How do you interpret the results of a Complete Blood Count (CBC)?

How to Interpret a Complete Blood Count (CBC)

Begin by systematically evaluating the three main cell lines—red blood cells, white blood cells, and platelets—using specific parameters and their clinical context to identify patterns that suggest underlying pathology. 1, 2

Red Blood Cell (RBC) Parameters

Hemoglobin and Hematocrit Assessment

• Evaluate hemoglobin (Hb) and hematocrit (Hct) first to assess oxygen-carrying capacity, with low values indicating anemia and high values suggesting polycythemia or dehydration 1, 2
• For cancer patients specifically, an Hb level of 11 g/dL or below should prompt evaluation for anemia, or a decrease of 2 g/dL or more from baseline regardless of absolute value 3
• Laboratory testing should include CBC alongside thyroid-stimulating hormone (TSH), B12, calcium, electrolytes, creatinine, alanine transaminase (ALT), lipid panel, and hemoglobin A1c (HbA1c) when investigating potential contributing factors 3

Mean Corpuscular Volume (MCV) Classification

• Use MCV to classify anemia type as the critical next step: microcytic (<80 fL), normocytic (80-100 fL), or macrocytic (>100 fL) 3, 1, 2
• Microcytic anemia most commonly results from iron deficiency, but also consider thalassemia, anemia of chronic disease, and sideroblastic anemia 3
• Normocytic anemia may be caused by hemorrhage, hemolysis, bone marrow failure, anemia of chronic inflammation, or renal insufficiency—the reticulocyte count becomes the key follow-up test 3
• Macrocytic anemia is most often megaloblastic, indicating vitamin B12 or folate deficiency from insufficient uptake or inadequate absorption 3

Reticulocyte Assessment

• A low reticulocyte index indicates decreased red blood cell production, while a high index suggests blood loss or hemolysis 2
• This parameter helps distinguish between production defects and destruction/loss mechanisms 2

White Blood Cell (WBC) Parameters

Total WBC Count and Differential

• Assess total white blood cell count for leukocytosis or leukopenia as the initial step 2
• Neutrophilia with left shift strongly indicates bacterial infection, with a likelihood ratio of 14.5 when band count exceeds 1,500 cells/mm³ 2
• Do not overlook the significance of left shift even in the absence of leukocytosis, as this is a common pitfall 2

Specific Cell Line Interpretation

• Lymphocytosis may indicate viral infection, while eosinophilia suggests allergic or parasitic conditions 2
• When reporting immunophenotyping results, correct data for lymphocyte purity of the gate (e.g., if lymphocyte purity is 94% and CD3 value is 70%, divide 0.70 by 0.94 and multiply by 100 to give 74%) 3

Platelet Parameters

Platelet Count Assessment

• Assess platelet count for thrombocytopenia or thrombocytosis, with low counts increasing bleeding risk and high counts potentially indicating inflammation or myeloproliferative disorders 1, 2
• Consider potential causes of thrombocytopenia including bone marrow suppression, immune destruction, or consumption 2

Critical Reporting and Documentation Practices

Proper Data Reporting

• Report both percentages and absolute counts when available, particularly for lymphocyte subsets 3, 2
• Calculate absolute values by multiplying the lymphocyte subset percentage by the absolute number of lymphocytes from the WBC differential 3
• Include both percentage and absolute number when reporting CD4+ T-cell counts 2

Common Pitfalls to Avoid

• Do not fail to correct for lymphocyte purity when reporting immunophenotyping results, as this leads to inaccurate values 2
• Avoid ordering additional tests when results will not change management decisions 2
• Be aware that CBC results are easily influenced by specimen collection procedures, transportation, and environmental factors 4
• Ensure blood is drawn at the same time for both CBC and immunophenotyping when absolute counts are needed 3

Contextual Considerations

Clinical Integration

• Always perform a detailed history and physical examination focusing on duration and onset of symptoms, comorbidities, family history, and exposure to antineoplastic drugs or radiation 3
• Look for specific signs: syncope, exercise dyspnea, headache, vertigo, chest pain, fatigue, pallor, jaundice, splenic enlargement, neurologic symptoms, blood in stool, petechiae, or heart murmur 3
• Visual review of the peripheral blood smear is critical to confirm size, shape, and color of RBCs beyond automated parameters 3