How do you interpret a Complete Blood Count (CBC) result in a patient?

How to Interpret a Complete Blood Count (CBC)

Systematically evaluate CBC components in the following order: red blood cell parameters (hemoglobin, hematocrit, MCV), white blood cell parameters (total count, differential with absolute values), and platelet count, correlating each abnormality with clinical context to guide diagnosis and management. 1, 2

Red Blood Cell Parameters

Hemoglobin and Hematocrit Assessment

• Evaluate hemoglobin and hematocrit first to assess oxygen-carrying capacity, with low values indicating anemia and elevated values suggesting polycythemia or dehydration 1, 2
• Normal hemoglobin varies by age and sex; abnormalities require correlation with clinical presentation 2

Mean Corpuscular Volume (MCV) Classification

• Use MCV to classify anemia type immediately after identifying low hemoglobin: microcytic (MCV <80 fL), normocytic (80-100 fL), or macrocytic (MCV >100 fL) 1, 2
• Microcytic anemia commonly indicates iron deficiency, thalassemia, or anemia of chronic disease 2
• Macrocytic anemia suggests vitamin B12/folate deficiency, alcohol use, or bone marrow disorders 2

Reticulocyte Count Interpretation

• A low reticulocyte index indicates decreased red blood cell production (bone marrow failure, nutritional deficiency), while an elevated index suggests blood loss or hemolysis 1
• This distinction is critical for determining whether anemia is due to production failure versus destruction/loss 1

White Blood Cell Parameters

Total WBC and Differential

• Always assess both total white blood cell count AND absolute values of each cell type—percentages alone are insufficient 1
• Leukocytosis (elevated WBC) or leukopenia (decreased WBC) requires evaluation of the differential to identify which cell line is affected 1

Neutrophil Assessment

• Neutrophilia with left shift (band count >1,500 cells/mm³) strongly indicates bacterial infection with a likelihood ratio of 14.5 1
• Do not overlook left shift even when total WBC is normal—this finding alone suggests significant bacterial infection 1
• Severe neutropenia (<500 cells/mm³) indicates high infection risk requiring urgent evaluation 3

Lymphocyte Evaluation

• Lymphocytosis typically indicates viral infection, while severe lymphopenia may suggest immunodeficiency 1
• In infants, severe lymphopenia (<2,500 cells/mm³) should prompt evaluation for severe combined immunodeficiency (SCID), especially if accompanied by absent thymus on imaging 3
• Always report both percentage and absolute lymphocyte count for accurate interpretation 1

Eosinophils and Other Cell Types

• Eosinophilia suggests allergic conditions, parasitic infections, or drug reactions 1
• Monocytosis may indicate chronic infection or inflammatory conditions 1

Platelet Parameters

Platelet Count Assessment

• Evaluate platelet count for thrombocytopenia (<150,000/μL) or thrombocytosis (>450,000/μL) 1, 2
• Thrombocytopenia increases bleeding risk, particularly when <50,000/μL for procedures or <10,000/μL for spontaneous bleeding 1
• Thrombocytosis may indicate inflammation, iron deficiency, or myeloproliferative disorders 2

Causes of Thrombocytopenia

• Consider three main mechanisms: decreased production (bone marrow suppression), increased destruction (immune thrombocytopenia, DIC), or sequestration (hypersplenism) 1
• Platelet count >400,000/μL in giant cell arteritis has a positive likelihood ratio of 3.75 for diagnosis 3

Critical Integration Points

Baseline Requirements for Clinical Trials

• For research protocols, obtain CBC within 48 hours of enrollment, including WBC with differential (percent and absolute lymphocytes/prolymphocytes), hemoglobin, hematocrit, platelet count, and reticulocyte count 3
• Repeat at appropriate intervals to assess treatment response 3

Anemia Workup in Specific Conditions

• In chronic lymphocytic leukemia evaluation, hemoglobin should be measured alongside immunoglobulin levels and direct/indirect Coombs testing 3
• In multiple myeloma workup, obtain CBC with differential and platelets alongside BUN, creatinine, calcium, and albumin 3

Common Pitfalls to Avoid

• Never rely on percentages alone—always calculate and report absolute counts for all cell types 1
• Do not overlook left shift in the differential even when total WBC is normal, as this indicates significant bacterial infection 1
• Avoid ordering additional tests when results will not change clinical management decisions 1
• Do not fail to correct for lymphocyte purity when interpreting immunophenotyping results 1
• Recognize that CBC values have circadian variation, with lymphocytes peaking overnight (23:54) and neutrophils peaking in late afternoon 4