Immunochromatographic Test for Rapid Detection of Streptococcus pneumoniae in the Nasopharynx

Abstract

Faden et al. recently reported the use of the NOW immunochromatographic test to rapidly detect nasopharyngeal colonization with Streptococcus pneumoniae (1). While the specificity of the test in this study was relatively high (97.7%), it is important to be aware that oropharyngeal colonizing bacteria other than S. pneumoniae may also produce positive results with this test. Specifically, Streptococcus mitis shares the antigen against which the NOW test is directed. This cross-reaction is recorded by the manufacturer in the product instructions and is supported by the findings from our own research.We noted a relatively high rate of positive results when evaluating the NOW test as an alternative method for rapidly identifying S. pneumoniae isolates. We tested 96 S. pneumoniae isolates and 83 alpha-hemolytic nonpneumococcal streptococcal isolates obtained mainly from blood cultures and respiratory samples. All S. pneumoniae isolates were positive by the NOW test, as well as 20 of the nonpneumococcal isolates. The latter isolates were all bile insoluble and optochin resistant and were identified to species level by the API 20 Strep identification system (bioMerieux Vitek, Inc., Hazelwood, Mo.), supplemented by conventional test procedures. Eighteen of these isolates were identified as belonging to the S. mitis group, and the remaining two isolates were identified as Gemella morbillorum and Gemella haemolysans.When the NOW test is used to detect S. pneumoniae antigen in urine from patients with pneumonia, this cross-reaction with S. mitis is unlikely to be important. In contrast, anyone using the NOW test to detect nasopharyngeal colonization should be aware of this possible source of false-positive results. Indeed, it is perhaps surprising that the false-positive rate was not higher in the study by Faden et al. Author's Reply I thank Drs. Murdoch and Reller for their timely comments. We were aware of the potential for cross-reactivity with S. mitis when we began the study, and we were very surprised at the lack of false-positive results. All of our specimens were collected from the nasopharynx as opposed to the oropharynx or throat. It is quite possible that S. mitis may have been relatively uncommon in our population in comparison to S. pneumoniae. It is also quite possible that S. mitis is found relatively less frequently in the nasopharynx than in the oropharynx. I think that the test deserves further clinical study as a rapid test for S. pneumoniae in the airway to validate the high sensitivity and specificity demonstrated in our study.

Streptococcus pneumoniae Antigen Test Using Positive Blood Culture Bottles as an Alternative Method To Diagnose Pneumococcal Bacteremia

“…Also, pneumococcal antigen was detected for 30 days following initial subculture for all bottles. False-positive reactions may occur, and several authors have demonstrated that viridans group streptococci, in particular the Streptococcus mitis group, may cross-react with the pneumococcal C polysaccharide antigen (1,4,8,11). All three false-positive results in our study were due to organisms within the S. mitis group.…”

Characteristics of Streptococcus pseudopneumoniae Isolated from Purulent Sputum Samples

“…Although sharing many features in common with S. pneumoniae and S. mitis, S. pseudopneumoniae can be readily identified in a clinical laboratory by the combination of some simple phenotypic tests, particularly tests for optochin susceptibility and bile solubility. Interestingly, S. pseudopneumoniae, like S. mitis (8), produces a positive result with the NOW S. pneumoniae antigen test, indicating that the antigen is shared between the species. Unlike the findings previously reported by Arbique et al (1), who detected the pneumolysin gene in all of their S. pseudopneumoniae strains, five of our isolates tested negative for this gene.…”

Diagnostic Usefulness of Ribosomal Protein L7/L12 for Pneumococcal Pneumonia in a Mouse Model

“…Testing for the presence of pneumococcal pneumonia with the CAD kit has been reported to exhibit longterm antigen-positive results regardless of antibiotic treatment (21,30). We therefore evaluated the effect of antibiotic treatment on RP-L7/L12 production in the mouse pneumonia model to determine whether this was a more sensitive and accurate readout of infection.…”