Misdiagnosis Due to False-Positive Detection of Pneumococcal Urinary Antigen

Abstract

Introduction: The detection of pneumococcal antigens in urine is an alternative to gram staining, and their culture is central to the diagnosis of pneumococcal pneumonia. We present a case of the false-positive detection of urinary Streptococcus species pneumococcal antigen with a BinaxNOW test. This resulted in delayed diagnosis of a liver abscess. Case description: A 70-year-old woman presented to the emergency department with a 1-day history of chills and difficulty walking. She had a fever and her physical examination was normal. Non-contrast chest computed tomography (CT) revealed a slight ground-glass opacity in the left lower lobe. Laboratory tests revealed liver injury and elevated C-reactive protein levels. A urinary pneumococcal antigen test was positive, and she was diagnosed with acute bronchopneumonia caused by Streptococcus pneumoniae. She was treated with ceftriaxone. However, abdominal contrast-enhanced CT performed the next day revealed portal vein thrombus and a left lobe liver abscess. Streptococcus constellatus was detected in a puncture specimen of the liver abscess. It was concluded that the positive urinary pneumococcal antigen test was a false-positive owing to Streptococcus infection. Discussion: False-positive results might be explained by the presence of C-polysaccharide antigens in the cell wall of S. pneumoniae. The positive urinary antigen test together with the finding of slight ground-glass opacity in the left lung on chest CT initially led to misdiagnosis. False positives may result in misdiagnosis and unnecessary antimicrobial therapy. Conclusion: The overuse of the pneumococcal urinary antigen tests can lead to false positives and misdiagnosis. LEARNING POINTS • False-positive pneumococcal urinary antigen results may lead to the misdiagnosis of pneumococcal pneumonia caused by Streptococcus pneumoniae and unnecessary antimicrobial therapy. • False-positive results can occur in patients with infections caused by other Streptococcus species (e.g., liver abscess caused by Streptococcus constellatus). • The optimal use of pneumococcal urinary antigen tests needs to be studied in clinical practice settings.