What is the diagnosis for a 17-year-old male presenting with fever, headache, and increased respiratory rate, despite normal laboratory results and imaging studies?

Metabolic Acidosis from Hyperventilation (Likely Viral Illness with Compensatory Tachypnea)

The most likely diagnosis is a viral respiratory infection with compensatory tachypnea, potentially progressing to metabolic acidosis or early sepsis, and you must immediately obtain an arterial blood gas (ABG) to assess for metabolic acidosis and lactate levels to rule out occult sepsis. 1, 2

Immediate Diagnostic Priorities

Obtain arterial blood gas immediately to assess:

• pH status (metabolic vs respiratory acidosis)
• Lactate levels (elevated in sepsis/tissue hypoperfusion)
• Base deficit (marker of metabolic acidosis) 2

Check additional urgent labs not yet obtained:

• Procalcitonin (elevated in bacterial infections, though not definitive) 1
• C-reactive protein and erythrocyte sedimentation rate (elevated in inflammatory/infectious processes) 1, 3
• Blood cultures if sepsis suspected (though yield is low without hemodynamic instability) 3
• Repeat complete blood count focusing on lymphocyte count (lymphopenia suggests viral infection including COVID-19) 1

Critical Differential Diagnosis

Primary considerations given isolated tachypnea with normal vital signs:

• Metabolic acidosis with respiratory compensation - The respiratory rate of 28/min in a 17-year-old (normal <20/min) with normal SpO2 and hemodynamics suggests compensatory hyperventilation for metabolic acidosis 1, 2

• COVID-19 or other viral respiratory infection - Fever, headache, and tachypnea (≥30/min indicates severe pneumonia in adults per WHO criteria) are characteristic, even with normal chest X-ray initially 1

• Early sepsis with occult hypoperfusion - Normal blood pressure and pulse do not exclude sepsis; lactate elevation and metabolic acidosis may precede hemodynamic collapse 1, 2

• Viral encephalitis - The combination of fever, headache, and altered respiratory drive warrants consideration, though normal neuroimaging is reassuring 1

Why Normal Initial Workup Doesn't Exclude Serious Pathology

COVID-19 and viral pneumonias can present with:

• Normal chest X-ray early in disease (CT more sensitive showing ground-glass opacities) 1
• Normal or decreased leukocyte counts (characteristic of viral infections) 1
• Normal liver and kidney function initially 1

The tachypnea is the critical warning sign - In patients >5 years old, respiratory rate ≥30/min indicates severe pneumonia even without other findings 1

Essential Next Steps

Perform RT-PCR testing for:

• SARS-CoV-2 (COVID-19) - throat swab, nasopharyngeal swab 1
• Influenza A/B 1, 4
• Other respiratory viral panel (RSV, parainfluenza, adenovirus, metapneumovirus) 1

Consider chest CT scan if:

• Tachypnea persists or worsens
• Clinical deterioration occurs
• ABG shows hypoxemia not reflected in pulse oximetry 1

Lumbar puncture indications (if no contraindications):

• Persistent fever with headache and any mental status changes
• Send CSF for bacterial cultures, viral PCR panel (including SARS-CoV-2 if COVID suspected), cell count, protein, glucose 1, 2

Critical Pitfalls to Avoid

Do not dismiss isolated tachypnea - This is often the earliest sign of respiratory compensation for metabolic acidosis or early respiratory failure 1, 2

Normal pulse oximetry does not exclude significant pathology - Early COVID-19 and other viral pneumonias may show "silent hypoxia" where patients maintain adequate oxygenation initially despite significant lung involvement 1

Fever with headache and neurological signs requires aggressive evaluation - Even with normal CT head, viral encephalitis (including COVID-19 CNS involvement) can present this way, and CSF SARS-CoV-2 PCR may be positive when nasopharyngeal swabs are negative 1

Do not attribute tachypnea to fever alone - While fever can increase respiratory rate, a rate of 28/min in a 17-year-old exceeds what fever alone would cause and demands investigation for metabolic or respiratory pathology 1, 2

Empiric Treatment Considerations

Aggressive fever control:

• Administer acetaminophen immediately when temperature reaches 37.5°C, as fever is independently associated with poor neurological outcomes 2

If influenza suspected and within 48 hours of symptom onset:

• Consider oseltamivir 75 mg twice daily for 5 days (reduces illness duration by 1.5 days in adolescents) 4

Do NOT start empiric antibiotics unless:

• Evidence of bacterial superinfection develops
• Patient becomes hemodynamically unstable
• Specific bacterial pathogen identified 1, 5, 3

Monitor closely for deterioration:

• Increasing respiratory rate
• Declining mental status
• Hemodynamic instability
• Development of hypoxemia 1, 2