Red Blood Cell Parameters
The CBC evaluates the cellular components of blood. Red blood cell (RBC) parameters include hematocrit (HCT), hemoglobin (Hgb), RBC count, and red cell indices (MCV, MCHC). These reflect oxygen-carrying capacity and help classify anemias.
Anemia is defined as HCT below reference range (typically 37–55% dogs, 30–45% cats). Classify by MCV (mean corpuscular volume) and MCHC (mean corpuscular hemoglobin concentration):
| Anemia Type | MCV | MCHC | Common Causes |
|---|---|---|---|
| Regenerative | High or normal | Normal | Hemorrhage, hemolysis |
| Iron deficiency | Low | Low | Chronic blood loss, GI parasitism |
| Non-regenerative | Normal | Normal | CKD, inflammation, bone marrow disease |
Reticulocyte count confirms regeneration. In cats, aggregate reticulocytes indicate recent response; punctate reticulocytes have a longer half-life.
White Blood Cell Parameters
WBC count and differential (neutrophils, lymphocytes, monocytes, eosinophils, basophils) reflect inflammatory and immune status.
Neutrophilia with left shift (band neutrophils) suggests acute bacterial infection. Neutrophilia without left shift may indicate stress, corticosteroids, or chronic inflammation. Neutropenia can indicate overwhelming infection, bone marrow suppression, or immune-mediated destruction.
Lymphocytosis may be reactive (chronic antigenic stimulation) or neoplastic (lymphoma, CLL). Lymphopenia is common with stress, corticosteroids, or viral immunosuppression. Eosinophilia suggests parasitism, allergy, or eosinophilic disease.
Platelet Count
Thrombocytopenia (low platelets) can cause bleeding when severe (below 50,000/µL). Causes include immune-mediated destruction, consumption (DIC), sequestration, or bone marrow failure. Always check a blood smear—clumping can falsely lower automated counts.
Case-Based Examples
Case 1: 8-year-old dog, HCT 22%, reticulocytes 120,000/µL. Regenerative anemia—consider hemolysis (IMHA, toxins) or hemorrhage (GI, trauma).
Case 2: 5-year-old cat, WBC 25,000/µL, 85% neutrophils, 10% bands. Left shift neutrophilia—acute bacterial infection likely. Consider abscess, pyometra, or septic focus.
Case 3: 12-year-old dog, HCT 18%, reticulocytes 15,000/µL. Non-regenerative anemia—investigate CKD, chronic disease, or bone marrow evaluation.
When to See a Veterinarian
Abnormal CBC results require clinical correlation. Anemia, leukopenia, or thrombocytopenia warrant prompt investigation. Always interpret in context of physical exam, history, and other diagnostics. Consider referral for bone marrow aspirate when cause is unclear.