What is a complete blood count (CBC) and what does it include?

Complete Blood Count: Definition and Components

A complete blood count (CBC) is a comprehensive laboratory panel that evaluates the total numbers and characteristics of all cellular components in the blood, including red blood cells, white blood cells, and platelets, along with their associated indices and differential counts. 1

Core Components of the CBC

The CBC consists of multiple distinct measurements that together provide a complete hematologic profile:

Red Blood Cell Parameters

• Red blood cell (RBC) count: Total number of erythrocytes per volume of blood 1
• Hemoglobin (Hb): Oxygen-carrying protein concentration, used to assess oxygen-carrying capacity 2, 1
• Hematocrit (Hct): Percentage of blood volume occupied by red blood cells 2, 1
• Mean corpuscular volume (MCV): Average size of red blood cells, critical for classifying anemia as microcytic, normocytic, or macrocytic 2, 3
• Mean corpuscular hemoglobin (MCH): Average hemoglobin content per red blood cell 1
• Mean corpuscular hemoglobin concentration (MCHC): Average hemoglobin concentration within red blood cells 1
• Red cell distribution width (RDW): Measure of variation in red blood cell size, useful for predicting cardiovascular disease and metabolic conditions 3, 1
• Reticulocyte count: Immature red blood cells, indicating bone marrow production capacity 4, 1

White Blood Cell Parameters

• Total white blood cell (WBC) count: Overall leukocyte concentration, essential for detecting leukocytosis or leukopenia 5, 2, 1
• Differential count: Breakdown of WBC subtypes including neutrophils, lymphocytes, monocytes, eosinophils, and basophils, reported as both percentages and absolute numbers 5, 2, 1
• Band neutrophils: Immature neutrophils indicating "left shift," with counts >1,500/mm³ having a likelihood ratio of 14.5 for bacterial infection 5, 2

Platelet Parameters

• Platelet count: Total number of thrombocytes, critical for assessing bleeding and clotting risk 4, 2, 1
• Mean platelet volume (MPV): Average platelet size, useful for predicting cardiovascular disease 3

Clinical Applications and Reporting Standards

Essential Reporting Requirements

• Both percentages and absolute counts must be reported for white blood cell differential results, particularly for lymphocyte subsets 5, 2
• Absolute values are calculated by multiplying the percentage by the total WBC count 5
• When performing immunophenotyping (such as CD4+ T-cell counts), the CBC must be drawn simultaneously to calculate absolute lymphocyte subset values 5

Mandatory Testing Scenarios

• All patients with suspected infection in long-term care facilities require CBC with differential 5
• Elevated WBC count (>14,000 cells/mm³) or left shift (band neutrophils ≥6% or >1,500/mm³) warrants immediate assessment for bacterial infection 5
• Acute ischemic priapism evaluation requires CBC to identify elevated white blood cell counts suggesting underlying malignancy such as leukemia 5
• Primary immune thrombocytopenia (ITP) diagnosis requires CBC showing isolated thrombocytopenia with otherwise normal counts 5

Critical Clinical Pitfalls

Peripheral Blood Smear Evaluation

• Manual review by a qualified hematologist or pathologist is mandatory when abnormalities are suspected, as automated analyzers may miss critical findings like schistocytes (helmet cells) 5, 4
• Helmet cells always warrant urgent investigation for potentially fatal conditions including thrombotic thrombocytopenic purpura (TTP), hemolytic uremic syndrome (HUS), or disseminated intravascular coagulation (DIC) 4
• Pseudo-thrombocytopenia from EDTA-dependent platelet agglutination must be excluded 5

Interpretation Errors to Avoid

• Do not overlook left shift even without leukocytosis, as band neutrophils >1,500/mm³ strongly indicate bacterial infection 5, 2
• Do not dismiss anemia as proportional to bleeding without checking reticulocyte count—a low reticulocyte index indicates production failure rather than blood loss 5, 4
• Severe lymphopenia (<2,500 cells/mm³) in infants requires urgent evaluation for severe combined immunodeficiency (SCID) 2
• Severe neutropenia (<500 cells/mm³) indicates high infection risk requiring urgent evaluation 2

Quality Assurance Standards

Laboratory performing CBC must meet specific regulatory requirements:

• Participation in proficiency testing programs approved by regulatory authorities (CLIA '88 requirements) 5
• Data retention for minimum 2 years or per local regulation 5
• Proper specimen handling with analysis within 1-2 hours of collection when culture is needed 5