infectious diseases
Streptococcal Infections
Diagnosis of Streptococcal Infections
Culture Sometimes rapid antigen tests or antibody titers Streptococci are readily identified by culture on a sheep blood agar plate. 95%) but vary considerably in sensitivity (55% to 80 to 90% for the newer optical immunoassay test) ( Diagnosis reference ). Thus, positive results can establish the diagnosis, but negative results, at least in children, should be confirmed by culture. Because streptococcal pharyngitis is less common among adults and adults are unlikely to have poststreptococcal complications, many clinicians do not confirm a negative rapid screening result in adults by culture unless use of a macrolide is being considered; in such cases, culture with susceptibility testing to detect macrolide resistance should be done. The Centor criteria can be used to guide decisions about testing for GABHS or selecting empiric antibiotic treatment for patients with pharyngitis. The criteria are age, presence of tonsillar exudate, anterior cervical lymphadenopathy, fever > 40° C, and absence of cough. Demonstratingantistreptococcal antibodiesin serum during convalescence provides only indirect evidence of infection. Antistreptococcal antibody tests are not useful in diagnosing acute GABHS infection because antibody first develops several weeks after GABHS infection begins and a single high antibody titer is more likely to reflect a long antecedent infection. Antibodies are most useful in diagnosis of poststreptococcal diseases, such as rheumatic fever and glomerulonephritis. Antistreptolysin O (ASO) and antideoxyribonuclease B (anti-DNase B) titers begin to increase about 1 week after the GABHS infection and peak about 1 to 2 months after the infection. Both titers may remain elevated for several months, even after uncomplicated infections. The ASO titer increases in only 75 to 80% of infections. For completeness in difficult cases, any one of the other tests (antihyaluronidase, antinicotinamide adenine dinucleotidase, antistreptokinase) can also be used. Titers are measured in the acute phase and in the convalescent phase 2 to 4 weeks later; a positive result is defined as a ≥ 2-fold increase in the titer. A single titer greater than the upper limit of normal suggests an antecedent streptococcal infection or high streptococcal endemicity in the community. Penicillin given within the first 5 days for symptomatic streptococcal pharyngitis may delay the appearance and decrease the magnitude of the ASO response. Patients with streptococcal pyoderma usually do not have a significant ASO response but may have a response to other antigens (ie, anti-DNAase, antihyaluronidase). 1.Plainvert C, Duquesne I, Touak G, et al: In vitro evaluation and comparison of 5 rapid antigen detection tests for the diagnosis of beta-hemolytic group A streptococcal pharyngitis.Diagn Microbiol Infect Dis83(2):105–111, 2015. doi: 10.1016/j.diagmicrobio.2015.06.012 