What is a Complete Blood Cell Count (CBC)?

What is a Complete Blood Cell Count (CBC)?

A Complete Blood Cell Count (CBC) is a fundamental laboratory test that measures the quantity and characteristics of the three major cellular components of blood: white blood cells (WBCs), red blood cells (RBCs) with hemoglobin, and platelets, serving as a comprehensive assessment of bone marrow function and hematologic status. 1

Core Components of the CBC

The CBC encompasses several distinct measurements that together provide a comprehensive hematologic profile:

Red Blood Cell Parameters

• Hemoglobin concentration: The preferred measure over hematocrit due to superior reproducibility across laboratories and lack of interference from storage time or patient variables like serum glucose 1
• Hematocrit: The percentage of blood volume occupied by red blood cells 1
• Red blood cell count: The absolute number of RBCs per volume of blood 2
• Mean corpuscular volume (MCV): Indicates red cell size and helps classify anemia as microcytic (low MCV suggesting iron, folate, or B12 deficiency), normocytic, or macrocytic 1
• Mean cell hemoglobin (MCH) and mean cell hemoglobin concentration (MCHC): Quantify hemoglobin content within red cells 2
• Red cell distribution width (RDW): Measures variation in red cell size 2
• Reticulocyte count: Evaluates bone marrow response to anemia, with low counts suggesting inadequate erythropoietin production or inflammation in CKD patients 1

White Blood Cell Parameters

• Total white blood cell count: Identifies leukocytosis (elevated) or leukopenia (decreased) 1
• Differential count: Reports both percentage and absolute numbers of lymphocytes, neutrophils, monocytes, eosinophils, and basophils 1
• Absolute lymphocyte count: Critical for immunologic assessment, particularly in conditions like chronic lymphocytic leukemia 1
• Band neutrophil count: An elevated band count ≥1,500 cells/mm³ has the highest diagnostic value (likelihood ratio 14.5) for bacterial infection 3

Platelet Parameters

• Platelet count: Assesses for thrombocytopenia or thrombocytosis 1
• Mean platelet volume (MPV): Indicates platelet size and activation status 4

Clinical Utility and Interpretation

Assessment of Bone Marrow Function

Abnormalities in two or more cell lines (pancytopenia or bicytopenia) warrant careful evaluation and likely require hematology consultation, as this suggests primary bone marrow dysfunction rather than isolated peripheral destruction or consumption. 1

Diagnostic Applications

• Infection detection: Leukocytosis ≥14,000 cells/mm³ has a likelihood ratio of 3.7 for bacterial infection, while neutrophil percentage ≥90% increases this to 7.5 3
• Anemia evaluation: The CBC distinguishes anemia types and guides further workup, with normochromic normocytic anemia being typical of chronic kidney disease 1
• Bleeding disorders: Platelet count abnormalities indicate potential coagulation problems requiring investigation 1

Reporting Standards

Results must include both percentages and absolute values for all cell types, with correction for lymphocyte purity when performing immunophenotyping. 1 For CD4+ T-cell determinations specifically, the absolute lymphocyte count from the CBC must be multiplied by the CD4+ percentage from flow cytometry 1.

Technical Considerations

Sample Requirements

Blood specimens for CBC should ideally be analyzed within 48 hours of collection for clinical trials, though routine clinical samples can be processed with slightly longer delays 1. Hemoglobin measurements are not affected by storage time, unlike hematocrit 1.

Quality Assurance

Analytical variability exists even with automated counters: 2.2%-7.7% for WBC counts and 1.9%-5.3% for lymphocyte counts, with manual counting showing even higher variability (9.3%-17.6% and 12.5%-27% respectively) 3. Biological variability adds approximately 10% within-day and 13% within-week variation 3.

Common Clinical Pitfalls

• Overlooking left shift: A left shift (band neutrophils ≥16%) indicates bacterial infection even when total WBC count is normal, with a likelihood ratio of 4.7 3
• Ignoring reticulocyte response: In anemic CKD patients with adequate iron, folate, and B12, an inappropriately low reticulocyte count most commonly indicates insufficient erythropoietin production or inflammation 1
• Misinterpreting isolated findings: Single cell line abnormalities have different implications than multi-lineage cytopenias and require distinct diagnostic approaches 1