Total Leucocyte Count (TLC) in Veterinary Science

1. Definition

2. Sample Collection

• Blood is collected in EDTA (preferred anticoagulant - preserves cell morphology without causing significant swelling/clumping).
• Heparin is unsuitable for TLC because it can cause leukocyte clumping and gives a blue background stain on smears.
• Counts should ideally be performed within 1-2 hours of collection since degenerative cell loss and morphological changes begin quickly, especially in warm climates.

3. Methods of Estimation

A. Manual (Hemocytometer) Method

1. Diluting fluid: Blood is diluted (commonly 1:20) in a WBC diluting fluid (e.g., 2-3% acetic acid or dilute hydrochloric acid with a trace of gentian/methylene violet). The acid lyses red blood cells while the leucocyte nuclei are stained, making them visible against a clear background.
2. Natt and Herrick's solution is the veterinary-specific alternative, especially critical in species with nucleated red blood cells (birds, reptiles, amphibians, fish). Because RBC lysis does not occur with this stain, a differential correction is needed - true WBCs are distinguished from nucleated RBCs based on nuclear/cytoplasmic staining characters, and the leucocyte percentage from a stained smear is used to correct the total nucleated cell count down to a true TLC.
3. Procedure: The diluted sample is loaded into a Neubauer counting chamber (hemocytometer); leucocytes are counted in the 4 large corner squares (each 1 mm², 0.1 mm deep) under low power.
4. Formula: TLC (cells/µL) = Number of cells counted x Dilution factor / Volume counted (mm³) For the standard Neubauer chamber with 1:20 dilution counting 4 corner squares: TLC = N x 50 (where N = total cells counted in 4 squares), giving cells/µL directly with the standard set-up.

B. Automated/Electronic Methods

• Impedance (Coulter principle): cells suspended in a conductive diluent pass through an aperture; each cell interrupts current flow, generating a pulse proportional to cell volume. Particles larger than ~36 fL are counted as leukocytes (- Quick Compendium of Clinical Pathology, 5th ed.).
• Laser flow cytometry / light-scatter analyzers: can generate a 5-part differential simultaneously using forward/side scatter and fluorescence.
• Automated counts have much lower coefficients of variation (1-3%) compared to manual hemocytometer counts (6.5% in normal ranges, up to 15% in leukopenic samples), but many analyzers are only validated for specific species and can misclassify nucleated RBCs as WBCs in avian/reptilian/piscine blood - this is why manual/Natt-Herrick methods are still taught and used in these species.

4. Normal TLC Reference Ranges (approximate, x10³/µL)

5. Physiological Variations

• Age: Neonates and young animals typically show higher lymphocyte proportions; TLC can be higher in young growing animals.
• Excitement/fear (physiologic leukocytosis): Epinephrine-mediated demargination of neutrophils and lymphocytosis, seen prominently in cats and horses during stressful handling or blood draw.
• Stress leukogram (cortisol-mediated): Classic in dogs, cattle, and other species under illness, corticosteroid therapy, or chronic stress - mature neutrophilia, lymphopenia, eosinopenia, and sometimes monocytosis, WITHOUT a left shift.
• Pregnancy/parturition: Mild physiologic leukocytosis is common near parturition in most domestic species.
• Exercise, breed, and diurnal variation can also cause minor fluctuations.
• Species differences in baseline differential: Dogs, cats, and horses are normally neutrophil-predominant; cattle, sheep, and goats are normally lymphocyte-predominant - this matters when interpreting whether a given TLC/DLC combination is "abnormal."

6. Pathological Variations

• Inflammatory/infectious leukocytosis - bacterial infections, abscesses, pyometra, mastitis (often with a left shift - increased band neutrophils)
• Leukemic leukocytosis - lymphoid or myeloid leukemias, producing very high counts with immature/abnormal cell forms
• Physiologic (see above)

• Viral infections causing bone marrow suppression or consumption: canine parvovirus, feline panleukopenia virus, Bovine Viral Diarrhea (BVD), Feline Leukemia Virus (FeLV)
• Overwhelming bacterial sepsis/endotoxemia (peracute, consumption exceeds production)
• Bone marrow disease, myelophthisis, certain drug toxicities (e.g., estrogen toxicity, chemotherapeutic agents)
• Anaphylaxis in some species

7. Sources of Error (Manual Method)

• Improper mixing or dilution errors
• Clots or platelet clumps miscounted as leukocytes
• Fragmented leukocyte cytoplasm in leukemic blood falsely elevating counts (phase-contrast correction needed)
• Uneven chamber filling, bubbles, or improper coverslip placement
• In nucleated-RBC species: failure to apply the differential correction with Natt-Herrick's method leads to marked overestimation of TLC.

8. Clinical/Diagnostic Significance

• Detect and monitor infection or inflammation
• Support diagnosis of leukemias and myeloproliferative disease
• Assess response to therapy
• Differentiate stress/physiologic responses from true pathology
• Guide prognosis in critically ill or febrile animals