What is a complete blood count (CBC) and what does it evaluate?

Complete Blood Count (CBC): Definition and Components

A complete blood count (CBC) is a comprehensive laboratory panel that quantifies and characterizes all cellular components of blood—red blood cells, white blood cells, and platelets—along with their associated indices, providing essential information about oxygen-carrying capacity, immune function, and hemostatic status. 1, 2

Core Components Evaluated

Red Blood Cell Parameters

• Hemoglobin and hematocrit measure oxygen-carrying capacity, with abnormalities indicating anemia, polycythemia, or dehydration 3
• Mean corpuscular volume (MCV) classifies anemia morphologically into three categories 1, 3:
  • Microcytic (<80 fL): typically iron deficiency, thalassemia, or chronic disease
  • Normocytic (80-100 fL): hemorrhage, hemolysis, bone marrow failure, or renal insufficiency
  • Macrocytic (>100 fL): vitamin B12/folate deficiency or myelodysplastic syndromes
• Reticulocyte count (when corrected for anemia as reticulocyte index) distinguishes production defects from destruction or loss 1:
  • Low reticulocyte index (1.0-2.0): decreased RBC production from iron/vitamin deficiency, aplastic anemia, or marrow suppression
  • High reticulocyte index: blood loss or hemolysis despite anemia

White Blood Cell Parameters

• Total white blood cell count identifies leukocytosis or leukopenia 3
• Differential count (both percentages and absolute numbers) evaluates specific cell populations 3:
  • Neutrophilia with left shift (band count >1,500 cells/mm³) strongly suggests bacterial infection with likelihood ratio of 14.5 3
  • Severe neutropenia (<500 cells/mm³) indicates high infection risk requiring urgent evaluation 3
  • Lymphocytosis may indicate viral infection; severe lymphopenia (<2,500 cells/mm³) in infants warrants SCID evaluation 3
  • Eosinophilia suggests allergic or parasitic conditions 3

Platelet Parameters

• Platelet count assesses for thrombocytopenia or thrombocytosis 3
• Mean platelet volume (MPV) and platelet-to-lymphocyte ratio provide additional cardiovascular and metabolic risk information 4

Peripheral Blood Smear: Critical Companion Test

Visual review of the peripheral blood smear by a qualified hematologist or pathologist is paramount when CBC abnormalities are detected. 1

• Schistocytes (helmet cells) indicate microangiopathic hemolytic anemia requiring urgent evaluation for TTP, HUS, DIC, or HELLP syndrome 1, 5
• Giant or small platelets may indicate inherited thrombocytopenia 1
• Leukocyte inclusion bodies suggest specific genetic disorders like MYH9-related disease 1
• Blasts or dysplastic changes mandate urgent hematology referral 6

Clinical Applications Beyond Basic Screening

Baseline Assessment Scenarios

• HIV-infected individuals require CBC at care initiation to detect cytopenias, calculate CD4 count, and establish baseline before myelosuppressive antiretroviral therapy 6
• Chronic kidney disease patients need at least annual CBC as anemia prevalence increases with declining renal function 6
• Immune checkpoint inhibitor recipients should obtain CBC at treatment start and regular intervals to monitor for rare hematologic immune-related adverse events 6

Preoperative Testing

The CBC is not routinely indicated for all surgical patients 1. Specific indications include:

• Patients with history of anemia or bleeding 1
• Cardiovascular surgery candidates 1
• ASA class 2-3 patients with cardiovascular or respiratory disease undergoing major surgery 1
• Patients with liver disease, extremes of age, or hematologic disorders 1

Monitoring High-Risk Hematologic Conditions

• High-risk leukemia-predisposing syndromes (e.g., Fanconi anemia) require CBC every 3-4 months even with stable counts 6
• New or worsening cytopenias warrant repeat CBC in 2-4 weeks; if abnormalities persist across two measurements, proceed to bone marrow evaluation rather than continued serial monitoring 6

Critical Pitfalls to Avoid

• Never dismiss helmet cells as insignificant—they always warrant investigation for potentially fatal conditions like TTP 5
• Do not delay treatment for TTP while awaiting confirmatory testing—initiate plasma exchange immediately when clinical suspicion is high 5
• Avoid overlooking left shift significance even without leukocytosis, as it strongly indicates bacterial infection 3
• Always review medication lists first before extensive workup, as corticosteroids, lithium, β-agonists, and growth factors commonly cause CBC abnormalities 6
• Confirm schistocyte presence with manual differential examination rather than relying solely on automated counts 5
• Report both percentages and absolute counts for differential results, as absolute numbers provide more clinically meaningful information 3

When Urgent Hematology Consultation Is Mandatory

Immediate subspecialty referral is required when: 5, 6

• Peripheral smear shows blasts or significant dysplastic changes
• Multiple cell-line abnormalities suggest bone marrow failure
• Severe leukocytosis without obvious reactive cause
• Helmet cells present with thrombocytopenia, elevated LDH, decreased haptoglobin, acute kidney injury, neurological symptoms, or fever without clear source
• Hemoglobin <10 g/dL with elevated reticulocyte count