Misdiagnosis Due to False-Positive Detection of Pneumococcal Urinary Antigen

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.