Hematology: CBC Interpretation, Blood Smear Preparation, and Manual Cell Counting

Components of the Complete Blood Count

The CBC is a panel of hematologic measurements that evaluates the cellular components of blood. Modern automated analyzers (impedance, laser flow cytometry, or optical systems) provide most values, but the technician must verify results by examining a blood smear.

Erythron (Red Blood Cell Parameters)

• RBC count — number of erythrocytes per volume of blood (×10^12/L). Decreases in anemia; increases in polycythemia or dehydration.
• Hematocrit (Hct) / PCV — percent of blood volume occupied by RBCs. The spun microhematocrit (PCV) is a fast in-house check; the analyzer Hct is calculated from RBC count × MCV. They should agree within ~3 percentage points.
• Hemoglobin (Hgb) — concentration of hemoglobin in g/L or g/dL. Measured by lysing RBCs and measuring light absorbance. Hgb and Hct should correlate: Hgb (g/dL) × 3 ≈ Hct (%).
• MCV (mean corpuscular volume) — average RBC size in femtoliters. High MCV = macrocytic (reticulocytes are larger); low MCV = microcytic (iron deficiency in some species).
• MCHC (mean corpuscular hemoglobin concentration) — hemoglobin per unit of RBC volume. Low MCHC = hypochromic. High MCHC usually indicates hemolysis or lipemia artifact.
• Reticulocytes — immature, anucleate RBCs with residual RNA, stained with new methylene blue supravital stain. Presence confirms a regenerative bone marrow response to anemia. Cats report aggregate (recently released) and punctate (older) reticulocytes separately; dogs report only aggregate.

Leukon (White Blood Cell Parameters)

• Total WBC count — all leukocytes per volume (×10^9/L). Leukocytosis = increased; leukopenia = decreased.
• WBC differential — relative percentages and absolute counts of each leukocyte type. Absolute counts (×10^9/L) are more clinically useful than percentages.
• Left shift — increased band (immature) neutrophils, indicating inflammation or infection that is consuming neutrophils faster than the marrow can produce segmented forms.
• Toxic change — basophilic cytoplasm, Dohle bodies, and vacuolation in neutrophils, signaling severe systemic inflammation.

Thrombon (Platelets)

• Platelet count — thrombocytes per volume (×10^9/L). Species reference ranges vary; dogs typically 200,000–500,000/µL. Counts below ~30,000–50,000/µL risk spontaneous bleeding.
• Estimation from smear: average platelets per 100x oil field × 15,000 (some labs use 20,000) gives an approximate count/µL.

Blood Smear Preparation: The Push/Wedge Technique

A good blood smear is essential for morphologic verification and manual differentials.

Steps

1. Materials: clean glass slides, a spreader slide (with polished or ground corners), fresh well-mixed EDTA blood, lint-free wipes.
2. Drop placement: place a small drop of blood (about 2–3 µL) near one end of the slide, about one-quarter of the way in from the short edge.
3. Spreader angle: hold the spreader slide at a 30 to 45 degree angle to the slide. Lower angle makes a longer smear (good for high-Hct samples); higher angle makes a shorter smear (good for anemic samples).
4. Pull back: draw the spreader slide back into the drop and allow blood to spread along its edge by capillary action.
5. Push forward: with a smooth, steady, quick motion, push the spreader slide forward to the end of the slide. Do not press down — let the blood spread itself.
6. Result: the smear should be bullet or tongue-shaped, reach near the slide edges laterally, and have a thin feathered edge at the end. The body of the smear should contain a monolayer where RBCs are evenly spaced without overlap — this is where cells are counted.
7. Air dry quickly and label the frosted end.

Common Smear Problems

• Too thick / too long: angle too low, drop too large, or spread too slow.
• Too short / uneven: angle too high, drop too small, or jerky motion.
• Holes in smear: dirty slide or grease on the slide.
• Cells dragged to feathered edge: pushing too hard or too slowly.

Staining with Diff-Quik

Diff-Quik is a rapid Romanowsky-type stain with three solutions:

1. Fixative (Solution 1, blue/clear methanol) — dip 5 times (~15 seconds) to fix cells to the slide.
2. Eosinophilic (Solution 2, orange/red) — dip 3 to 5 times (~10–15 seconds); stains cytoplasm and eosinophil granules.
3. Basophilic (Solution 3, blue/purple, buffered thiazine dye) — dip 3 to 5 times (~10–15 seconds); stains nuclei and basophilic cytoplasm.
4. Rinse gently with buffer or distilled water, air dry, and examine under oil immersion.

Staining time varies with stain freshness and species; over-staining makes everything blue, under-staining makes nuclei too pale. Adjust dips to achieve crisp nuclear detail and visible cytoplasmic granularity.

Manual WBC Differential: The Battlement Method

When automated analyzers flag results or when cell counters are unavailable, the technician performs a manual differential.

Counting Procedure

1. At low power (10x), scan the smear to locate the monolayer region — where RBCs are near each other but not overlapping.
2. Switch to 40x or 100x oil immersion for counting.
3. Use the battlement (crenellation) pattern: start at the edge of the smear, move inward one field, then back out one field, creating a zigzag or castle-wall track. This prevents the bias of counting only the smear's center, where larger cells (monocytes, granulocytes) tend to distribute.
4. Count 100 leukocytes, classifying each by type. The result is reported as a percentage of each type.
5. Calculate absolute counts: percent of each type × total WBC count / 100 = absolute count for that type (×10^9/L). Absolute counts are clinically more meaningful than percentages.
6. Note morphologic abnormalities: toxic change in neutrophils, reactive lymphocytes, atypical cells, parasites (e.g., Mycoplasma haemofelis on RBCs, Hepatozoon or Babesia in leukocytes).
7. Estimate platelet count: average platelets per 100x oil field × 15,000–20,000 = approximate count/µL. Confirm the automated platelet count is plausible.
8. Evaluate RBC morphology: anisocytosis (size variation), poikilocytosis (shape variation — acanthocytes, schistocytes, echinocytes), polychromasia (bluish reticulocytes), and RBC parasites.

Cell Identification Basics

Accurate leukocyte classification is a core VTNE skill. Each cell type has distinct nuclear and cytoplasmic features visible with Diff-Quik or Wright's stain.

Neutrophil (Segmented)

• Nucleus: multilobed with 3 to 5 lobes connected by thin chromatin bridges.
• Cytoplasm: pale pink, with fine granulation.
• Function: first-line phagocyte for bacterial infection; the most abundant WBC in healthy dogs.
• Increase (neutrophilia): inflammation, infection, stress (corticosteroids), physiologic epinephrine.
• Decrease (neutropenia): overwhelming infection, bone marrow suppression, immune-mediated destruction.

Band Neutrophil

• Nucleus: unsegmented, horseshoe or sausage-shaped, with smooth parallel chromatin edges (no filament between lobes).
• Significance: presence of bands indicates a left shift — the marrow is releasing immature cells because demand outstrips supply.

Lymphocyte

• Nucleus: round, densely chromatic, fills most of the cell in small lymphocytes.
• Cytoplasm: scant, rim of pale blue; large granular lymphocytes have azurophilic granules.
• Function: adaptive immunity (T cells, B cells/plasma cells).
• Increase (lymphocytosis): chronic infection, antigenic stimulation, epinephrine response in cats.
• Decrease (lymphopenia): stress (corticosteroids), immunosuppression.

Monocyte

• Nucleus: large, variable shape (kidney bean, lobed, or amorphous), lacy chromatin.
• Cytoplasm: abundant, gray-blue, often with fine granules and clear vacuoles.
• Function: tissue macrophage precursor; phagocytosis of debris and organisms.
• Increase: chronic inflammation, tissue necrosis, granulomatous disease.

Eosinophil

• Nucleus: lobed (often bilobed in dogs and cats).
• Cytoplasm: filled with orange-red granules. Granule morphology varies by species: dogs have round granules; cats have rod-shaped granules; horses have large donut-shaped granules; cattle have small, densely packed granules.
• Function: allergic and parasitic responses, mast cell degranulation.
• Increase (eosinophilia): parasites, hypersensitivity, eosinophilic syndromes.

Basophil

• Nucleus: lobed or irregular, often obscured by granules.
• Cytoplasm: dark purple/blue granules that can overlie the nucleus.
• Function: mediator of immediate hypersensitivity (histamine release).
• Significance: the rarest WBC; basophilia is uncommon but suggests parasitism or hypersensitivity.

Correlating CBC With Blood Smear

The technician's role is to verify analyzer results: confirm the WBC count by estimating leukocyte density at 40x (approximately 1 WBC per 40x field per 1,000 cells/µL), confirm the platelet count by smear estimate, and identify morphologic abnormalities the analyzer cannot detect. Discrepancies between automated values and smear findings require repeat analysis or manual methods.