What is the significance of a leukocytosis with a total white blood cell count of 15.5 ×10⁹/L, neutrophils 9,440/µL, lymphocytes 4,433/µL, and monocytes 1,225/µL in an adult, and what initial evaluation and management steps are recommended?

Evaluation of Leukocytosis with Elevated Neutrophils, Lymphocytes, and Monocytes

Your laboratory findings show a moderate leukocytosis with elevations across all white blood cell lineages—this pattern strongly suggests an acute bacterial infection and requires immediate clinical assessment to identify the source and determine if empiric antibiotics are needed. 1

Immediate Clinical Assessment Required

Check vital signs immediately to rule out sepsis, which is the most critical consideration: 2

• Temperature ≥38.3°C (single measurement) or ≥38.0°C sustained for ≥1 hour
• Blood pressure <90 mmHg systolic (hypotension)
• Heart rate >90 bpm (tachycardia)
• Respiratory rate >20 breaths/min (tachypnea)
• Altered mental status or confusion

Obtain a serum lactate level urgently—if >3 mmol/L, this confirms severe sepsis requiring immediate broad-spectrum antibiotics and aggressive fluid resuscitation within 1 hour. 2

Interpretation of Your Specific Laboratory Values

Your WBC count of 15.5 × 10⁹/L carries a likelihood ratio of 3.7 for underlying bacterial infection, even without fever. 1, 3 The pattern shows:

• Absolute neutrophil count 9,440/µL (elevated): This degree of neutrophilia warrants careful assessment for bacterial infection. 1
• Absolute lymphocyte count 4,433/µL (elevated): The concurrent lymphocytosis is somewhat unusual for acute bacterial infection, which typically causes lymphopenia. 4
• Absolute monocyte count 1,225/µL (elevated): Monocytosis can occur with certain bacterial infections but also raises consideration of other processes. 5

Critical next step: Obtain a manual differential count immediately—automated analyzers miss band forms and toxic granulations that are essential for diagnosis. 1, 3, 2 Specifically assess for:

• Absolute band count: If ≥1,500 cells/mm³, this has the highest likelihood ratio (14.5) for documented bacterial infection 1, 3
• Left shift: If ≥16% band neutrophils, this has a likelihood ratio of 4.7 for bacterial infection, even with normal total WBC 1, 3
• Neutrophil percentage: If >90%, this has a likelihood ratio of 7.5 for bacterial infection 1

Systematic Evaluation for Infection Source

Respiratory tract: Assess for cough, dyspnea, chest pain, sputum production—consider chest X-ray if any respiratory symptoms present. 3

Urinary tract: Evaluate for dysuria, flank pain, frequency, urgency—obtain urinalysis with culture for any urinary symptoms. 5, 3 However, do not treat asymptomatic bacteriuria based solely on positive urine culture, as 15-50% of older adults have asymptomatic colonization. 5

Skin and soft tissue: Examine for erythema, warmth, purulent drainage, cellulitis, or abscess. 3

Gastrointestinal tract: Check for abdominal pain, peritoneal signs, diarrhea suggesting intra-abdominal infection. 3

Bloodstream: Obtain blood cultures before antibiotics if any systemic signs are present (fever, hypotension, tachycardia, altered mental status). 3, 2

Management Algorithm

If hemodynamically stable with no sepsis criteria:

• Complete the diagnostic workup first before initiating antibiotics 3
• Obtain manual differential, blood cultures, urinalysis, and site-specific cultures as indicated 3
• Reassess within 12-24 hours with repeat CBC and clinical evaluation 5

If sepsis criteria present (fever + hypotension OR lactate >3 mmol/L OR organ dysfunction):

• Initiate broad-spectrum empiric antibiotics within 1 hour of recognition 3, 2
• Aggressive fluid resuscitation for hypotension 3
• Vasopressor support if hypotension persists despite fluids 3
• Source control measures (drainage of abscesses, removal of infected catheters) 3

Alternative Diagnoses to Consider

Medication-induced leukocytosis: Review current medications—corticosteroids, lithium, and beta-agonists commonly cause neutrophilia. 1, 6

Physiologic stress: Recent surgery, trauma, seizures, or intense physical/emotional stress can elevate WBC counts. 6

Myeloproliferative disorder: The concurrent elevation of all three cell lines (neutrophils, lymphocytes, monocytes) is somewhat atypical for simple reactive leukocytosis. If no infection source is identified and leukocytosis persists, consider peripheral blood smear examination for dysplasia, basophilia (suggests chronic myeloid leukemia), or immature cells. 2, 6, 7 However, your WBC count of 15.5 is not in the range typically seen with primary bone marrow disorders (usually >30,000-50,000/mm³). 2, 6

Critical Pitfalls to Avoid

Do not ignore elevated absolute neutrophil count when total WBC is only mildly elevated—left shift can occur with normal or near-normal WBC and still indicate serious bacterial infection. 1, 3, 2

Do not rely on automated differential alone—manual review is essential to identify band forms, toxic granulations, and immature cells that automated analyzers miss. 1, 3, 2

Do not delay antibiotics in severe sepsis or septic shock while awaiting culture results—every hour of delay increases mortality. 3, 2

Do not treat asymptomatic patients with antibiotics based solely on laboratory findings—if the patient is asymptomatic and hemodynamically stable, complete the diagnostic workup first. 3