How do you interpret a complete blood count (CBC) report?

How to Interpret a Complete Blood Count (CBC) Report

Systematically evaluate the CBC by analyzing red blood cell parameters, white blood cell differential with absolute counts, and platelet indices, while always correlating abnormal findings with clinical context to determine if urgent intervention or subspecialty consultation is needed. 1

Initial Assessment Framework

When reviewing a CBC, organize your interpretation into three main compartments, examining each for abnormalities that require action:

Red Blood Cell Parameters

• Evaluate hemoglobin and hematocrit first to assess oxygen-carrying capacity, with abnormalities indicating anemia, polycythemia, or dehydration 1
• Use Mean Corpuscular Volume (MCV) to classify any anemia as microcytic (<80 fL), normocytic (80-100 fL), or macrocytic (>100 fL) 1
• Check the reticulocyte count when anemia is present: a low reticulocyte index indicates decreased red blood cell production (bone marrow problem), while a high index suggests blood loss or hemolysis 1
• Examine the peripheral smear carefully for abnormal red cell morphology, particularly helmet cells (schistocytes), which always warrant investigation for potentially fatal conditions 2

White Blood Cell Parameters

• Assess total white blood cell count for leukocytosis (>11,000/µL) or leukopenia (<4,000/µL) 1
• Always report and interpret both percentages AND absolute counts for the differential—percentages alone are misleading 1
• Look for neutrophilia with left shift (increased band forms), which strongly indicates bacterial infection with a likelihood ratio of 14.5 when band count exceeds 1,500 cells/mm³ 1
• Evaluate lymphocyte count: lymphocytosis may indicate viral infection, while lymphopenia can suggest immunosuppression 1
• Check eosinophil count: eosinophilia suggests allergic or parasitic conditions 1

Platelet Parameters

• Assess platelet count for thrombocytopenia (<150,000/µL) or thrombocytosis (>450,000/µL) 1
• When thrombocytopenia is present, consider bone marrow suppression, immune destruction, or consumption as potential causes 1

Critical Red Flags Requiring Urgent Action

Microangiopathic Hemolytic Anemia (MAHA)

If helmet cells (schistocytes) are identified on smear, immediately initiate the following workup without delay:

• Perform urgent reticulocyte count: an elevated reticulocyte index suggests hemolysis 2
• Check lactate dehydrogenase (LDH): markedly elevated levels indicate hemolysis 2
• Measure haptoglobin: decreased or absent levels indicate intravascular hemolysis 2
• Assess indirect bilirubin: elevated levels indicate RBC breakdown 2
• Order coagulation studies (PT, PTT, fibrinogen, D-dimer) to evaluate for disseminated intravascular coagulation (DIC) 2
• Perform direct antiglobulin test (DAT/Coombs): a negative result helps exclude immune hemolysis 2

Consider specific diagnoses based on clinical context:

• Thrombotic thrombocytopenic purpura (TTP) requires urgent plasma exchange—check ADAMTS13 activity but do not delay treatment while awaiting results 2
• Hemolytic uremic syndrome (HUS) in children with bloody diarrhea—perform renal function tests 2
• DIC when coagulation studies are abnormal 2
• HELLP syndrome in pregnant or postpartum women 2

When to Obtain Urgent Hematology Consultation

Mandatory immediate hematology consultation when helmet cells are present with any of the following: 2

• Thrombocytopenia
• Elevated LDH with decreased haptoglobin
• Acute kidney injury
• Neurological symptoms
• Fever with no clear infectious source
• Hemoglobin <10 g/dL with elevated reticulocyte count

Specific Clinical Scenarios

Preoperative Screening

For asymptomatic patients undergoing surgery, the CBC is part of initial hemostasis testing (along with PT and PTT) to screen for coagulation factor deficiency or thrombocytopenia that might cause bleeding 3

Suspected Bleeding Disorders

When evaluating patients with bleeding history, the CBC helps identify thrombocytopenia or thrombocytosis as potential causes, though it does not evaluate for von Willebrand disease specifically 3

Infection Evaluation

The CBC with differential is essential for distinguishing bacterial from viral infections, though it lacks perfect specificity 1. A complete blood count including white blood cell count, hemoglobin, hematocrit, platelet count, differential (with both percent and absolute lymphocyte and prolymphocyte counts), and reticulocyte count should be obtained 3

Hematologic Malignancy Monitoring

For patients with chronic lymphocytic leukemia or other hematologic conditions, serial CBCs track disease progression and treatment response 3

Common Pitfalls to Avoid

• Never dismiss helmet cells as insignificant—they always warrant investigation for potentially fatal conditions 2
• Do not delay treatment for TTP while awaiting confirmatory testing; initiate plasma exchange immediately when clinical suspicion is high 2
• Avoid relying on percentages alone for white blood cell differential—always calculate and interpret absolute counts 1
• Do not overlook left shift even in the absence of leukocytosis, as it strongly suggests bacterial infection 1
• Avoid ordering additional tests when results will not change management decisions 1
• Do not confuse helmet cells with other RBC abnormalities—ensure the laboratory performs manual differential examination to confirm schistocyte presence 2

Structured Approach to Abnormal Results

When confronted with abnormal CBC results, use this algorithmic approach: 4

1. Identify which compartment is abnormal (RBC, WBC, or platelets)
2. Determine severity (mild, moderate, or severe deviation from normal)
3. Review the peripheral smear for morphologic abnormalities
4. Correlate with clinical presentation and symptoms
5. Decide if subspecialty consultation is needed based on severity and clinical context
6. Initiate appropriate workup based on the specific abnormality identified