At a Glance
Differentiation between viral and bacterial etiologies based on physical exam alone may be difficult. Acute pharyngitis may present with a variety of signs and symptoms, including redness and edema of the throat and pharyngeal mucosa, sore throat and pain when swallowing, exudation of tonsils, pseudomembrane formation, edema of the uvula, and gray coating of the tongue (strawberry tongue). Fever, cervical node enlargement, and leukocytosis are present in both viral and streptococcal (strep) pharyngitis, but may be more marked in strep.
What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?
Gram stain and bacterial culture of the posterior pharynx (throat culture); a swab of tonsils, if abscess or exudate is present; or a nasopharyngeal swab will be useful to determine the etiology. A rayon or dacron swab, not cotton, will enhance organism isolation.
Rapid detection tests with variable sensitivity and specificity are available for detection of group A streptococci and influenza.
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Rapid direct (nonculture) detection of virus-infected cells using immunofluorescence [direct fluorescent antibody (DFA)] has a turnaround of less than 1 day and may be diagnostically helpful, since viral culture results can take several days to weeks; however, the test may still be negative in the presence of viral infection.
To preserve sensitivity, viral culture specimens, such as a swab of the posterior wall of the nasopharynx or throat, bronchial washings or lavage, or lung biopsy, should be collected as soon as possible after onset of the disease.
Polymerase chain reaction (PCR) testing is not available at all institutions; however, if a rapid diagnosis would provide immediate changes in clinical therapy, as in severely ill or immunocompromised patients, the increased cost of testing may be warranted.
Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications – OTC drugs or Herbals – that might affect the lab results?
Results should be interpreted with caution, because a variety of potentially pathogenic normal flora and the potential presence of infection in adjacent sites may exist. Nasal cultures for bacteria rarely provide useful clinical information. Specimens for viral culture should be brought to the laboratory in viral holding media as quickly as possible to ensure sensitivity. Negative viral culture does not rule out viral etiology.
What Lab Results Are Absolutely Confirmatory?
Culture isolation is absolutely confirmatory. A negative rapid test must be confirmed via culture. Culture isolation is confirmatory of a positive rapid screen result. Although usually viral in origin, pharyngitis may also be caused by β-hemolytic group A Streptococcus (Streptococcus pyogenes), Corynebacterium diphtheriae, Neisseria gonorrhoeae, Bordetella pertussis, and Candida albicans. Respiratory viruses, such as influenza, parainfluenza, adenovirus, enterovirus, Epstein-Barr virus (EBV), reovirus, rhinovirus, respiratory syncytial virus (RSV), herpes simplex virus (HSV), and varicella zoster virus (VZV) may also cause pharyngitis.
What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?
An antistreptolysin O (ASO) titer may indirectly demonstrate antistreptococcal antibodies in serum during convalescence by showing a significant rise of greater than 2 serial dilutions over not less than a 2-week period.
Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications – OTC drugs or Herbals – that might affect the lab results?
ASO titers may rise in only 75-80% of strep infections.
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