Complete Blood Count (CBC)
A Complete Blood Count (CBC) is a fundamental laboratory test that evaluates the cellular components of blood, including red blood cells, white blood cells, and platelets, providing essential information for diagnosing and monitoring various medical conditions. 1, 2
Components of a CBC
A CBC consists of several key measurements:
Red Blood Cell (RBC) Parameters:
- Hemoglobin (Hb): Measures oxygen-carrying capacity
- Hematocrit: Percentage of blood volume occupied by RBCs
- RBC count: Number of red blood cells per volume of blood
- RBC indices: Mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC), and red cell distribution width (RDW)
White Blood Cell (WBC) Parameters:
- Total WBC count: Number of white blood cells per volume of blood
- WBC differential: Percentages of different types of white blood cells (neutrophils, lymphocytes, monocytes, eosinophils, basophils)
Platelet Parameters:
- Platelet count: Number of platelets per volume of blood
- Mean platelet volume (MPV): Average size of platelets
Clinical Significance
The CBC provides crucial information for:
Diagnosing Hematologic Disorders:
- Anemia: Identified by low hemoglobin levels
- Polycythemia: Characterized by elevated RBC counts
- Leukemia and other blood cancers: May show abnormal WBC counts or presence of immature cells
- Thrombocytopenia or thrombocytosis: Abnormal platelet counts
Monitoring Disease Progression and Treatment Response:
- Chronic kidney disease: Regular monitoring of anemia
- Chemotherapy: Assessing bone marrow suppression
- Infections: Tracking WBC counts and differentials
Evaluating Systemic Conditions:
- Inflammatory disorders: Often show elevated WBC counts
- Autoimmune diseases: May present with various CBC abnormalities
- Nutritional deficiencies: Can affect RBC parameters
Interpretation Considerations
When interpreting CBC results:
Hemoglobin is preferred over hematocrit for assessing anemia due to its greater reproducibility across laboratories and lower susceptibility to variables like sample storage time and patient-specific factors such as serum glucose 1
Abnormalities in multiple cell lines (two or more of WBC, RBC, platelets) warrant careful evaluation and may require hematology consultation 1
Low MCV typically suggests iron, folate, or vitamin B12 deficiencies, or inherited hemoglobin disorders 1
Reticulocyte count helps evaluate bone marrow response to anemia 1
Clinical Applications
The CBC has numerous applications:
- Screening: Often included in routine health examinations
- Diagnosis: Helps identify various hematologic and non-hematologic conditions
- Monitoring: Tracks disease progression and treatment response
- Preoperative evaluation: Assesses surgical risk and need for blood products
- Medication management: Monitors for medication side effects
Calculation of Absolute Values
When immunophenotyping is performed alongside a CBC, absolute lymphocyte subset values can be calculated by multiplying the lymphocyte subset percentage (from flow cytometry) by the absolute number of lymphocytes (from CBC and differential) 1
Technological Advances
Modern CBC testing has evolved significantly:
- Automated analyzers now provide rapid, accurate results with minimal sample volume
- Point-of-care devices are being developed for resource-limited settings 3
- Advanced parameters beyond the traditional CBC components are increasingly available
Limitations and Pitfalls
Important considerations when using CBC results:
- A normal CBC does not rule out certain conditions like early-stage lymphoma 4
- CBC results should be interpreted in clinical context, as values can be affected by numerous factors including pregnancy, altitude, smoking, age, and race 1
- For suspected hematologic malignancies, additional testing beyond CBC is often necessary, including bone marrow examination, tissue biopsy, and specialized testing 4
The CBC remains one of the most commonly ordered laboratory tests in medicine, providing valuable information for diagnosing and managing a wide range of medical conditions while being relatively inexpensive and minimally invasive.