Complete Blood Count (CBC) Components and Clinical Significance
A CBC assesses bone marrow function through three primary cell lines—white blood cells, hemoglobin, and platelets—and abnormalities in two or more cell lines warrant hematology consultation. 1
Red Blood Cell Parameters
Hemoglobin and Hematocrit
- Hemoglobin is the preferred measure over hematocrit because it demonstrates superior reproducibility across laboratories with lower variability, and unlike hematocrit, is not affected by sample storage time or patient-specific variables like serum glucose 1
- These parameters assess oxygen-carrying capacity, with abnormalities indicating anemia, polycythemia, or dehydration 2
Mean Corpuscular Volume (MCV)
- Low MCV indicates microcytic anemia and typically suggests iron deficiency, folic acid deficiency, vitamin B12 deficiency, or inherited hemoglobin synthesis disorders 1
- MCV serves as a late marker of iron deficiency and helps classify anemia as microcytic, normocytic, or macrocytic 1, 2
Red Cell Distribution Width (RDW)
- RDW measures variation in red blood cell size and serves as a useful marker to predict cardiovascular disease and metabolic conditions 3
Reticulocyte Count
- A low reticulocyte count indicates inadequate bone marrow response and may represent absent or unavailable iron for erythropoiesis, defects in red cell production, or in CKD patients with adequate iron/folate/B12, likely reflects insufficient erythropoietin production or inflammation 1
- A high reticulocyte index suggests active blood loss or hemolysis 2
White Blood Cell Parameters
Total White Blood Cell Count
- Assess for leukocytosis (elevated) or leukopenia (decreased) as indicators of infection, inflammation, bone marrow disorders, or immunosuppression 2
- Severe neutropenia (<500 cells/mm³) indicates high infection risk requiring urgent evaluation 2
Differential Count with Left Shift
- Neutrophilia with left shift (band count >1,500 cells/mm³) strongly indicates bacterial infection with a likelihood ratio of 14.5 2
- Do not overlook the significance of left shift even when total WBC is normal 2
Lymphocyte Count
- Lymphocytosis may indicate viral infection, while lymphopenia can suggest immunodeficiency 2
- In infants, severe lymphopenia (<2,500 cells/mm³) should prompt immediate evaluation for severe combined immunodeficiency (SCID), especially with absent thymus on imaging 2
Eosinophils and Basophils
- Eosinophilia suggests allergic reactions or parasitic infections 2
- Increased basophils may indicate chronic myeloid leukemia 1
Neutrophil-to-Lymphocyte Ratio (NLR)
- NLR serves as a useful marker to predict cardiovascular disease and metabolic conditions 3
Platelet Parameters
Platelet Count
- Assess for thrombocytopenia (low) or thrombocytosis (elevated) 2
- Platelet count >400,000/μL has a positive likelihood ratio of 3.75 for giant cell arteritis 2
- Thrombocytopenia causes include bone marrow suppression, immune destruction, or consumption 2
Mean Platelet Volume (MPV) and Platelet-to-Lymphocyte Ratio (PLR)
- These parameters are useful markers to predict cardiovascular disease and metabolic conditions 3
Critical Clinical Integration Points
Multi-Lineage Abnormalities
- When abnormalities occur in two or more cell lines (WBC, hemoglobin, platelets), carefully evaluate and discuss with a hematologist 1
Reporting Standards
- Always report both percentages AND absolute counts for accurate interpretation, particularly for lymphocyte subsets and immunophenotyping 2
- Correct for lymphocyte purity when reporting immunophenotyping results 2
Common Pitfalls to Avoid
- Do not order additional tests when results will not change management decisions 2
- Recognize that CBC is easily influenced by collection techniques, specimen handling, and transportation—ensure proper specimen management to avoid erroneous values leading to inappropriate treatment 4
- The CBC can unintentionally detect leukemias in asymptomatic individuals, as it quantifies all three cell lines originating from bone marrow where most leukemias develop 5