When to Order a Complete Blood Count (CBC)
A CBC should be ordered when there is clinical suspicion of hematologic abnormality, infection, or when monitoring specific treatments—not as routine screening in asymptomatic patients without risk factors. The test is most valuable when targeted to specific clinical scenarios rather than ordered reflexively.
Initial Assessment Upon Entry to Care
Order a CBC with differential at initiation of care for HIV-infected patients to establish baseline hematologic status, as anemia, leukopenia, and thrombocytopenia are common in this population 1. This baseline is essential for calculating total CD4 lymphocyte count and assessing overall immune function 1.
For patients entering hepatitis C treatment, obtain a CBC after 4 weeks of therapy and as clinically indicated to monitor for drug-related hematologic toxicity, particularly in those receiving ribavirin 1.
Suspected Infection
Long-Term Care Facility Residents
Obtain a CBC with manual differential within 12-24 hours of symptom onset (or sooner if seriously ill) for any LTCF resident with suspected infection 1. The presence of leukocytosis (WBC ≥14,000 cells/mm³) or left shift (band neutrophils ≥16% or absolute band count ≥1,500 cells/mm³) warrants careful assessment for bacterial infection, even without fever 1.
- Neutrophilia with left shift strongly indicates bacterial infection, with a likelihood ratio of 14.5 when band count exceeds 1,500 cells/mm³ 2
- In the absence of fever, leukocytosis, left shift, or focal infection signs, additional diagnostic tests may not be indicated due to low yield 1
- Do not overlook the significance of left shift even when total WBC count is normal 2
Congenital Syphilis Evaluation
Order CBC with differential and platelet count for infants born to mothers with reactive syphilis serology when evaluating for congenital syphilis, particularly those with abnormal physical examination, elevated nontreponemal titers, or inadequately treated mothers 1.
Preoperative Assessment
High-Risk Surgical Patients
Order preoperative CBC for patients with liver disease, extremes of age, history of anemia or bleeding, hematologic disorders, or when undergoing invasive procedures 1.
Specific indications include:
- Patients with history of anemia or recent blood loss 1
- Cardiovascular surgery (all patients) 1
- Neurosurgery in patients >60 years 1
- ASA class 2-3 patients with cardiovascular disease undergoing major (grade 3-4) surgery 1
- ASA class 2-3 patients with respiratory disease undergoing intermediate to major surgery 1
- ASA class 3 patients with renal disease undergoing any surgery 1
Low-Risk Patients
Do not order CBC for ASA class 1 patients under 40 years undergoing low-risk surgery, as the yield is extremely low and unlikely to change management 1.
Monitoring During Treatment
Hepatitis C Therapy
Obtain CBC after 4 weeks of treatment and as clinically indicated, with more frequent assessment for patients receiving ribavirin due to hemolytic anemia risk 1. Monitor for drug-related toxic effects throughout the treatment course 1.
HIV Management
Perform CBC upon initiation of care and periodically during antiretroviral therapy to monitor for drug-induced cytopenias 1. This is particularly important as many antiretroviral drugs can cause bone marrow suppression 1.
Disease Progression Monitoring
For patients with hepatitis C who fail to achieve sustained virologic response, obtain CBC every 6-12 months along with hepatic function panel to assess disease progression 1.
Key Interpretation Principles
When CBC is ordered, evaluate:
- Red cell parameters: Hemoglobin/hematocrit for oxygen-carrying capacity; MCV to classify anemia type (microcytic, normocytic, macrocytic); reticulocyte count to distinguish production defects from loss/destruction 2
- White cell parameters: Total WBC and differential for infection or immune dysfunction; left shift as strong indicator of bacterial infection even without leukocytosis 2
- Platelet parameters: Count for thrombocytopenia or thrombocytosis; consider bone marrow suppression, immune destruction, or consumption as causes 2
Common Pitfalls to Avoid
- Do not order CBC when results will not change management decisions 2
- Do not ignore left shift even when total WBC is normal, as this strongly suggests bacterial infection 2
- Do not fail to report both percentages and absolute counts for differential results, as absolute values are more clinically meaningful 2
- Do not order routine screening CBCs in asymptomatic, healthy patients without specific risk factors or clinical indications 1