How do you interpret a Complete Blood Count (CBC) and differential?

How to Interpret a Complete Blood Count (CBC) and Differential

Systematic Approach to CBC Interpretation

Interpret the CBC by evaluating each component systematically: white blood cell count with differential, red blood cell indices (hemoglobin, hematocrit, MCV, RDW), and platelet count, using both absolute values and percentages to identify patterns that suggest specific disease processes. 1, 2

White Blood Cell (WBC) Count and Differential

When to Order

• Order CBC with differential within 12-24 hours of symptom onset when infection is suspected based on clinical presentation (or sooner if the patient is seriously ill), not as routine screening 3
• Manual differential counting is preferred over automated methods for evaluating band neutrophils and other immature forms 4, 3

Interpreting Elevated WBC Count

• WBC ≥14,000 cells/mm³ has a likelihood ratio of 3.7 for bacterial infection and warrants thorough investigation even without fever 4, 3
• Neutrophil percentage ≥90% has a likelihood ratio of 7.5 for bacterial infection 4
• Band neutrophil count ≥1,500 cells/mm³ has the highest likelihood ratio (14.5) for documented bacterial infection 4
• Left shift (band neutrophils or metamyelocytes ≥16%) has a likelihood ratio of 4.7 for bacterial infection 4, 3

Interpreting Normal WBC Count

• In the absence of fever, normal WBC, no left shift, and no specific clinical manifestations of focal infection, additional diagnostic tests may not be indicated due to low potential yield 3
• Even with normal total WBC count, evaluate the differential for left shift, which may indicate bacterial infection despite normal total count 3
• High percentage of neutrophils or left shift with normal total WBC still indicates high likelihood of bacterial infection 4

Understanding Laboratory Variability

• Analytical variability in WBC determination is 2.2%-7.7% using automated counters and 9.3%-17.6% using hemocytometer 4
• Biological variability is approximately 10% within a day and 13% within a week 4
• Results should include both percentage and absolute values of blood cells 4

Red Blood Cell (RBC) Indices

Key Parameters to Evaluate

• Hemoglobin and hematocrit assess oxygen-carrying capacity 1
• Mean corpuscular volume (MCV) helps classify anemia type and predicts cardiovascular disease and metabolic syndrome 5
• Red cell distribution width (RDW) is a useful marker to predict cardiovascular disease and metabolic conditions 5
• The CBC contains considerable information regarding the cause of anemia when present, which assists in formulating differential diagnosis 6

Platelet Count and Indices

Platelet Parameters

• Platelet count identifies thrombocytopenia or thrombocytosis 2
• Mean platelet volume (MPV) is a useful marker to predict cardiovascular disease 5
• Platelet-to-lymphocyte ratio (PLR) can predict cardiovascular disease and metabolic diseases 5

Clinical Applications in Specific Populations

Older Adults in Long-Term Care Facilities

• Do not order routine or scheduled CBC testing in asymptomatic residents, as this leads to unnecessary costs and potential false positives 3
• Do not rely solely on CBC results to rule out infection, as typical symptoms and signs are frequently absent in older adults 3
• Temperature readings of >100°F (37.8°C), >2 readings of >99°F (37.2°C), or an increase of 2°F (1.1°C) over baseline should prompt further evaluation even with normal WBC 3
• Review advance directives prior to any intervention, including laboratory testing 3

Suspected Infections

• For suspected UTI: Order CBC with urinalysis and urine culture only for residents with acute onset of UTI-associated symptoms (fever, dysuria, gross hematuria, new/worsening incontinence) 3
• For suspected pneumonia: Order CBC with pulse oximetry and possibly chest radiography 3
• For suspected GI infection: CBC may be indicated for severely ill residents or if symptoms persist beyond 7 days 3

Neonatal Evaluation

• CBC with differential and platelet count is part of the recommended evaluation for infants with proven or highly probable congenital syphilis 7
• Full diagnostic evaluation for suspected neonatal sepsis includes CBC with differential and platelet counts at birth and/or at 6-12 hours of life 7

Common Pitfalls to Avoid

• Avoid ordering CBCs when results will not change management decisions 3
• Do not order CBC as routine screening without clinical indication, as the test lacks specificity in many patients and does not reliably distinguish between bacterial and viral infections 8
• Do not ignore left shift or elevated band count even when total WBC is normal 4, 3
• Basal body temperature decreases with age and frailty, making classic definitions of fever less reliable in older populations 3
• Only perform tests if they have reasonable diagnostic yield, are low risk, reasonable in cost, and will improve patient management 3