A patient with community-acquired pneumonia has a urinary antigen test (UAT) that is positive for Legionella. Which serogroup of Legionella pneumophila does the widely used Binax NOW urinary antigen test detect, and what is its sensitivity limitation?
- A All 15 serogroups of L. pneumophila equally
- B All Legionella species including L. micdadei and L. longbeachae
- C Serogroups 1 and 6 only, covering 95% of community-acquired cases
- D Serogroup 1 only — sensitivity 70–80% for Legionella pneumophila serogroup 1 (responsible for ~80% of cases) but misses other serogroups and other Legionella species ✓
Correct answer: D. Serogroup 1 only — sensitivity 70–80% for Legionella pneumophila serogroup 1 (responsible for ~80% of cases) but misses other serogroups and other Legionella species
Explanation
The Legionella urinary antigen test (Binax NOW, Sofia) is specific for L. pneumophila serogroup 1 (Lp1), which causes approximately 80–85% of community-acquired Legionnaires' disease. The sensitivity for Lp1 is approximately 74–80% with specificity >99%; it misses all other 14 serogroups of L. pneumophila and all other Legionella species (e.g., L. micdadei). A negative UAT does not rule out Legionellosis from non-Lp1 serotypes; culture on BCYE agar and/or PCR should be considered in clinically suspected cases.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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