EVIDENCE-BASED ANSWER
Standardized clinical decision rules, such as the Centor criteria, can identify patients with low likelihood of group A beta-hemolytic streptococ-cal (GABHS) pharyngitis who require no further evaluation or antibiotics (strength of recommendation [SOR]: A, based on validated cohort studies). For patients at intermediate and higher risk by clinical prediction rules, a positive rapid anti-gen detection (RAD) test is highly specific for GABHS (SOR: A, based on systematic reviews of diagnostic trials).
A negative RAD test result, using the best technique, approaches the sensitivity of throat culture (SOR: B, based on retrospective cohort studies). In children and populations with an increased prevalence of GABHS and GABHS complications, adding a backup throat culture reduces the risk of missing GABHS due to false-negative RAD results (SOR: C, based on expert opinion).
Evidence summary
In the US, GABHS is the cause of acute pharyn-gitis in 5% to 10% of adults and 15% to 30% of children. It is the only commonly occurring cause of pharyngitis with an indication for antibiotic therapy.1 The main benefit of antibiotic treatment in adults is earlier symptom relief—1 fewer day of fever and pain if antibiotics are begun within 3 days of onset.
Antibiotic treatment also reduces the incidence of acute rheumatic fever, which complicates 1 case per 100,000 in most of the US and Europe (relative risk reduction [RRR]=0.28).2 The risk of acute rheumatic fever is higher in some populaHawaiians (13–45 per 100,000).3 Treatment may also reduce suppurative complications (peritonsil-tions, particularly Native Americans and lar or retropharyngeal abscess), which occur in 1 case out of 1000.2,4
A systematic review of the diagnosis of GABHS evaluated the accuracy of history and physical exam elements.5 Clinical prediction rules based on selected symptoms and signs can identify patients at low risk for GABHS. The 4 Centor criteria (history of fever, anterior cervical adenopathy, tonsillar exudates, absence of cough) are well validated in adult populations ( Table 1 ), while other clinical prediction rules (such as McIssac) are validated in populations with children and adults ( Table 2 ). The number of criteria present determines the likelihood ratio (LR), with which to calculate the posttest probability of GABHS.
The usefulness of clinical prediction rules depends on knowing how prevalent GABHS is among cases of pharyngitis in a particular community. In a typical US adult population, GABHS comprises 5% to 10% of cases. The presence of only 1 Centor criterion would reduce the probability of GABHS pharyngitis to 2% to 3%, while meeting all 4 criteria would raise the probability to 25% to 40%, an intermediate value ( Table 1 ). If the prevalence of GABHS pharyngitis were 50%, as in some Native communities in Alaska, meeting all 4 criteria would predict an 86% probability of pharyngitis due to GABHS. Performing additional testing for patients with intermediate or high probability based on clinical prediction rules reduces the likelihood of unnecessary antibiotic treatment.1
A systematic review6 of RAD testing demonstrates that the newer techniques (optical immunoassay, chemiluminescent DNA probes) have a sensitivity of 80% to 90%, which compares closely with that of throat culture (90%–95%). Both have a specificity greater than 95%, so false-positive test results are uncommon (LR+ =16–19). Treatment based on a positive RAD test would result in few unnecessary antibiotic prescriptions.1