RADTs developed for the identification of GAS directly from throat swabs are more expensive than blood agar cultures, but offer speed in providing results. Rapid identification and treatment of patients with GAS pharyngitis can reduce the risk of the spread of GAS, allow the patient to return to school or work sooner, and speed clinical improvement.21,30 In addition, in certain environments (e.g., emergency departments) the use of RADTs compared with throat cultures has significantly increased the number of patients appropriately treated for GAS pharyngitis.31,32
The majority of currently available RADTs have specificities of ≥95% compared with blood agar cultures.33 Therapeutic decisions, therefore, can be made with confidence on the basis of a positive RADT result. However, the sensitivity of RADTs is between 70% and 90%.33 Although some patients with falsely negative RADT results merely are GAS carriers, a large proportion truly are infected with GAS.34
The first RADTs utilized latex agglutination methodology, were relatively insensitive, and had unclear endpoints.33 Subsequent tests based on enzyme immunoassay techniques had a more sharply defined endpoint and increased sensitivity. RADTs using optical immunoassay (OIA) and chemiluminescent DNA probes may be more sensitive than other RADTs and perhaps even as sensitive as blood agar plate cultures,33 but because of conflicting and limited data about the OIA and other commercially available RADTs, advisory groups still recommend a confirmatory blood agar culture for children and adolescents who are suspected on clinical grounds of having GAS pharyngitis and have a negative RADT result.
The relative sensitivities of different RADTs can only be determined by direct comparisons in the same study. There have been only five reports of direct comparisons of different RADTs.35–39 Only a handful of studies have investigated the performance of RADTs in actual clinical practice and physician investigators have concluded differently about adequacy of test performance.29,36–41 In one study,29 performed over three winter periods and using on-site office testing in a pediatric group practice, RADT had a sensitivity of approximately 85% compared with a single blood agar plate culture. Investigators in a different pediatric group practice reviewed their experience with 11,427 RADTs performed between 1996 and 1999.42 Only 2.4% of specimens negative by RADT were positive by culture.42 A retrospective review of over 19,000 clinical RADTs performed in a heterogeneous inpatient and outpatient group demonstrated a negative predictive value (NPV) ranging from 90% to 96% and a maximum sensitivity of 77% to 86%.39 Physicians electing to use any RADT in children and adolescents without culture backup of negative results should do so only after demonstrating with adequate sample size calculation that the RADT is as sensitive as throat culture in their own practice.14,15
Neither blood agar culture nor RADT accurately differentiates individuals with GAS pharyngitis from carriers. However, use facilitates withholding antimicrobial therapy in the great majority of patients with GAS sore throat. There are an estimated 6.7 million visits to primary care providers by adults who complain of sore throat each year in the U.S.; antimicrobial therapy historically was prescribed at 73% of these visits.43 With encouragement for judicious use of antibiotics, trends show a modest decline in the use in children and adolescents diagnosed with pharyngitis to 69% in one study in 1999 to 2000,44 and to 54% in another study in 2003.45