What is the purpose of a complete blood cell (CBC) count?

Purpose and Clinical Applications of Complete Blood Cell Count (CBC)

A complete blood count (CBC) is a fundamental laboratory test that evaluates blood cell components to diagnose and monitor various medical conditions, assess organ function, and screen for underlying diseases. 1

Components of a CBC

• CBC includes hemoglobin, white blood cell count with differential, platelet count, and detailed red blood cell indices 2
• The test evaluates both the quantity and characteristics of cellular blood components 3
• Components typically include:
  • Red blood cell (RBC) count, hemoglobin, and hematocrit 3
  • RBC indices (MCV, MCH, MCHC, and RDW) 3
  • White blood cell (WBC) count with differential 1
  • Platelet count 3
  • Reticulocyte count (when specifically ordered) 3

Clinical Applications

Hematologic Assessment

• Detects anemia, leukopenia, and thrombocytopenia, which are common in various patient populations including HIV-infected persons 1, 2
• Provides baseline information necessary before initiating medications with potential myelosuppressive, nephrotoxic, or hepatotoxic effects 1
• Helps calculate the total CD4 cell count in HIV patients, which is crucial for immune status assessment 1, 2

Infection and Inflammation Detection

• Elevated WBC count (≥14,000 cells/mm³) or a left shift (≥16% band neutrophils or ≥1,500 cells/mm³ band count) warrants careful assessment for bacterial infection 1
• Helps evaluate suspected infections in long-term care facility residents, particularly when fever is present 1
• Abnormal WBC counts, even without fever, can indicate underlying bacterial infections 1, 2

Disease Monitoring and Diagnosis

• Essential for monitoring patients with leukemia-predisposing conditions, with frequency determined by specific disease risk 1
• Required for evaluation of chronic lymphocytic leukemia and monitoring treatment response 1
• Helps identify potential underlying malignancies in conditions like acute ischemic priapism 1

Screening and Risk Assessment

• Identifies patients who might need screening for genetic conditions like G6PD deficiency 1
• Can help predict risk for cardiovascular disease, arteriosclerosis, type 2 diabetes, and metabolic syndrome 4
• Provides critical information for pretreatment evaluation in clinical trials 1

Special Considerations

• For patients with suspected bone marrow disorders, CBC results should be compared with baseline values to detect significant changes 1
• In patients with sickle cell disease and acute ischemic priapism, CBC may show lower hemoglobin and elevated lactate dehydrogenase, bilirubin, and other markers 1
• Specimen collection and handling can significantly affect CBC results, requiring proper technique and timely processing 5

Limitations and Pitfalls

• CBC abnormalities are often nonspecific and require clinical correlation 6
• Results can be affected by timing of collection, patient hydration status, and medication use 5
• Automated CBC results may need manual review when significant abnormalities are present 7
• CBC should be interpreted in conjunction with patient history, physical examination findings, and other laboratory tests for accurate diagnosis 6, 7