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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Upper Respiratory Tract Infections and Stomatitis

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General Diagnostic Approaches

Although viruses cause the vast majority of upper respiratory infections, they are gener-ally not amenable to specific therapy, and laboratory tests for viral infections are usually reserved for investigating outbreaks or in cases in which the illness seems unusually severe or atypical.

GENERAL DIAGNOSTIC APPROACHES

Although viruses cause the vast majority of upper respiratory infections, they are gener-ally not amenable to specific therapy, and laboratory tests for viral infections are usually reserved for investigating outbreaks or in cases in which the illness seems unusually severe or atypical.

The primary diagnostic approach in pharyngitis and tonsillitis is to determine whether there is a bacterial cause requiring specific treatment. The only reliable method is to col-lect a throat swab for culture, taking care to thoroughly swab the tonsillar fauces as well as the posterior pharynx, and to include any purulent material from inflamed areas. Cul-tures are usually made only to detect the presence or absence of group A streptococci. Di-rect antigen tests for rapidly detecting S. pyogenes in throat swabs have gained popularity in recent years. These are usually enzyme immunoassay or latex agglutination – based methods. The most common limitation of such tests is lack of sensitivity; that is, false-negative results can occur.

For the laboratory diagnosis of diphtheria or pharyngeal gonorrhea, the clinical suspi-cion should be indicated to the laboratory so that specific cultures for C. diphtheriae or N. gonorrhoeae may be made. Candida species, fusospirochetal bacteria, Pseudomonas species, and other Gram-negative organisms are often found in pharyngeal or oral speci-mens from healthy individuals as well as in certain infections. Their probable pathogenic significance in association with disease in these sites, largely based on the appearance of the lesions and the presence of the organisms in large numbers, can be supported by his-tologic demonstration of tissue invasion by the organisms. It is important to remember that other bacterial pathogens such as Streptococcus pneumoniae, Staphylococcus aureus,Haemophilus influenzae, and even Neisseria meningitidis may be present in the pharynx.These organisms are not primary etiologic agents in rhinitis, pharyngitis, and tonsillitis, and their presence in the throat does not implicate them as causes of the illnesses; they should instead be regarded as colonizers.

The laboratory diagnosis of causes of peritonsillar and retropharyngeal abscesses is based on Gram staining and culture of purulent material obtained directly from the lesion, including anaerobic cultures.


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