Although pharyngitis caused by the group A β-hemolytic streptococcus (GAS) is one of the most common human infections and has been studied intensively for decades, considerable debate remains about the most appropriate method of diagnosis and treatment. Most cases of acute pharyngitis seen in primary care practice are viral in cause; GAS is the only commonly occurring cause of sore throat for which antimicrobial therapy is indicated. Moreover, the signs and symptoms of GAS pharyngitis and viral pharyngitis overlap so broadly that precise diagnosis on clinical grounds is difficult. For this reason, practice guidelines issued by the American Heart Association (1), American Academy of Pediatrics (2), and Infectious Diseases Society of America (3) advocate microbiological confirmation of the diagnosis by throat culture or a rapid antigen diagnostic test. Because the latter is generally less sensitive than throat culture, these guidelines suggest that a negative result be backed up by throat culture. The necessity for this has been questioned, however (4), especially because supposedly more sensitive rapid tests, such as optical immunoassay (OIA), have appeared on the market (5).