Understanding Hematology Blood Tests: CBC Parameters Explained
A Complete Blood Count (CBC) with Differential is the cornerstone of hematologic evaluation. It provides quantitative and qualitative data on all blood cell lines.
Red Blood Cell (RBC) Indices
| Parameter | What It Measures | Low Suggests | High Suggests |
|---|
| RBC Count | Number of RBCs/µL | Anemia | Polycythemia (primary/secondary) |
| Hemoglobin (Hgb) | Oxygen-carrying protein | Anemia | Polycythemia, dehydration |
| Hematocrit (Hct) | % blood volume = RBCs | Anemia | Polycythemia, dehydration |
| MCV | Average RBC size (fL) | Microcytic (Fe deficiency, thalassemia, anemia chronic disease) | Macrocytic (B12/folate deficiency, alcohol, liver disease, MDS, drugs) |
| MCH | Average Hgb per RBC (pg) | Hypochromic (Fe deficiency, thalassemia) | Hyperchromic (rare; macrocytosis, cold agglutinins) |
| MCHC | Hgb concentration in RBCs (g/dL) | Hypochromic (Fe deficiency, thalassemia) | Hereditary spherocytosis, cold agglutinins (artifact) |
| RDW | Variation in RBC size (%) | — | Anisocytosis: Fe deficiency (early), mixed anemias, recent transfusion |
| Reticulocyte Count | Young RBCs (%) → marrow output | Hypoproliferative (marrow failure, deficiency) | Hemolysis, blood loss, response to therapy |
White Blood Cell (WBC) Indices
| Parameter | Significance |
|---|
| Total WBC | Leukopenia (<4K): marrow failure, sequestration, drugs, autoimmune. Leukocytosis (>11K): infection, inflammation, stress, steroids, malignancy. |
| Differential (Absolute Counts!) | Neutrophils (bacterial, stress), Lymphocytes (viral, CLL), Monocytes (chronic inflam, TB, recovery), Eosinophils (allergy, parasites, drugs), Basophils (allergy, CML). |
| Neutrophil-to-Lymphocyte Ratio (NLR) | Systemic inflammation marker; prognostic in cancer, CVD, sepsis. |
Platelet Indices
| Parameter | Significance |
|---|
| Platelet Count | Thrombocytopenia (<150K): consumption (ITP, TTP, DIC, sepsis), sequestration (splenomegaly), decreased production. Thrombocytosis (>450K): reactive (iron def, inflam), clonal (ET, CML). |
| MPV (Mean Platelet Volume) | High: young platelets (increased production/destruction). Low: older platelets (marrow failure). |
Interpretation Pearls
- Always use absolute counts (not percentages) for differential.
- Trends > single values.
- Clinical context is everything. Isolated abnormality often benign.
- Peripheral smear review by pathologist adds morphology (schistocytes, blasts, toxic granulation, etc.).
CBC is a screening tool; abnormal results guide targeted workup.
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