What are the components and indications of a Complete Blood Count (CBC)?

Complete Blood Count (CBC): Components and Clinical Indications

Core Components of CBC

A CBC evaluates three primary cell lines—white blood cells, red blood cells, and platelets—to assess bone marrow function and detect hematologic abnormalities. 1

Red Blood Cell Parameters

• Hemoglobin is the preferred measure over hematocrit due to superior reproducibility across laboratories, lower coefficients of variation, and independence from storage time or patient variables like serum glucose 1
• Hematocrit reflects the percentage of blood volume occupied by red blood cells 2
• Mean Corpuscular Volume (MCV) classifies anemia: low MCV suggests iron, folate, or B12 deficiency or inherited hemoglobin synthesis disorders; normal MCV is typical in chronic kidney disease 1, 2
• Red cell distribution width (RDW) indicates variation in red blood cell size and helps predict cardiovascular disease risk 3
• Reticulocyte count (absolute or adjusted for anemia severity) evaluates bone marrow response appropriateness—low counts suggest iron unavailability, production defects, or erythropoietin deficiency in CKD patients 1

White Blood Cell Parameters

• Total WBC count identifies leukocytosis or leukopenia, with abnormalities suggesting infection, inflammation, or hematologic malignancy 2
• Neutrophil count with left shift strongly indicates bacterial infection, with band count >1,500 cells/mm³ having a likelihood ratio of 14.5 2
• Lymphocyte count (both percentage and absolute number) helps identify viral infections when elevated 2
• Eosinophil count elevation suggests allergic reactions or parasitic infections 2

Platelet Parameters

• Platelet count detects thrombocytopenia (bone marrow suppression, immune destruction, consumption) or thrombocytosis 2
• Mean platelet volume (MPV) and platelet-to-lymphocyte ratio (PLR) serve as markers for cardiovascular disease prediction 3

Clinical Indications for CBC Testing

Anemia Evaluation

Screen all chronic kidney disease patients yearly at minimum for anemia, with more frequent monitoring in diabetic patients who develop anemia earlier and have higher prevalence regardless of kidney function. 1

• Order CBC when evaluating unexplained fatigue, pallor, or suspected blood loss 1
• Abnormalities in two or more cell lines warrant hematology consultation for possible bone marrow pathology 1

Preoperative Assessment

Order preoperative CBC for patients with liver disease, extremes of age, history of anemia or bleeding, hematologic disorders, or those undergoing cardiovascular surgery. 1

Specific preoperative indications include:

• Patients >60 years undergoing neurosurgery 1
• ASA class 2-3 patients with cardiovascular or respiratory disease undergoing major surgery 1
• ASA class 3 patients with renal disease undergoing any surgery 1

Do not order preoperative CBC for healthy ASA class 1 patients <40 years undergoing low-risk surgery. 1

Infection Evaluation

• Obtain CBC when evaluating suspected neonatal sepsis, particularly in infants exposed to inadequate intrapartum antibiotics, with optimal timing 6-12 hours after birth for maximum sensitivity 1
• Order CBC for suspected chorioamnionitis or maternal fever in labor 1
• Clinical signs of sepsis are more sensitive than hematologic tests alone 1

Bleeding Assessment

In patients with anorectal bleeding or suspected hemorrhoids, check CBC including hemoglobin and hematocrit to evaluate bleeding severity, along with coagulation studies. 1

• Perform blood typing and cross-matching if severe bleeding is present 1
• Include BUN and creatinine in lower gastrointestinal bleeding evaluation 1

Congenital Syphilis Evaluation

• Obtain CBC with differential and platelet count for all infants with proven or highly probable congenital syphilis 1
• Include CBC in evaluation of infants born to inadequately treated mothers 1

Hematologic Malignancy Monitoring

• Perform CBC within 48 hours of initiating chronic lymphocytic leukemia treatment protocols, including WBC count, hemoglobin, hematocrit, platelet count, differential with absolute lymphocyte and prolymphocyte counts, and reticulocyte count 1

Cardiovascular and Metabolic Disease Risk Assessment

• Components including WBC, neutrophil-to-lymphocyte ratio (NLR), hemoglobin, MCV, RDW, platelet count, and MPV predict cardiovascular disease, arteriosclerosis, type 2 diabetes, and metabolic syndrome risk 3

Critical Reporting and Interpretation Considerations

Report both percentages and absolute counts for all differential results, as absolute numbers provide more clinically meaningful information than percentages alone. 2

Common Pitfalls to Avoid

• Do not overlook left shift significance even without leukocytosis—it remains a strong bacterial infection indicator 2
• Avoid ordering additional tests when results will not change management decisions 2
• Do not fail to correct for lymphocyte purity when interpreting immunophenotyping results 2
• Recognize that CBC sensitivity for neonatal sepsis is lowest immediately after birth—delay testing 6-12 hours when possible 1
• Understand that blood culture sensitivity decreases in newborns exposed to intrapartum antibiotics, and CBC negative predictive value is high but positive predictive value is low in healthy-appearing term infants 1