What is a Complete Blood Cell Count (CBC)?

What is a Complete Blood Cell Count (CBC)?

A Complete Blood Cell Count (CBC) is a comprehensive laboratory test that measures the quantity and characteristics of all three major blood cell lines—white blood cells (WBC), red blood cells (RBC) with hemoglobin and hematocrit, and platelets—to assess bone marrow function, oxygen-carrying capacity, immune status, and clotting ability. 1

Core Components of the CBC

The CBC consists of several essential measurements that evaluate different aspects of blood health:

Red Blood Cell Parameters

• Hemoglobin and hematocrit: Hemoglobin is the preferred measure over hematocrit because it is more reproducible across laboratories with lower variability and is not affected by sample storage time or patient-specific variables like serum glucose 1
• Red blood cell count: Provides the total number of oxygen-carrying cells 2
• Mean corpuscular volume (MCV): Classifies red blood cells by size; low MCV suggests iron, folic acid, or vitamin B12 deficiencies, or inherited hemoglobin synthesis disorders 1
• Red cell distribution width (RDW): Measures variation in red blood cell size 3
• Reticulocyte count: Evaluates bone marrow response to anemia; a low count may indicate absent or unavailable iron, defects in red cell production, or insufficient erythropoietin production 1

White Blood Cell Parameters

• Total white blood cell count: Assesses for leukocytosis (elevated) or leukopenia (decreased) 4
• Differential count: Includes both percentage and absolute numbers of lymphocytes, neutrophils, eosinophils, basophils, monocytes, and prolymphocytes 1
• Band neutrophil count: An elevated band count (≥1,500 cells/mm³) has the highest diagnostic value with a likelihood ratio of 14.5 for bacterial infection 5
• Neutrophil-to-lymphocyte ratio (NLR): Useful marker for predicting cardiovascular disease and metabolic conditions 3

Platelet Parameters

• Platelet count: Evaluates clotting capacity and screens for thrombocytopenia or thrombocytosis 1, 4
• Mean platelet volume (MPV): Provides information about platelet size and function 3
• Platelet-to-lymphocyte ratio (PLR): Additional marker for cardiovascular risk assessment 3

Clinical Purpose and Applications

Assessment of Bone Marrow Function

Abnormalities in two or more cell lines (WBC, RBC, or platelets) warrant careful evaluation and likely require hematology consultation, as this suggests a primary bone marrow disorder. 1

Disease Detection and Monitoring

• Anemia evaluation: The CBC identifies anemia type (normochromic/normocytic in chronic kidney disease) and guides further workup 1
• Infection diagnosis: Leukocytosis with left shift (≥16% band neutrophils) has a likelihood ratio of 4.7 for bacterial infection, even without fever 5
• Baseline monitoring: Required before initiating certain medications such as biologics for psoriasis treatment 6
• Cancer screening: Used in chronic lymphocytic leukemia to evaluate absolute lymphocyte and prolymphocyte counts 1, 5

Reporting Standards and Quality Assurance

Required Reporting Elements

• Both percentages and absolute counts must be reported when available 6, 4
• Reference ranges corresponding to laboratory-specific, age-specific, and sex-specific normal values must be included 6
• Absolute lymphocyte subset values are calculated by multiplying the lymphocyte subset percentage by the absolute lymphocyte count from the WBC differential 6

Quality Control Requirements

• Automated differentials must achieve at least 90% lymphocyte purity within the lymphocyte gate (minimum 85%) 6
• The sum of CD3+CD4+ and CD3+CD8+ cells should equal total CD3+ cells within ±5% (maximum variability ≤10%) 6

Critical Pitfalls to Avoid

Technical Considerations

• Specimen handling: CBC values are easily influenced by collection techniques, transportation conditions, and storage, requiring careful attention to pre-analytical variables 7
• Analytical variability: Automated WBC counts have 2.2%-7.7% variability, while manual hemocytometer counts have 9.3%-17.6% variability 5
• Biological variability: Normal fluctuation is approximately 10% within a day and 13% within a week 5

Interpretation Errors

• Do not overlook left shift even when total WBC count is normal, as elevated neutrophil percentage (≥90%) alone has a likelihood ratio of 7.5 for bacterial infection 5
• Do not fail to correct for lymphocyte purity when interpreting immunophenotyping results 4
• Do not use hematocrit alone as it is less reliable than hemoglobin for anemia assessment 1

Clinical Decision-Making

• Manual differential counting is preferred over automated methods when evaluating band forms and other immature neutrophils for infection diagnosis 5
• In chronic kidney disease patients, anemia is typically normochromic and normocytic, making it indistinguishable from anemia of other chronic conditions without additional testing 1
• Timing matters: CBC with differential should be performed within 12-24 hours of symptom onset when infection is suspected, or earlier if the patient is seriously ill 5