A retrospective outcome study4 reviewed the frequency of suppurative complications of GABHS among 30,036 patients with pharyngitis diagnosed with either RAD testing or throat culture. Patients included adults and children in a primary care setting. Complication rates were identical. A prospective study of 465 suburban outpatients with pharyngitis assessed the accuracy of RAD diagnosis using throat culture as a reference. The RAD accuracy was 93% for pediatric patients and 97% for adults.5 In another retrospective review of RAD testing, investigators performed 11,427 RAD tests over 3 years in a private pediatric group. There were 8385 negative tests, among which follow-up cultures detected 200 (2.4%) that were positive for GABHS. In the second half of the study, a newer RAD test produced a false-negative rate of 1.4%.7 Because of the possibility of higher false-negative RAD test rates in some settings, unless the physician has ascertained that RAD testing is comparable to throat culture in their own setting, expert opinion recommends confirming a negative RAD test in children or adolescents with a throat culture.1 Patients at higher risk of GABHS or GABHS complications may also warrant throat culture back up of RAD testing.1
TABLE 1
Centor clinical prediction rules for diagnosis of GABHS (for adults)
| One point for each: History of fever, anterior cervical adenopathy, tonsillar exudates, absence of cough | |||||
| Points | LR+ | Pretest prevalence of GABHS (%) | |||
| 5 | 10 | 25 | 50 | ||
| Post-test probability of GABHS (%) | |||||
| 0 | 0.16 | 1 | 2 | 5 | 14 |
| 1 | 0.3 | 2 | 3 | 9 | 23 |
| 2 | 0.75 | 4 | 8 | 20 | 43 |
| 3 | 2.1 | 10 | 19 | 41 | 68 |
| 4 | 6.3 | 25 | 41 | 68 | 86 |
| GABHS, group A beta-hemolytic streptococcus; LR+, positive likelihood ratio. | |||||
| Adapted from data in Ebell et al 2000.5 | |||||
TABLE 2
McIssac clinical prediction rules for diagnosis of GABHS (for adults and children)
| One point for each: History of fever (or measured temperature >38°C), absence of cough, tender anterior cervical adenopathy, tonsillar swelling or exudates, age <15. Subtract 1 point if age 45 or more | |||||
| Points | LR+ | Pretest prevalence of GABHS (%) | |||
| 5 | 10 | 25 | 50 | ||
| Post-test probability of GABHS (%) | |||||
| –1 or 0 | 0.05 | <1 | 1 | 2 | 5 |
| 1 | 0.52 | 3 | 5 | 15 | 33 |
| 2 | 0.95 | 5 | 10 | 24 | 47 |
| 3 | 2.5 | 12 | 22 | 45 | 56 |
| 4 or 5 | 4.9 | 20 | 35 | 62 | 71 |
| GABHS, group A beta-hemolytic streptococcus; LR+, positive likelihood ratio. | |||||
| Adapted from data in Ebell et al 2000.5 | |||||
Recommendations from others
The Infectious Diseases Society of America recommends that if the physician is unable to exclude the diagnosis of GABHS on epidemiological or clinical grounds, either RAD testing or throat culture should be done. A positive result warrants treatment for patients with signs and symptoms of acute pharyngitis. A negative RAD result for a child or adolescent should be confirmed by throat culture unless the physician has ascertained that the sensitivity of RAD testing and throat culture are comparable in his or her practice setting.1
The American Academy of Pediatrics also recommends laboratory confirmation of GABHS pharyngitis in children with throat culture or RAD testing. If a patient suspected clinically of GABHS has a negative RAD test, a throat culture should be done. Since some experts believe RAD tests using optical immunoassay are sufficiently sensitive to be used without throat culture backup, physicians who wish to use them should validate them by comparison to throat culture in their practice.8
Clinical commentary
The RAD test helps to avoid overprescribing antibiotics
Peter Danis, MD
St. John’s Mercy Medical Center, St. Louis, Mo
The patient with a sore throat presents a diagnostic dilemma at 8:00 in the evening or on a Sunday morning. Patients (or parents) want something done, and frequently request antibiotics. Most of the time, they appreciate accurate information on the likelihood of a sore throat being a “strep throat” and the benefit or lack of benefit of antibiotics. The “in-between” cases are the toughest to manage, and the RAD test gives us the additional information needed to avoid overprescribing antibiotics. Empathetic reassurance and symptomatic treatment still suffice in most cases.