False-positive rapid antigen detection test results: reduced specificity in the absence of group A streptococci in the upper respiratory tract.

Rapid antigen detection tests (ADTs) are promoted for identification of group A streptococci (GAS) in the upper respiratory tract. Although debate persists about their sensitivity, most investigators consider ADTs to be highly specific. Nevertheless, reports continue to describe false-positive ADT results (positive ADT result, negative culture result). This study examined culture results from 522 adult patients with acute pharyngitis and a positive ADT result; unexpectedly, 15% had throat culture results negative for GAS. Normal bacterial flora from 30% of these "negative" culture results produced a positive ADT result. An ADT-reactive organism belonging to the Streptococcus milleri group was isolated from 1 culture; it carried group A carbohydrate antigen. Although S. milleri previously has been associated with false-positive ADT reactions, the frequency of this phenomenon is unknown. The present, large study suggests that false-positive ADT reactions may be more common than previously reported, an observation that must be precisely explained and epidemiologically and clinically defined.