How should I interpret a CBC in a female patient that shows leukocytosis (WBC 15.7 ×10³/µL) with neutrophil predominance (ANC 11.8 ×10³/µL) and mild immature granulocytosis, with normal red cell and platelet indices?

Interpretation of CBC with Leukocytosis and Neutrophilia

This CBC demonstrates a significant neutrophilic leukocytosis (WBC 15.7 with ANC 11.8) that strongly suggests an acute bacterial infection requiring immediate clinical evaluation for the source and consideration of empiric antibiotics if signs of systemic infection are present. 1, 2

Primary Interpretation: Acute Bacterial Infection Most Likely

The laboratory findings meet multiple high-probability criteria for bacterial infection:

• The absolute neutrophil count of 11.8 ×10³/µL is markedly elevated (reference 1.4-7.0), which carries a likelihood ratio of 3.7 for bacterial infection when WBC ≥14,000 cells/mm³ 1, 2

• The presence of immature granulocytes (1% with absolute count 0.2 ×10³/µL) represents a "left shift" that, when ≥16% bands or absolute band count ≥1,500 cells/mm³, has a likelihood ratio of 4.7-14.5 for documented bacterial infection 1, 2

• The neutrophil percentage of 75% approaches the threshold (>90%) that carries a likelihood ratio of 7.5 for serious bacterial infection 2

Immediate Clinical Actions Required

Assess for Systemic Infection Signs

• Check for fever: A single temperature ≥38.3°C or sustained ≥38.0°C for ≥1 hour markedly increases bacterial infection probability and mandates microbiologic work-up 2

• Evaluate for sepsis indicators: Altered mental status (especially in older adults), hypotension, tachycardia, or hemodynamic instability warrant immediate empiric antibiotics without delay 2

• Physical examination focus: Look specifically for respiratory symptoms (consider chest imaging per American Thoracic Society), urinary symptoms (obtain urinalysis with culture), abdominal pain/peritoneal signs (evaluate gastrointestinal sources), or skin/soft tissue findings 2, 1

Special Population Considerations

• If patient has cirrhosis with ascites: Any neutrophilia mandates immediate diagnostic paracentesis to rule out spontaneous bacterial peritonitis (SBP), which is diagnosed when ascitic fluid neutrophil count >250 cells/µL and requires urgent antibiotics regardless of culture results 2

• Recent tick exposure: Consider tick-borne rickettsial diseases if headache, fever, or confusion present 2

Differential Diagnosis Beyond Infection

While bacterial infection is most likely, other causes must be considered:

Physiologic/Reactive Causes

• Stress-related: Recent surgery, trauma, intense exercise, seizures, or emotional stress can produce neutrophilia with normal RBC/platelet indices 3, 2

• Medications: Corticosteroids, lithium, and beta-agonists commonly cause leukocytosis 3

Hematologic Malignancies (Less Likely but Critical to Exclude)

The presence of immature granulocytes raises concern for myeloproliferative disorders, though several features argue against this:

• Normal RBC and platelet counts make primary bone marrow disorders less likely, as concurrent cytopenias typically occur 3, 4

• Chronic myeloid leukemia (CML) criteria NOT met: CML requires WBC typically >100 ×10⁹/L with basophilia, immature granulocytes (metamyelocytes, myelocytes, promyelocytes), and no immature granulocytes in differential for complete hematologic response 1

• The modest elevation (15.7) and absence of basophilia, eosinophilia, or splenomegaly make CML unlikely 1

Diagnostic Work-Up Algorithm

Immediate (Within 12-24 Hours)

1. Obtain blood cultures if fever, systemic infection signs, or hemodynamic instability present 2, 1

2. Site-specific cultures based on clinical findings (urine, sputum, wound) 2

3. Imaging studies directed at suspected infection source 2

If No Clear Infection Source Identified

• Peripheral blood smear review to confirm automated differential, assess for dysplasia, and evaluate blast morphology 4

• Repeat CBC in 1-2 weeks to assess if leukocytosis persists or resolves with treatment 3

Red Flags Requiring Hematology Referral

• Progressive leukocytosis despite treatment of presumed infection 3

• Development of anemia or thrombocytopenia 3, 4

• Persistent leukocytosis >20 ×10⁹/L without identified infection 3

• Weight loss, hepatosplenomegaly, lymphadenopathy, or unexplained bleeding/bruising 3

• Dysplastic features on peripheral smear 4

Common Pitfalls to Avoid

• Do not dismiss the significance of absolute neutrophil count elevation even when total WBC is only moderately elevated—left shift can occur with WBC <10,000 cells/mm³ and still indicate bacterial infection 2, 1

• Do not treat asymptomatic patients with antibiotics based solely on elevated neutrophil counts without clinical evidence of infection 2

• Do not overlook non-infectious causes such as medications (corticosteroids, lithium, beta-agonists) or physiologic stress 3

• In the absence of fever, leukocytosis/left shift, or specific focal infection manifestations, additional diagnostic tests may have low yield for bacterial infection, though nonbacterial infections cannot be excluded 1