What is the purpose of a Complete Blood Count (CBC) test?

Purpose of a Complete Blood Count (CBC)

A CBC is a fundamental laboratory test that measures the quantity and characteristics of blood cells—including white blood cells, red blood cells, and platelets—to detect infections, anemia, bleeding disorders, immune dysfunction, and hematologic malignancies. 1

Core Components and What They Measure

A CBC provides several critical measurements that form the foundation of clinical assessment 1:

• Hemoglobin and red blood cell indices detect anemia and characterize its type 2, 1
• White blood cell count with differential identifies infections, immune disorders, and calculates absolute lymphocyte subsets (such as CD4 counts in HIV patients) 2, 1
• Platelet count assesses bleeding risk and thrombocytopenia 2, 1
• Red cell distribution width (RDW) and mean corpuscular volume (MCV) provide additional diagnostic information about red blood cell abnormalities 3

Primary Clinical Applications

Infection Detection and Monitoring

The CBC is essential for identifying bacterial infections, even in the absence of fever. 2

• An elevated white blood cell count (≥14,000 cells/mm³) or left shift (band neutrophils ≥16% or absolute band count ≥1,500 cells/mm³) warrants careful assessment for bacterial infection 2
• An elevated total band count >1,500/mm³ has a likelihood ratio of 14.5 for documented bacterial infection 1
• Neutrophil percentage >90% has a likelihood ratio of 7.5 for bacterial infection 1
• For suspected infections in long-term care facilities, a CBC with manual differential should be performed within 12-24 hours of symptom onset 2

Hematologic Disease Screening

The CBC serves as a critical screening tool for blood disorders 2, 1:

• Detects cytopenias that may indicate leukemia or myelodysplastic syndrome (MDS) in children with leukemia-predisposing conditions 2
• Identifies anemia, leukopenia, and thrombocytopenia common in HIV-infected persons 2, 1
• Screens for hematologic abnormalities requiring bone marrow evaluation when progressive fatigue, pallor, fever, petechiae, or bruising are present 2

Baseline Assessment Before Treatment

A CBC must be obtained upon initiation of care for specific conditions to establish baseline values before starting potentially myelosuppressive, nephrotoxic, or hepatotoxic therapies. 2

• Required before initiating HIV antiretroviral therapy to calculate CD4 cell counts and detect preexisting cytopenias 2
• Necessary for preoperative evaluation in patients with liver disease, extremes of age, history of anemia or bleeding, or undergoing major surgery 2
• Essential for evaluating infants with suspected congenital syphilis to detect anemia, thrombocytopenia, or leukocytosis 2

Surveillance and Monitoring

For high-risk populations, serial CBCs track disease progression 2:

• Annual CBC recommended for patients with stable blood counts and lower risk of MDS/AML 2
• CBC every 3-4 months for higher-risk patients to determine trajectory of blood counts 2
• Repeat CBC within 2-4 weeks if counts worsen or become abnormal 2

Special Populations Requiring CBC

HIV-Infected Patients

• CBC with differential required at entry into care to calculate absolute CD4 counts (by multiplying lymphocyte subset percentage by absolute lymphocyte count from automated CBC) 2
• Identifies common complications including anemia, leukopenia, and thrombocytopenia 2, 1

Suspected Infection in Elderly

• Manual differential preferred over automated to assess bands and immature forms 2
• Even without fever, leukocytosis or left shift indicates need for bacterial infection workup 2

Preoperative Assessment

• Recommended for patients with liver disease, history of anemia or bleeding, cardiovascular disease undergoing major surgery, or respiratory disease patients undergoing high-risk procedures 2

Infants with Congenital Syphilis

• CBC with differential and platelet count required as part of evaluation for proven or probable disease 2

Important Technical Considerations

The CBC requires proper specimen collection and handling to provide accurate results. 4

• Blood should be collected in appropriate anticoagulant tubes (typically EDTA) 4
• Specimens are easily influenced by collection technique, transportation conditions, and storage 4
• For CD4 calculations in HIV patients, the hematology laboratory must participate in HCFA-approved proficiency testing programs 2
• Absolute lymphocyte subset values should only be reported when automated CBC is performed from blood drawn simultaneously with immunophenotyping samples 2

Common Pitfalls to Avoid

• Do not use umbilical cord blood for infant syphilis screening as it can be contaminated with maternal blood and yield false-positive results; use infant serum instead 2
• Do not order urinalysis or blood cultures reflexively in long-term care residents without specific clinical indications, as CBC abnormalities should guide further testing 2
• Do not ignore normal CBC results in symptomatic patients—nonbacterial infections and early hematologic malignancies may not initially show CBC abnormalities 2
• Recognize that 10-20% of CBC results are reported as abnormal, requiring structured interpretation rather than reflexive subspecialty referral 5