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Diagnostic Utility of Pneumococcal Urinary Antigen Assay

Two highly promisingstudies examine a 15-minute immunochromatographic assay designed to detect Streptococcus pneumoniae C polysaccharide in urine.

Given the difficulty in identifying precise microbial etiologies in patients with pneumonia, there is a clear need for new diagnostic tests in this area. Fortunately, several recent studies suggest that a 15-minute immunochromatographic assay designed to detect Streptococcus pneumoniae C polysaccharide in urine may address this need.

Smith and colleagues performed a partially manufacturer-sponsored, prospective, controlled trial in which they collected urine samples from hospitalized adult patients with bacteremia either before or during the first 24 hours of antibiotic treatment. The pneumococcal urinary antigen assay was positive in 88 of 107 patients with pneumococcal bacteremia and in 3 of 106 patients with nonpneumococcal bacteremia. The frequency of antigen positivity was significantly higher for pneumococcal pneumonia than for other forms of pneumococcal infection (87% vs. 70%; P=0.04). The 3 false-positive assays were in patients with bacteremias due to Escherichia coli, Klebsiella pneumoniae, and enterobacter sp.; none of the 3 had apparent pneumococcal infection. In 45 patients with bacteremic pneumococcal infections, the assay was repeated during the first week of antibiotic therapy, and the results remained positive in the majority of patients through 7 days.

Murdoch and coworkers report additional information on this assay's duration of positivity and specificity. They obtained urine samples approximately 6 weeks after presentation in 80 individuals who had community-acquired pneumonia and a positive initial urine antigen assay. The assay remained positive in 38 (48%) patients. The longest duration of positivity was 89 days. They also tested urine samples from 97 patients with COPD; 49 patients were tested during acute exacerbations, and 48 were tested during routine outpatient visits. There were 3 positive assays, all from patients with acute COPD exacerbations, one of whom had a positive nasopharyngeal/sputum culture for S. pneumoniae.

Comment: This assay will be a valuable adjunct to the diagnostic work-up of adult patients with presumed community-acquired pneumonia. As noted by the authors of both papers, however, other studies have found the sensitivity and specificity to be inadequate for use in children. In addition, the prolonged duration of assay positivity will limit this assay's usefulness in patients with a recent episode of pneumonia.

— Richard T. Ellison III, MD

Published in Journal Watch Infectious Diseases August 25, 2003

Citation(s):

Smith MD et al. Rapid diagnosis of bacteremic pneumococcal infections in adults by using the Binax NOW Streptococcus pneumoniae urinary antigen test: A prospective, controlled clinical evaluation. J Clin Microbiol 2003 Jul; 41:2810-3.

• Original article (Subscription may be required)
• Medline abstract (Free)

Murdoch DR et al. The NOW S. pneumoniae urinary antigen test positivity rate 6 weeks after pneumonia onset and among patients with COPD. Clin Infect Dis 2003 Jul 1; 37:153-4.

• Medline abstract (Free)