A 64-year-old man with pneumonia has hyponatremia on laboratory testing. Which finding most supports Legionella as the causative pathogen?

• A. Hypoglycemia
• B. Hyponatremia
• C. Infiltrate in the upper lobes
• D. Pleuritic chest pain

Answer: (B)

Hyponatremia in pneumonia is a classic clue pointing toward Legionella infection. Legionella pneumophila often causes an atypical pneumonia with an SIADH-like syndrome, leading to low sodium levels. This electrolyte abnormality is more characteristic of Legionella than of many other common pneumonias, so its presence strongly supports this pathogen in the setting of pneumonia.

Other features described are less specific. Infiltrates can appear in various parts of the lungs and aren’t reliably confined to upper lobes. Pleuritic chest pain is a common symptom in many pneumonias and doesn’t distinguish Legionella. Hypoglycemia is not a typical hallmark of Legionella infection. So the hyponatremia itself is the finding that best points to Legionella as the cause.