What is a Complete Blood Count (CBC)?

Complete Blood Count (CBC): Definition and Description

A complete blood count (CBC) is a laboratory test that measures the three major cellular components of blood—white blood cells, red blood cells (including hemoglobin), and platelets—to assess bone marrow function and detect a wide range of hematologic and systemic disorders. 1

Core Components Measured

The CBC evaluates multiple parameters across three cell lines:

Red Blood Cell Parameters

• Hemoglobin: The preferred measure over hematocrit due to superior reproducibility across laboratories, with lower within-assessment and between-assessment coefficients of variation, and no interference from storage time or patient variables like serum glucose 1
• Hematocrit: Represents the percentage of blood volume occupied by red blood cells 2
• Red blood cell count: Total number of erythrocytes per volume of blood 2
• Mean corpuscular volume (MCV): Classifies red blood cells by size; low MCV suggests iron, folic acid, or vitamin B12 deficiency, or inherited hemoglobin synthesis disorders 1
• Mean cell hemoglobin (MCH) and mean cell hemoglobin concentration (MCHC): Assess hemoglobin content within red blood cells 2
• Red cell distribution width (RDW): Measures variation in red blood cell size 2

White Blood Cell Parameters

• Total white blood cell count: Identifies leukocytosis or leukopenia 1
• Differential count: Breaks down WBC types (neutrophils, lymphocytes, monocytes, eosinophils, basophils) to identify specific patterns of infection, inflammation, or malignancy 2

Platelet Parameters

• Platelet count: Detects thrombocytopenia or thrombocytosis 1
• Mean platelet volume (MPV): Provides information about platelet production and function 3

Clinical Utility and Bone Marrow Assessment

The CBC serves as a fundamental assessment of bone marrow function, and abnormalities in two or more cell lines warrant careful evaluation and likely consultation with a hematologist. 1 This multi-lineage involvement suggests primary bone marrow pathology rather than isolated peripheral destruction or consumption.

The test provides baseline information necessary before initiating therapeutic agents that may have myelosuppressive, nephrotoxic, or hepatotoxic effects, or those requiring dosage adjustment in patients with organ dysfunction 1

Specific Clinical Applications

In Chronic Kidney Disease

• The CBC is recommended yearly at minimum for CKD patients to screen for anemia, which is associated with high rates of death and progression to end-stage renal disease 1
• More frequent monitoring is indicated in diabetic patients, who develop anemia at earlier CKD stages and have higher prevalence regardless of kidney function level 1

In HIV Care

• A CBC with differential white blood cell count should be obtained upon initiation of HIV care, as anemia, leukopenia, and thrombocytopenia are common in this population 1
• The CBC is used to calculate total CD4 cell count, a critical marker for HIV disease progression 1

In Acute Ischemic Priapism

• A CBC may identify elevated white blood cell counts suggesting underlying malignancy such as leukemia as the cause of priapism 1
• In sickle cell disease patients with priapism, the CBC shows characteristic patterns including lower hemoglobin and elevated reticulocyte counts 1

Reporting Standards

When reporting CBC results with immunophenotyping data, both percentages and absolute counts should be included, with corrections made for lymphocyte purity. 1, 4 For example, if lymphocyte purity is 94% and CD3 value is 70%, the corrected T-lymphocyte value is calculated by dividing 0.70 by 0.94 and multiplying by 100, yielding 74% 1

Absolute lymphocyte subset values should be reported when an automated CBC with differential has been performed from blood drawn simultaneously with immunophenotyping samples 1

Common Pitfalls to Avoid

• Do not overlook the significance of abnormalities in multiple cell lines, which strongly suggests bone marrow pathology requiring hematology consultation 1
• Avoid using hematocrit instead of hemoglobin for anemia assessment, as hemoglobin provides more reliable and reproducible results 1
• Do not fail to correct for lymphocyte purity when reporting immunophenotyping results alongside CBC data 1, 4
• Recognize that 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