A 73-year-old man was admitted to the hospital because of breathing difficulties, chest pain, chills, and fever of several days’ duration. He had been well until 1 week before admission, when he noted...

A 73-year-old man was admitted to the hospital because of breathing difficulties, chest pain, chills, and fever of several days’ duration. He had been well until 1 week before admission, when he noted the onset of a persistent headache and a productive cough. The patient smoked two packs of cigarettes a day for more than 50 years and drank a six-pack of beer daily; he also had a history of bronchitis. Physical examination results revealed an elderly man in severe respiratory distress with a temperature of 39° C, pulse of 120 beats/min, respiratory rate of 36 breaths/ min, and blood pressure of 145/95 mm Hg. A chest radio graph revealed an infiltrate in the middle and lower lobes of the right lung. The white blood cell count was 14,000 cells/mm³
(80% polymorphonuclear neutrophils). Gram stain of the sputum showed neutrophils but no bacteria, and routine bacterial cultures of sputum and blood were negative for organisms. Infection with L. pneumophila was suspected.

7. What laboratory tests can be used to confirm this diagnosis? Why were the routine culture and Gram-stained specimen negative for Legionella organisms?