Should You Check a CBC?
The decision to check a CBC depends entirely on the clinical context—it is not a routine screening test for healthy, asymptomatic individuals, but is indicated for specific clinical scenarios including preoperative risk assessment, evaluation of bleeding symptoms, monitoring of high-risk hematologic conditions, and investigation of suspected blood disorders.
Clinical Scenarios Where CBC Is Indicated
Preoperative Assessment
Check a CBC preoperatively if the patient has specific risk factors, not routinely for all surgical patients. 1
The American Society of Anesthesiologists and UK National Institute for Clinical Excellence recommend CBC for:
- Patients with liver disease 1
- Patients at extremes of age (>60 years for major surgery, >80 years for intermediate surgery with comorbidities) 1
- History of anemia or bleeding 1
- Hematologic disorders 1
- Cardiovascular surgery (all patients) 1
- Major surgery (grade 3-4) in ASA class 2-3 patients with cardiovascular, respiratory, or renal disease 1
Do not order CBC for healthy ASA class 1 patients undergoing low-risk procedures—this represents unnecessary testing that does not improve outcomes. 1
Bleeding Disorder Evaluation
Order CBC when evaluating for von Willebrand disease or other bleeding disorders as part of the initial hemostasis workup. 1
The NHLBI/NIH guidelines recommend CBC alongside PT and PTT when:
- Patient answers positively to bleeding history screening questions 1
- Physical examination reveals ecchymoses, hematomas, petechiae, or other bleeding signs 1
- Mucocutaneous bleeding history is present 1
The CBC helps identify thrombocytopenia, thrombocytosis, or anemia that may explain bleeding symptoms, though it does not directly evaluate for von Willebrand disease itself. 1
Leukemia-Predisposing Conditions
For children with leukemia-predisposing syndromes, CBC surveillance is essential and should be performed at specific intervals based on risk stratification. 1
The Clinical Cancer Research guidelines recommend:
- At least annual CBC for lower-risk conditions (Li-Fraumeni syndrome, Down syndrome, PAX5, ETV6, CMMRD, Bloom syndrome, ataxia-telangiectasia, RASopathies, dyskeratosis congenita, Diamond-Blackfan anemia) with stable blood counts 1
- CBC every 3-4 months for higher-risk conditions (Fanconi anemia, severe congenital neutropenia) to determine trajectory of blood counts 1, 2
- CBC should include manual differential, reticulocyte count, and blood smear with morphology 1
If CBC worsens or becomes abnormal, repeat within 2-4 weeks and consider bone marrow examination. 1, 2
Suspected Leukemia
CBC is a powerful tool for detecting leukemia, though it is not a screening test for asymptomatic populations. 3
When leukemia is suspected, CBC findings include:
- Chronic Myeloid Leukemia: Leukocytosis with left shift (WBC often >100 × 10⁹/L), basophilia (≥200/mm³), elevated platelets, mild anemia 4
- Acute Myeloid Leukemia: ≥20% blasts in peripheral blood or marrow 4
- Chronic Lymphocytic Leukemia: Lymphocytosis with mature-appearing lymphocytes 4
The presence of new cytopenias, unexplained leukocytosis, or abnormal differential warrants immediate CBC with manual differential and blood smear review. 1, 4
When NOT to Check CBC
Avoid routine CBC screening in healthy, asymptomatic individuals without specific risk factors—this practice is not cost-effective and lacks evidence for improving outcomes. 5
Do not use CBC as the sole determinant for distinguishing bacterial from viral infections, as WBC and differential counts lack both sensitivity and specificity for this purpose. 5
Do not continue frequent CBC monitoring indefinitely once stability is established—lengthen intervals appropriately to avoid unnecessary testing. 2, 6
Monitoring Intervals After Initial Abnormal CBC
For new or worsening cytopenias, repeat CBC within 2-4 weeks. 1, 2
For high-risk hematologic conditions with stable counts, monitor every 3-4 months initially, then lengthen to 6-12 months if stability continues. 2, 6
If CBC remains abnormal over two or more measurements, proceed to bone marrow evaluation rather than continued CBC monitoring alone. 2
Key Clinical Pitfalls to Avoid
Do not overlook medication-induced CBC abnormalities—methotrexate, anticonvulsants, and other drugs commonly cause macrocytosis or cytopenias. 6
Do not assume normal platelet counts exclude bleeding risk in leukemia—functional platelet abnormalities are common despite normal/elevated counts. 4
Do not order CBC without clinical context—the test must be interpreted alongside history, physical examination, and other diagnostic information to be clinically useful. 7, 8