A 66-year-old man is admitted to the intensive care unit in respiratory distress. He was attending a conference in a local hotel when he developed fevers and shortness of breath, followed by mild confusion. He also has end-stage renal disease managed with thriceweekly hemodialysis and is anuric. Chest X-ray demonstrates a patchy infiltrate in the left upper lobe. As you admit him to the intensive care unit, you are called by the emergency department for a second admission—this time for a 52-year-old woman with chronic obstructive pulmonary disease who also attended the same conference. In addition to fever and tachypnea, the second patient has a serum sodium of 119 mEq/L, diarrhea, and vomiting. Her chest X-ray demonstrates diffuse bilateral patchy infiltrates.
What is the MOST sensitive test to diagnose the organism causing these patients’ symptoms?
A. Urinary antigenCorrect Answer: B
These two patients present with Legionnaire’s disease, pneumonia due to the intracellular pathogen Legionella pneumophilia. The scenario portrayed here likely represents an outbreak because both patients were attending the same conference. Although certain features such as gastrointestinal symptoms and hyponatremia increase suspicion of Legionella pneumonia, the presentation can also closely mimic other types of CAP. PCR testing of lower respiratory tract specimens is the most sensitive diagnostic assay, although exact sensitivity and specificity are difficult to determine owing to lack of a perfect reference standard. Urinary antigen testing appears to be 70% to 80% sensitive and nearly 100% specific for Legionella disease, and it is a reasonable alternative when PCR is not available or a lower respiratory tract specimen cannot be obtained. Culture performed on lower respiratory tract specimens is nearly 100% specific for Legionella, but sensitivity varies widely from <10% up to 80% and is thought to be significantly lower than PCR. Because Legionella is an intracellular pathogen, Gram stain plays no role in diagnosis of Legionnaire disease.
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