What is a Complete Blood Count (CBC)?
A complete blood count (CBC) is a fundamental laboratory test that measures white blood cells, hemoglobin, and platelets to assess bone marrow function and detect hematologic abnormalities. 1
Core Components of the CBC
The CBC evaluates three primary cell lines that reflect overall bone marrow health:
Red blood cell parameters include hemoglobin, hematocrit, red blood cell count, and red blood cell indices (mean corpuscular volume, mean corpuscular hemoglobin, mean corpuscular hemoglobin concentration, and red cell distribution width). 2
White blood cell parameters include total white blood cell count and differential count (neutrophils, lymphocytes, monocytes, eosinophils, and basophils). 2
Platelet count assesses the number of platelets circulating in the blood. 2
Clinical Utility and Interpretation
Assessing Bone Marrow Function
Abnormalities in two or more cell lines warrant careful evaluation and likely require hematology consultation, as this suggests a primary bone marrow disorder. 1
Red Blood Cell Assessment
Hemoglobin is the preferred measure over hematocrit because it demonstrates superior reproducibility across laboratories with lower coefficients of variation and is not affected by sample storage time or patient variables like serum glucose. 1
Mean corpuscular volume (MCV) helps classify anemia: low MCV suggests iron, folate, or vitamin B12 deficiency, or inherited hemoglobin synthesis disorders. 1
The reticulocyte count (often ordered separately but conceptually part of comprehensive red cell assessment) evaluates bone marrow response to anemia—a low count indicates inadequate production while a high count suggests blood loss or hemolysis. 1
White Blood Cell Assessment
Total white blood cell count identifies leukocytosis or leukopenia, which may indicate infection, inflammation, malignancy, or bone marrow suppression. 1
The differential count provides critical diagnostic information: neutrophilia suggests bacterial infection, lymphocytosis indicates viral infection, and eosinophilia points to allergic or parasitic conditions. 3
Platelet Assessment
- Platelet count identifies thrombocytopenia or thrombocytosis, which may result from bone marrow suppression, immune destruction, consumption, or reactive processes. 3
Reporting Standards
Both percentages and absolute counts should be reported when calculating lymphocyte subsets (such as CD4+ T-cells in HIV management), as this provides the most complete clinical picture. 1, 3
- Absolute values are calculated by multiplying the lymphocyte subset percentage by the absolute lymphocyte count from the CBC. 1
Common Clinical Applications
The CBC serves multiple clinical purposes beyond basic screening:
Routine baseline assessment upon initiation of care for chronic conditions like HIV infection or chronic kidney disease. 1
Monitoring for drug toxicity before and during treatment with myelosuppressive, nephrotoxic, or hepatotoxic agents. 1
Evaluating anemia in chronic kidney disease, where it is recommended at least yearly given the high prevalence and association with mortality. 1
Identifying underlying malignancy, as elevated white blood cell counts may indicate leukemia or other hematologic malignancies. 1
Predicting cardiovascular and metabolic disease risk, as components like neutrophil-to-lymphocyte ratio, red cell distribution width, and mean platelet volume correlate with disease outcomes. 4
Critical Pitfalls to Avoid
Do not overlook the significance of abnormalities in multiple cell lines, as this strongly suggests bone marrow pathology requiring specialist evaluation. 1
Avoid using hematocrit when hemoglobin is available, as hemoglobin provides more reliable and reproducible results. 1
Do not fail to consider the clinical context: the CBC must be interpreted alongside patient history, as many conditions (pregnancy, high altitude, smoking, age ≥70 years, non-Caucasian race, chronic lung disease, hemoglobinopathy) alter normal reference ranges. 1