How to interpret a complete hemogram (complete blood count, CBC)?

How to Interpret a Complete Hemogram (CBC)

A complete blood count should be systematically interpreted by evaluating three main cell lines—red blood cells, white blood cells, and platelets—using both quantitative values and morphologic characteristics to identify patterns that suggest specific pathologic conditions. 1

Initial Assessment Framework

Begin by obtaining a CBC with differential that includes hemoglobin, hematocrit, platelet count, white blood cell count with differential (both percentages and absolute numbers), and red blood cell indices (MCV, MCH, MCHC, RDW). 2 A visual review of the peripheral blood smear is critical to confirm the size, shape, and color of RBCs and identify any morphologic abnormalities. 2

Abnormalities in two or more cell lines warrant careful evaluation and likely require hematology consultation, as this suggests bone marrow dysfunction rather than isolated peripheral processes. 2

Red Blood Cell Interpretation

Hemoglobin and Hematocrit Assessment

• Hemoglobin is preferred over hematocrit because it is more reproducible across laboratories with lower coefficients of variation and is not affected by storage time or patient variables like serum glucose. 2
• Evaluate hemoglobin and hematocrit together to assess oxygen-carrying capacity, with abnormalities indicating anemia, polycythemia, or dehydration. 1

Morphologic Classification by MCV

Use Mean Corpuscular Volume (MCV) to classify anemia into three categories: 2, 1

• Microcytic (<80 fL): Most commonly iron deficiency; also consider thalassemia, anemia of chronic disease, or sideroblastic anemia 2
• Normocytic (80-100 fL): May indicate hemorrhage, hemolysis, bone marrow failure, anemia of chronic inflammation, or renal insufficiency—the reticulocyte count becomes the key follow-up test 2
• Macrocytic (>100 fL): Most commonly megaloblastic from vitamin B12 or folate deficiency; non-megaloblastic causes include alcoholism, myelodysplastic syndrome, or drugs like hydroxyurea 2

Kinetic Assessment with Reticulocyte Index

The reticulocyte count corrected for the degree of anemia (reticulocyte index) distinguishes production versus destruction/loss mechanisms: 2

• Low reticulocyte index (<1.0): Indicates decreased RBC production, suggesting iron deficiency, vitamin B12/folate deficiency, aplastic anemia, bone marrow suppression from chemotherapy/radiation, or insufficient erythropoietin in CKD 2
• High reticulocyte index (>2.0): Indicates appropriate bone marrow response to anemia, suggesting ongoing blood loss or hemolysis 2, 1

A common pitfall is failing to calculate the reticulocyte index rather than using the raw reticulocyte percentage, which can be misleadingly elevated in severe anemia. 2

White Blood Cell Interpretation

Total WBC Count and Differential

• Assess total white blood cell count for leukocytosis (>11,000/mm³) or leukopenia (<4,000/mm³). 2, 1
• Always report both percentages AND absolute counts for each cell type, as a normal percentage can mask an abnormal absolute count. 1

Neutrophil Assessment

• Neutrophilia with left shift (increased band forms >1,500/mm³) strongly indicates bacterial infection with a likelihood ratio of 14.5. 1
• A critical pitfall is overlooking the significance of left shift even when total WBC is normal—the left shift itself is diagnostically important. 1

Other WBC Components

• Lymphocytosis: Typically indicates viral infection, chronic lymphocytic leukemia, or other lymphoproliferative disorders 1
• Eosinophilia: Suggests allergic conditions, parasitic infections, or drug reactions 1
• Monocytosis: May indicate chronic infection, inflammatory conditions, or certain malignancies 2

Platelet Interpretation

• Assess platelet count for thrombocytopenia (<150,000/mm³) or thrombocytosis (>450,000/mm³). 1
• When thrombocytopenia is present, consider three main mechanisms: decreased production (bone marrow suppression), increased destruction (immune-mediated like ITP), or increased consumption (DIC, TTP). 1
• In CKD patients, thrombocytopenia unrelated to therapy (<100,000/mm³) may indicate progression to accelerated phase disease. 2

Critical Reporting Considerations

When reporting immunophenotyping or lymphocyte subset results, always include both the percentage and absolute number (e.g., CD4+ T-cells), and correct for lymphocyte purity to avoid misinterpretation. 1 This is particularly important in HIV management where CD4 counts guide treatment decisions. 2

Common Clinical Pitfalls

• Do not order additional confirmatory tests when results will not change clinical management—this increases costs without improving outcomes. 1
• In anemia evaluation, the finding of iron deficiency in non-dialysis CKD patients without obvious blood loss should prompt evaluation for gastrointestinal bleeding. 2
• Anemia in CKD is typically normochromic and normocytic, making it indistinguishable from anemia of chronic disease without additional iron studies (ferritin, transferrin saturation). 2
• When evaluating cancer patients, a decrease of 2 g/dL or more in hemoglobin from baseline warrants investigation even if the absolute value remains above the lower limit of normal. 2