Understanding Complete Blood Count (CBC) Results
A Complete Blood Count (CBC) is a comprehensive blood test that evaluates three major cell lines—red blood cells, white blood cells, and platelets—to assess bone marrow function and detect a wide range of hematologic, infectious, inflammatory, and systemic diseases. 1, 2
Core Components and Their Clinical Significance
Red Blood Cell (RBC) Parameters
Hemoglobin and hematocrit are the primary measures for assessing oxygen-carrying capacity, with abnormalities indicating anemia, polycythemia, or dehydration. 2 Hemoglobin is preferred over hematocrit because it demonstrates superior reproducibility across laboratories with lower variability, and unlike hematocrit, it is not affected by sample storage time or patient-specific variables such as serum glucose. 1
Mean Corpuscular Volume (MCV) classifies anemia into three categories: 1, 2
- Microcytic (low MCV): Suggests iron deficiency, thalassemia, or inherited hemoglobin synthesis disorders
- Normocytic (normal MCV): Common in chronic kidney disease, chronic inflammatory conditions, or acute blood loss
- Macrocytic (high MCV): Indicates folate or vitamin B12 deficiency
Reticulocyte count evaluates bone marrow response to anemia. 1 A low reticulocyte index indicates decreased red blood cell production (bone marrow failure, nutritional deficiency, or insufficient erythropoietin), while an elevated index suggests appropriate marrow response to blood loss or hemolysis. 2
White Blood Cell (WBC) Parameters
Total WBC count identifies leukocytosis (elevated) or leukopenia (decreased), both of which require further investigation. 2 The differential count breaks down WBC subtypes:
Neutrophilia with left shift (increased immature neutrophils/bands) strongly indicates bacterial infection, with a band count >1,500 cells/mm³ having a likelihood ratio of 14.5 for bacterial infection. 2 This finding is clinically significant even without overall leukocytosis. 2
Lymphocytosis typically suggests viral infection, while eosinophilia points toward allergic reactions or parasitic infections. 2
Abnormalities in two or more cell lines warrant careful evaluation and likely require hematology consultation, as this suggests primary bone marrow pathology rather than reactive changes. 1
Platelet Parameters
Platelet count identifies thrombocytopenia (low) or thrombocytosis (elevated). 2 When thrombocytopenia is present, consider three major mechanisms: 2
- Decreased production (bone marrow suppression from medications, infections, or infiltrative processes)
- Increased destruction (immune-mediated destruction, such as ITP)
- Increased consumption (DIC, TTP, or massive bleeding)
Critical Reporting Standards
Both percentages and absolute counts must be reported when available, particularly for lymphocyte subsets and immunophenotyping results. 1, 2 Absolute values are calculated by multiplying the subset percentage by the absolute lymphocyte count from the differential. 1
Data must be corrected for lymphocyte purity when reporting flow cytometry results. For example, if lymphocyte purity is 94% and CD3 value is 70%, the corrected T-lymphocyte value is (0.70 ÷ 0.94) × 100 = 74%. 1, 2
Clinical Applications Beyond Basic Hematology
The CBC has expanded utility in predicting cardiovascular disease, metabolic syndrome, and type 2 diabetes through derived ratios: 3
- Neutrophil-to-lymphocyte ratio (NLR)
- Platelet-to-lymphocyte ratio (PLR)
- Red cell distribution width (RDW)
- Mean platelet volume (MPV)
The CBC can unintentionally detect leukemias in asymptomatic individuals because it quantifies all three cell lines originating from bone marrow, where most leukemias arise. 4 Elevated WBC counts with abnormal differential, unexplained cytopenias, or presence of immature cells (blasts) on peripheral smear mandate urgent hematology referral. 1
Common Pitfalls to Avoid
Never fail to correct for lymphocyte purity when interpreting immunophenotyping data, as this leads to inaccurate reporting of lymphocyte subset percentages. 2
Do not overlook left shift (increased band forms) even when total WBC count is normal, as this finding alone strongly suggests bacterial infection requiring antimicrobial therapy. 2
Avoid ordering additional tests when results will not change clinical management, as this increases costs without improving patient outcomes. 2
Do not assume anemia in CKD is solely due to erythropoietin deficiency—patients may have concurrent iron deficiency, folate/B12 deficiency, or gastrointestinal bleeding that requires separate evaluation and treatment. 1
When to Consult Hematology
Immediate hematology consultation is warranted for: 1, 5
- Abnormalities in two or more cell lines
- Presence of blasts or other immature cells on peripheral smear
- Severe unexplained cytopenias (hemoglobin <7 g/dL, platelets <20,000/μL, ANC <500/μL)
- Suspected acute leukemia or bone marrow failure syndromes