To the Editor.—Previously, we reported on the laboratory practices used to identify group A streptococci among 790 customers of the College of American Pathologists Excel Proficiency Testing Program, a program targeted primarily at physician office laboratories and small hospital laboratories.1 In our report, the majority (57.9%) of respondents who performed bacitracin testing applied the disk to a primary plate. Because we were unable to distinguish between those who used selective media primary plates and those who used blood agar (BA) primary plates, we conducted a follow-up survey of current Excel Proficiency Testing subscribers and asked, “What method do you use to confirm group A streptococcus on throat cultures?” Among 1086 respondents, 557 (51.3%) used the bacitracin test and applied the disks to selective agar primary plates, 390 (35.9%) applied the disks to BA primary plates, and 45 (4.1%) applied the disks to subculture plates. Other methods, not specified, were used by 94 (8.7%).

This finding confirms that most office laboratories do not perform subcultures for group A streptococci, presumably because it adds another day to the testing process and is categorized as a highly complex procedure. Although selective media primary plates have been shown to effectively identify group A β-hemolytic streptococci,2 false negatives may occur if BA primary plates are used. Overgrowth with normal flora can obscure the inhibition of group A β-hemolytic streptococci, and too few organisms may be present to accurately assess inhibition zones. Accordingly, the use of BA primary plates for the presumptive diagnosis of group A β-hemolytic streptococci generally is not recommended,3 although this has been disputed.4 We believe additional studies are needed to clarify this issue. In the absence of definitive studies, we recommend that if group A β-hemolytic streptococci identification is performed with a bacitracin disk on a primary plate (not a subculture), the plate used should be a selective media plate rather than a BA plate.

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J. C.
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P.
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Wahl
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