In a 48‑year‑old woman admitted for acute stroke who has fever, cough, a clear chest radiograph, normal abdominal/pelvic CT, and a rising leukocytosis (white blood cell count 19.4 ×10⁹/L increasing to 27.1 ×10⁹/L), what is the most likely cause of the elevated white blood cell count?

Elevated WBC in Acute Stroke: Most Likely Cause

In this 48-year-old woman with acute stroke, fever (100.2°F), cough, clear chest X-ray, and rapidly rising leukocytosis (19.4→27.1 ×10⁹/L), the most likely cause is stroke-associated pneumonia (SAP) that has not yet developed radiographic changes, representing "probable SAP" rather than definite SAP. 1

Primary Diagnostic Consideration: Stroke-Associated Pneumonia

Stroke-associated pneumonia is the leading infectious complication in acute stroke patients and accounts for 15-25% of stroke-related deaths. 1 The clinical presentation here strongly suggests SAP:

• Fever (100.2°F = 37.9°C) with cough in the acute stroke setting is highly suspicious for SAP, even with a clear initial chest X-ray. 1
• The rising WBC count (19.4→27.1) indicates progressive bacterial infection rather than simple stress response. 1, 2
• Chest X-rays are frequently normal early in pneumonia—typical changes were present in only 36% of initial radiographs in pneumonia patients. 1

Why This Meets "Probable SAP" Criteria

The modified CDC criteria for probable SAP require: 1

1. At least one of: Fever >38°C (she has 37.9°C, borderline), leukocytosis >12,000 (she has 27,100), or altered mental status (stroke itself)
2. At least two respiratory findings: New cough (present), purulent sputum/increased secretions, dyspnea, tachypnea, rales/crackles, or worsening gas exchange
3. Non-confirmatory CXR but no alternative diagnosis = Probable SAP 1

This patient clearly meets criteria for probable SAP, which is pneumonia that has not yet evolved radiographic changes or where CXR is inadequate. 1

Critical Next Step: Obtain Manual Differential Count

Before attributing leukocytosis solely to infection, immediately obtain a manual differential count to assess for left shift, as this has the highest diagnostic accuracy for bacterial infection. 1, 2, 3

• An absolute band count ≥1,500 cells/mm³ has a likelihood ratio of 14.5 for documented bacterial infection, even without fever. 1, 2
• Band percentage ≥16% (left shift) has a likelihood ratio of 4.7 for bacterial infection. 1, 2
• Automated analyzers cannot reliably detect band forms—manual differential is mandatory. 2, 3

Alternative Causes to Consider (But Less Likely)

Stress Response from Stroke Alone

While leukocytosis commonly occurs as a stress response in acute stroke, several factors argue against this being the sole explanation: 4, 5, 6

• Stress-induced leukocytosis typically occurs immediately at stroke onset and remains stable, not progressively rising (19.4→27.1). 6
• The presence of fever (100.2°F) and cough cannot be explained by stroke alone. 1
• Studies show that leukocytosis from stroke stress response reflects initial stroke severity but does not independently worsen or progress without infection. 6

Urinary Tract Infection

UTI is less likely despite being common in stroke patients (15-60% incidence): 1

• She has no urinary symptoms mentioned, and the CT abdomen/pelvis was normal. 1
• The presence of cough points toward respiratory rather than urinary source. 1
• However, obtain urinalysis if any urinary symptoms develop, as UTI independently predicts poor stroke outcome. 1

Aspiration Pneumonia

Aspiration is the most common cause of stroke-associated pneumonia due to dysphagia: 1

• Dysphagia occurs in stroke patients and increases aspiration risk—a bedside water swallow test should have been performed. 1
• Aspiration pneumonia may not show immediate radiographic changes. 1

Recommended Diagnostic Algorithm

Immediate Actions (Within Hours)

1. Obtain manual differential count with absolute band count and band percentage 1, 2, 3
2. Repeat chest X-ray at 24-48 hours, as CDC criteria recommend serial imaging on days 2-7 if initial CXR negative 1
3. Obtain blood cultures before antibiotics if not already done 3
4. Assess for sputum production and obtain sputum culture if productive cough present 3
5. Perform bedside swallow assessment to evaluate aspiration risk 1

If Left Shift Present (Bands ≥1,500 or ≥16%)

Initiate empiric antibiotics for probable SAP without waiting for radiographic confirmation, as delay increases mortality. 1, 3

• SAP typically occurs within the first 48-72 hours after stroke. 1
• Early antibiotic initiation is critical, as stroke-associated pneumonia increases length of stay, mortality, and hospital costs. 1

If No Left Shift and Patient Stable

Consider repeat CXR in 24-48 hours and continue close monitoring, as radiographic changes may evolve. 1

Common Pitfalls to Avoid

• Do not dismiss fever and cough as "just stress response" when WBC is rising progressively—this indicates active infection. 1, 2
• Do not wait for definitive CXR changes before treating probable SAP in a deteriorating patient—early pneumonia may not show radiographic findings. 1
• Do not rely on automated differential alone—manual differential is essential to detect left shift. 2, 3
• Do not ignore the cough—this is a key respiratory symptom that, combined with fever and leukocytosis, strongly suggests SAP. 1
• Do not obtain urine culture without urinary symptoms, as asymptomatic bacteriuria is extremely common (15-50%) in hospitalized patients and does not require treatment. 1

Monitoring and Prevention

Implement aspiration precautions immediately: 1

• Keep head of bed elevated 30-45 degrees 1
• NPO until formal swallow evaluation completed 1
• Early mobility when medically stable 1
• Aggressive pulmonary hygiene with incentive spirometry and deep breathing exercises 1