Discordant Rate of Simultaneous Duplicate Throat Swab Culture for Discovering Beta-hemolytic Streptococcal Carrier from Normal School Children

Cha, Sung Ho; Han, Mi Young; Choi, Yong Mook; Kil, Young Chul; Suh, Jin Tae

Abstract

PURPOSE
The most patients with acute streptococcal pharyngitis lack of classic clinical manifestations, therefore diagnostic laboratory test such as the throat culture or a rapid antigen detection test are frequently employed in primary practices of developed countries. We'd like to know the accuracy of the throat swab culture as gold standard for diagnosis of streptococcal infection with studying the discordant and concordant rate of duplicate culture.
METHODS
The study included 89 normal school children (boys:50, girls:39) who were attending Uljin primary school in Uljin, Kyong Sang Buk Do on March 1996. We obtained simultaneous 2 times of throat swab from each subject, and plating and streaking on 5-7% of sheep blood agar separately. We counted the characteristic beta-hemolytic colonies after overnight incubation.
RESULTS
1) The carrier rate of beta-hemolytic streptococci at first culture is 25.1% and second one is 29.2%. 2) Ten out of 89(11.2%) is discordant in duplicate culture. 3) Culture containing less than 50 colonies of beta-hemolytic streptococci (+2) in first culture is 70.4%, second one is 85.7%. 4) Number of colonies is less than 50 in all ten discordant children.
CONCLUSION
The discordant rate of duplicate throat swab cullture for beta-hemolytic streptococci is 11.2%, even if the subjects are normal school children. About 5% of individuals harboring beta-hemolytic streptococci in the pharynx may be missed by a single throat culture. If we are trying to examine the patients with pharyngitis, the discordant rate will be much lower than this results.