Microbiology and laboratory diagnosis of upper respiratory tract infections

In the article that follows, Carroll and Reimer address a number of issues related to the clinical and laboratory diagnosis of upper respiratory tract infections. These syndromes occur with great frequency in both adults and children and have tremendous economic impact, related not only to lost productivity in the workplace but also to the frequent prescription by physicians of antibiotics, even when the etiologic agents of infection almost certainly are not bacteria. Most of these infections are diagnosed clinically, and specimens for microbiological identification are not obtained. Indeed, the difficulty in obtaining microbiological specimens that are not contaminated by resident colonizing flora often results in laboratory culture reports of dubious clinical value. As the authors note, the most standardized procedures are for the diagnosis of pharyngitis due to Streptococcus pyogenes. The preferred culture methods are reviewed as are the sensitivities, specificities, and limitations of rapid direct tests for group A streptococcal antigens. Currently, as the authors emphasize, a negative direct test mandates a conventional culture for S. pyogenes. More problematic are requests for isolation of other streptococci, Haemophilus species, corynebacteria, and gram-negative bacteria. Given limited resources, cost-containment imperatives, and the absence of clear evidence that these organisms are pharyngeal pathogens associated with important sequelae, my laboratory does not attempt to isolate these bacteria unless the ordering physician has directly consulted with me (the laboratory director). Carroll and Reimer emphasize that nasopharyngeal cultures have no place in the microbiological diagnosis of otitis media and that diagnostic tympanocentesis is the only procedure for obtaining specimens that yield reliable microbiological findings. They also point out the futility of using swabs to obtain material for the diagnosis of otitis externa, since the external auditory canal cannot be decontaminated sufficiently to obtain a meaningful culture result. Finally, the authors address the available methods for obtaining specimens to establish the etiology of sinusitis. For microbiological diagnosis, direct antral puncture has been the method of choice for many years. However, otorhinolaryngologists now obtain many specimens endoscopically. It probably is not possible to obtain specimens by this method without contamination by normal upper respiratory flora. Thus, results of cultures of endoscopic specimens are more difficult to interpret. For patients with complicated illnesses, use of the diagnostic "gold standard" of antral puncture, as well as biopsy with histopathologic correlation, should be encouraged.