Immunochromatographic Test for Rapid Detection of Streptococcus pneumoniae in the Nasopharynx

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.