In a patient with an elevated white‑blood‑cell count on the complete blood count but only trace leukocytes on urinalysis, what is the likely source of leukocytosis and what is the appropriate next step in management?

Elevated WBC with Trace Urine Leukocytes: Source and Management

The leukocytosis is most likely from a systemic bacterial infection rather than a urinary source, and you should obtain a manual differential to calculate the absolute band count and assess for left shift, then pursue other infection sources based on clinical findings. 1, 2

Why the Urinary Tract is Unlikely the Source

• Trace leukocytes on urinalysis without significant pyuria makes symptomatic UTI unlikely as the cause of systemic leukocytosis 3
• In elderly patients particularly, asymptomatic bacteriuria is present in 15-50% of non-catheterized individuals and essentially 100% of those with chronic catheters, so positive urine findings often represent colonization rather than infection 3
• Do not obtain urine culture in truly asymptomatic patients even with leukocytosis, as bacteriuria does not indicate infection in this population 2

Immediate Diagnostic Steps

Calculate the Absolute Band Count

• An absolute band count ≥1,500 cells/mm³ has the highest likelihood ratio (14.5) for documented bacterial infection and should be your first priority 1, 2
• A band percentage ≥16% (left shift) has a likelihood ratio of 4.7 for bacterial infection, even when total WBC is normal 1, 2
• Manual differential count is mandatory—automated analyzers are insufficient for accurate band assessment 1, 2

Assess Clinical Parameters for Infection Source

• Respiratory: Check for cough, dyspnea, chest pain, hypoxemia; obtain pulse oximetry and chest radiography if respiratory symptoms present 1, 2
• Skin/soft tissue: Look for erythema, warmth, purulent drainage suggesting cellulitis or abscess 1
• Abdominal: Assess for peritoneal signs, diarrhea suggesting intra-abdominal infection or C. difficile colitis 1, 2
• Vital signs: Fever >38°C or hypothermia <36°C, hypotension <90 mmHg systolic, tachycardia, tachypnea 1

Laboratory Workup

• Obtain blood cultures if any signs of systemic infection are present (fever, hypotension, tachycardia, altered mental status) 1
• Check lactate level—if >3 mmol/L, this indicates severe sepsis requiring immediate intervention 1

Management Algorithm

If Hemodynamically Stable

• Complete diagnostic workup first before initiating antibiotics 1
• Target specific infection source based on clinical findings 1, 2

If Sepsis Criteria Present

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

Critical Pitfalls to Avoid

• Do not ignore elevated neutrophil count when total WBC is only mildly elevated—left shift can occur with normal WBC and still indicate serious bacterial infection 1, 2
• Do not rely on automated analyzer alone for differential count 1, 2
• Do not delay antibiotics in severe sepsis/septic shock while awaiting culture results 1
• Do not treat based solely on laboratory findings without clinical correlation if patient is asymptomatic and hemodynamically stable 1
• In older adults, absence of fever does not exclude bacterial infection due to decreased basal body temperature 2

Special Considerations

Non-Infectious Causes to Consider if No Source Found

• Physical or emotional stress (surgery, exercise, trauma) can double WBC count within hours 4, 5
• Medications: corticosteroids, lithium, beta-agonists 4, 5
• Smoking, obesity, chronic inflammatory conditions 4
• Asplenia 4

When to Suspect Malignancy

• Constitutional symptoms: fever, weight loss, bruising, fatigue 4
• Concurrent abnormalities in red blood cells or platelets 5
• Liver, spleen, or lymph node enlargement 5
• WBC >100,000/mm³ represents a medical emergency due to risk of brain infarction and hemorrhage 5, 6
• If malignancy cannot be excluded, refer to hematology/oncology 4