Hematology: Complete Blood Count & Blood Film Evaluation

Hematology: Complete Blood Count & Blood Film Evaluation

The complete blood count (CBC) is one of the most frequently ordered diagnostic tests in veterinary medicine. It gives the veterinarian a quantitative snapshot of the three major cell lines in peripheral blood: erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets). As a veterinary technician or assistant, you are responsible for running the analyzer, preparing quality blood smears, performing the manual differential, and recognizing values that fall outside the normal range.

The EDTA Sample

Blood for a CBC is collected into a tube containing the anticoagulant ethylenediaminetetraacetic acid (EDTA). EDTA chelates calcium and prevents clotting without distorting cell morphology the way other anticoagulants can. The tube must be filled to the correct volume mark and gently inverted several times to mix the anticoagulant thoroughly. Under-filling causes a high anticoagulant-to-blood ratio that can shrink red cells and produce falsely low hematocrit values. Samples should be processed promptly; if a delay is unavoidable, refrigerate but return the sample to room temperature and re-mix before analysis.

Safety note: Treat all blood samples as potentially infectious. Wear gloves, a lab coat, and eye protection throughout this exercise. Dispose of sharps and blood-contaminated materials in the appropriate biohazard containers.

Running the Hematology Analyzer

In-house hematology analyzers use impedance (cell counting by electrical resistance change), laser light scatter, or a combination of both to count and size cells. Before running patient samples, you must run quality control (QC) material — a prepared sample with known target values and acceptable ranges supplied by the manufacturer. If a QC result falls outside its acceptable range, the analyzer must not be used for patient samples until the problem is identified and corrected. Document all QC results in the laboratory log; this is a regulatory and accreditation requirement.

When patient results are printed, compare each value to the species-specific reference interval chart posted in your lab. Values outside the reference interval should be circled and flagged for the veterinarian. Key CBC parameters include:

• RBC count, hemoglobin (Hgb), hematocrit (HCT/PCV): Decreased values indicate anemia; increased values indicate polycythemia.
• MCV (mean corpuscular volume): Classifies red cells as normocytic, microcytic, or macrocytic.
• MCHC (mean corpuscular hemoglobin concentration): Decreased in iron-deficiency; increased MCHC can indicate hemolysis or spherocytosis.
• WBC total and differential: Increased total WBC (leukocytosis) or specific cell-line elevations guide the clinician toward infection, inflammation, stress, or neoplasia.
• Platelet count: Decreased count (thrombocytopenia) raises concern for bleeding disorders.

Preparing the Wedge Blood Smear

Analyzer differentials are estimates; a manual blood smear allows direct visual evaluation of cell morphology. To prepare a wedge smear:

1. Place a small drop of well-mixed EDTA blood near one end of a clean glass slide.

2. Hold a second slide (the spreader) at roughly a 30–45° angle, back it into the drop until blood spreads along the spreader edge by capillary action, then push forward in a single smooth motion.

3. Air-dry immediately — do not blow on the slide or heat-fix.

A well-made smear has three zones: a thick body, a gradual transition, and a feathered edge (thin trailing end). The monolayer zone — just behind the feathered edge — is where cells are spread enough to evaluate individually without overlapping.

Diff-Quik staining (standard timed-dip protocol):

1. Fix — dip the air-dried slide in the fixative solution for the manufacturer-specified number of seconds.

2. Stain solution 1 (eosinophilic/orange) — dip for the specified count.

3. Stain solution 2 (basophilic/blue) — dip for the specified count.

4. Rinse gently with tap or distilled water; allow to air-dry or blot carefully.

5. Apply a coverslip with mounting medium or use immersion oil directly on the stained smear.

Performing the 100-Cell Manual Differential

Using the 10× objective, locate the monolayer zone. Switch to 40× to scan, then to 100× oil immersion for definitive cell identification. Count 100 consecutive white blood cells using a battlement (back-and-forth) pattern to avoid sampling bias. Record the number of each cell type; those numbers become the percentage.

In dogs and cats the WBC differential includes:

• Segmented (mature) neutrophils — most numerous in health; multilobed nucleus.
• Band neutrophils — immature; U- or C-shaped unsegmented nucleus; increased in severe infection (left shift).
• Lymphocytes — small, round dark nucleus, scant cytoplasm.
• Monocytes — largest leukocyte; kidney-shaped nucleus, blue-gray cytoplasm.
• Eosinophils — pink-to-red granules; associated with parasitism and allergy.
• Basophils — rare; dark purple granules.

Recognizing Common Abnormalities

Anemia (decreased RBC, Hgb, HCT): Look for polychromasia (larger, blue-tinged RBCs called polychromatophils) and reticulocytes — signs of regeneration, indicating the bone marrow is responding. Regenerative anemia is common with blood loss or hemolysis. Non-regenerative anemia lacks these signs.

Leukocytosis (elevated WBC): A neutrophilia with a left shift (increased bands) points to infection or severe inflammation. A mature neutrophilia without a left shift may reflect physiologic or stress leukocytosis.

Thrombocytopenia (low platelet count): Scan the feathered edge for platelet clumps, which cause falsely low analyzer counts. True thrombocytopenia requires clinical correlation and may indicate immune-mediated disease, tick-borne illness, or bone marrow suppression.

Scope of Practice Reminder

Veterinary technicians and assistants perform and record CBC results and flag abnormalities, but interpretation and diagnosis are the veterinarian's responsibility. Always communicate critical or unexpected results to the supervising veterinarian promptly.