A 58-year-old man is ventilated for severe community-acquired pneumonia. Bronchoscopy and bronchoalveolar lavage (BAL) are performed to obtain microbiological samples. The BAL fluid returns 45% neutrophils with intracellular organisms on Gram stain. The BAL culture subsequently grows Legionella pneumophila. In addition to appropriate antibiotics, which laboratory test is most useful for rapid diagnosis of Legionella in the ICU?