2: Place the Blood Drop: Hold a clean glass slide on a flat surface. Using a Pasteur pipette or micro-pipette, place a small drop (2-3 mm in diameter, approximately 5-10 µL) of well-mixed EDTA blood about 1-2 cm from one end of the slide. The size of the drop is critical; too large will result in a thick film, too small, a thin film.
• Step 3: Spread the Blood:
  • Take the spreader slide (which should have a perfectly smooth edge) and place its edge on the surface of the first slide, just in front of the blood drop.
  • Draw the spreader slide backwards into the blood drop. The blood should spread rapidly by capillary action along the entire width of the spreader edge. This is a crucial step to ensure the film’s width is consistent.
  • Maintain an angle of approximately 30-45 degrees between the spreader slide and the bottom slide. The exact angle depends on the hematocrit and desired film thickness; a higher angle yields a thicker, shorter film, while a lower angle produces a thinner, longer film.
  • With a smooth, steady, and swift motion, push the spreader slide forward along the bottom slide, drawing the blood behind it. The single stroke should be continuous and uninterrupted.
• Step 4: Drying: Allow the blood film to air-dry rapidly by waving the slide in the air or using a fan. Rapid drying prevents crenation (shrinkage) of red blood cells and preserves morphology. Do not blow on the slide, as this introduces moisture and contaminants.
• Step 5: Fixation: Once completely dry, fix the film by immersing it in absolute methanol for 1-5 minutes. Fixation denatures proteins and “fixes” the cells to the slide, preventing them from washing off during subsequent staining steps. Remove the slide and allow it to air-dry completely.
• Step 6: Staining (e.g., Leishman’s or Wright’s Stain):
  • Place the fixed, dry slide on a level staining rack.
  • Flood the slide with the Romanowsky stain (e.g., Leishman’s). Allow it to act for 5-10 minutes (this fixes and stains simultaneously in some protocols, or primarily stains if already fixed).
  • Add an equal quantity of buffer solution (e.g., pH 6.8) to the stain on the slide. Gently mix by rocking the slide or using a pipette. A metallic sheen should appear on the surface. Allow this mixture to act for 10-20 minutes. The specific timing depends on the stain, batch, and desired intensity. This step, known as differentiation, allows the acidic and basic dyes to bind to cellular components.
  • Rinse the slide thoroughly with distilled water until all excess stain is removed and the film appears pinkish-purple. Rinse from the back of the slide to avoid washing off the film.
• Step 7: Final Drying: Stand the slide upright in a drying rack or wipe the back clean and allow it to air-dry completely.

4. Characteristics of a Good Blood Film:

A high-quality blood film should possess the following features:

• Optimal Length: It should cover approximately two-thirds to three-quarters of the slide.
• Feather Edge: It should gradually thin out to a distinct, feathery edge, with no lines or ridges.
• Smoothness: No streaks, holes, or clumps of cells.
• Even Distribution: Cells should be evenly distributed, especially in the “monolayer” or “zone of morphology” where RBCs are separate but just touching.
• Width: The film should be slightly narrower than the slide, with clear margins.
• Absence of Artifacts: No water marks, stain precipitates, or cellular distortion.

Identifying and Quantifying Different Types of White Blood Cells

Once a properly stained blood film is prepared, it is ready for microscopic examination and differential leukocyte counting.

1. Microscope Setup and Scanning Technique:

• Initial Scan (10x Objective): Place the stained blood film on the microscope stage. Begin by scanning the entire film under low power (10x objective). This allows for a general assessment of film quality, cell distribution, and the presence of any large abnormal cells or aggregates. Locate the “zone of morphology” – the area where RBCs are separate but close enough to touch, and central pallor is visible. This zone is typically just behind the feather edge.
• Intermediate Scan (40x Objective): Switch to the high-dry objective (40x). This provides a clearer view of individual cells and helps in selecting the optimal area for the differential count.
• Oil Immersion (100x Objective): Apply a drop of immersion oil directly onto the blood film in the zone of morphology. Switch to the oil immersion lens (100x objective). This magnification is essential for detailed morphological examination of WBCs, RBCs, and platelets.
• Systematic Scanning: To ensure a representative count, adopt a systematic scanning pattern, such as a “battlement” or “zig-zag” pattern, moving across the zone of morphology without missing any areas or recounting cells.

2. Identification of White Blood Cells (Leukocytes):

White blood cells are broadly classified into granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes). They vary significantly in size, nuclear morphology, cytoplasmic characteristics, and granule content.

• General Features of WBCs:
  • All WBCs are nucleated.
  • They are generally larger than red blood cells (which are typically 7-8 µm in diameter).
  • Their cytoplasm can range from pale blue to pink, and may contain specific granules.
• Detailed Identification:
  • Neutrophils (Polymorphonuclear Leukocytes – PMNs):
    • Size: 10-15 µm (1.5-2 times the size of an RBC).
    • Nucleus: Segmented, typically 2-5 distinct lobes connected by thin filaments of chromatin. Chromatin is dense and clumped. The number of lobes increases with cell age.
    • Cytoplasm: Pale pink or faint lilac, usually abundant.
    • Granules: Fine, numerous, light pink or violet (neutrophilic) granules throughout the cytoplasm. These are specific granules. Azurophilic (primary) granules are also present but not distinctly visible with Romanowsky stains.
    • Function: Phagocytosis of bacteria and cellular debris. Crucial in acute bacterial infections.
  • Eosinophils:
    • Size: 12-17 µm (slightly larger than neutrophils).
    • Nucleus: Usually bilobed (two distinct lobes), often appearing like spectacles or dumbbells. Chromatin is coarser than neutrophils.
    • Cytoplasm: Distinctly pink or orange, filled with large, uniformly-sized, round, refractile, bright red-orange (acidophilic) granules. These granules often obscure the nucleus.
    • Function: Involved in allergic reactions, parasitic infections, and modulating inflammatory responses.
  • Basophils:
    • Size: 10-14 µm (similar to neutrophils).
    • Nucleus: Irregularly shaped, often bilobed but frequently obscured by granules. Chromatin is relatively loose.
    • Cytoplasm: Scanty.
    • Granules: Large, dark blue to purple, coarse, unevenly sized granules that often overlie and obscure the nucleus. These granules contain histamine and heparin and are water-soluble, sometimes appearing as “punched-out” areas.
    • Function: Mediate hypersensitivity reactions, release histamine in allergic responses. Their presence is rare in peripheral blood.
  • Lymphocytes:
    • Size: Highly variable, 7-18 µm. Small lymphocytes (7-10 µm) are most common, large lymphocytes (10-18 µm) are less frequent.
    • Nucleus: Typically round or oval, often eccentrically placed. Dense, clumped chromatin with no visible nucleoli in typical small lymphocytes. High nuclear-to-cytoplasmic (N:C) ratio in small lymphocytes.
    • Cytoplasm: In small lymphocytes, cytoplasm is scanty, appearing as a thin rim of sky-blue cytoplasm around the nucleus. May contain a few azurophilic granules (non-specific). Large lymphocytes have more abundant cytoplasm.
    • Function: Central to adaptive immunity, including antibody production (B lymphocytes) and cell-mediated immunity (T lymphocytes).
  • Monocytes:
    • Size: 12-20 µm (largest WBC in peripheral blood).
    • Nucleus: Highly variable in shape; often kidney-shaped, horseshoe-shaped, or brain-like (convoluted). Chromatin is fine and lacy, giving a “brain-like” appearance, typically lighter than lymphocyte chromatin.
    • Cytoplasm: Abundant, dull grey-blue, often described as having a “ground-glass” or “frosted” appearance. May contain fine azurophilic granules and often vacuoles.
    • Function: Precursors to macrophages in tissues. Involved in phagocytosis, antigen presentation, and immune regulation.

3. Quantification (Differential Leukocyte Count):

The differential count determines the relative percentage of each type of WBC present in the peripheral blood.

• Counting Method: Using the 100x oil immersion objective, systematically scan the blood film in the zone of morphology. Identify and count at least 100-200 consecutive WBCs, categorizing each one into its specific type (neutrophil, eosinophil, basophil, lymphocyte, monocyte). Many laboratories use a “differential counter” device or software that allows for tallying each cell type.
• Reporting:
  • Relative Count: Expressed as a percentage of the total WBCs counted (e.g., 60% neutrophils).
  • Absolute Count: Calculated by multiplying the relative percentage of each cell type by the total WBC count (obtained from an automated hematology analyzer). For example, if total WBC count is 10 x 10^9/L and neutrophils are 60%, then absolute neutrophil count is 6 x 10^9/L. Absolute counts are generally more clinically significant than relative counts.
• Normal Ranges: Normal ranges vary slightly between laboratories and populations, but typical adult ranges are:
  • Neutrophils: 40-75% (Absolute: 2.0-7.5 x 10^9/L)
  • Lymphocytes: 20-45% (Absolute: 1.0-4.5 x 10^9/L)
  • Monocytes: 2-10% (Absolute: 0.2-1.0 x 10^9/L)
  • Eosinophils: 1-6% (Absolute: 0.02-0.6 x 10^9/L)
  • Basophils: 0-1% (Absolute: 0.00-0.1 x 10^9/L)

4. Clinical Significance:

Changes in the number and morphology of WBCs are indicative of various physiological and pathological states:

• Neutrophilia/Neutropenia: Increased/decreased neutrophils, often associated with bacterial infections or inflammation/bone marrow suppression.
• Lymphocytosis/Lymphopenia: Increased/decreased lymphocytes, seen in viral infections, chronic inflammation/immunodeficiencies.
• Monocytosis: Increased monocytes, often in chronic inflammation, tuberculosis, or some leukemias.
• Eosinophilia: Increased eosinophils, a hallmark of allergic reactions and parasitic infections.
• Basophilia: Increased basophils, seen in rare conditions like chronic myeloid leukemia.
• Abnormal Forms: The presence of immature forms (e.g., myelocytes, metamyelocytes, blast cells) or atypical lymphocytes suggests more serious conditions, including leukemia or severe viral infections.

Conclusion

The preparation and microscopic examination of a peripheral blood film, coupled with a meticulous differential leukocyte count, remain indispensable techniques in clinical hematology. While automated analyzers provide rapid and accurate cell counts, the manual blood film review offers critical morphological insights that machines cannot replicate. The ability to correctly prepare a high-quality film and accurately identify and quantify white blood cells requires skill, practice, and a thorough understanding of cellular morphology and staining principles. This expertise is vital for providing accurate diagnostic information and guiding patient management.

References

1. Bain, B. J., Bates, I., & Laffan, M. A. (2017). Dacie and Lewis Practical Haematology (12th ed.). Elsevier.
2. Rodak, B. F., Carr, J. H., & Smith, L. J. (2023). Clinical Hematology Atlas (6th ed.). Elsevier.
3. Harmening, D. M. (2009). Clinical Hematology and Fundamentals of Hemostasis (5th ed.). F.A. Davis Company.
4. Palmer, R., & D’Souza, L. (2013). Manual of Blood Film Morphology. John Wiley & Sons.
5. World Health Organization. (2010). Laboratory Manual for the Examination of Human Semen and Sperm-Cervical Mucus Interaction (5th ed.). WHO Press. (While focused on semen, it provides excellent general guidance on microscopy and slide preparation).