Objective: To determine the accuracy and utility of rapid antigen detection tests for group A beta-hemolytic Streptococcus [GAS] on ambulatory adult patients with a sore throat associated with acute respiratory infections at Siriraj Hospital.

Materials and Methods: The study was conducted on adult patients with a sore throat associated with acute respiratory infections within five days who visited the ambulatory care services at Siriraj Hospital between January 2016 and January 2017. All patients received regular care from their responsible physicians. Information on the patients’ illnesses, physical findings, diagnoses and antibiotic prescriptions were collected. Throat swabs were obtained from each patient for culture for GAS and for rapid antigen detection tests for GAS (QuickVue Dipstick Strep A test, and Sofia Strep A+ FIA). On days 3 and 7 after their hospital visit, each patient received a telephone follow-up to ascertain the clinical course of the acute respiratory infections. Several months after their hospital visits, those subjects who had had a positive throat swab culture for GAS received another telephone call to ascertain any new illness suspected of being acute rheumatic fever. The medical records of all patients with positive cultures for GAS or positive tests for rapid antigen detection of GAS at enrollment were reviewed to identify any subsequent illness suggestive of acute rheumatic fever.

Results: Three hundred sixty patients with a sore throat as their chief complaint or an accompanying symptom of an acute respiratory infection were included. The mean age of the patients was 41 years, and most were female. The median duration of sore throat was two days. Acute pharyngitis or tonsillitis was diagnosed in 42.5% of the patients. The overall prevalence of GAS in the throat swabs was 3.3%. With the presence of GAS in a throat swab being considered the gold standard for diagnosis of a GAS infection, the sensitivity and specificity of the QuickVue Dipstick Strep A test were 58.3% and 95.7%, respectively, whereas the sensitivity and specificity of Sofia Strep A+ FIA were 44.4% and 97.1%, respectively. Fifty-seven patients (15.8%) received antibiotics, of which amoxicillin was the most commonly prescribed (35.1%). For most patients, the duration of the antibiotic treatment was five to seven days. The clinical outcomes of the acute respiratory infections on day 3 after the hospital visit were “cured” in 23.3% of cases, “improved” in 72.2%, “unchanged” in 3.9%, and “become worse” in 0.6%. However, on day 7 after their hospital visits, 91.4% of the patients were cured of their acute respiratory infections, while 8.1% and 0.6% had improved or remained the same, respectively. During the several months after the initial hospital visits for a sore throat, none of the patients with a positive throat swab culture for GAS (including two patients who were not prescribed antibiotics), and none of the patients with positive rapid antigen detection tests for GAS, had a subsequent hospital visit with an illness suggestive of acute rheumatic fever. If the study patients received a rapid test, the cost of the tests to detect a case of positive throat swab cultures of GAS was between 8,200 and 9,000 Baht. Antibiotics were administered to 57 patients, but two other patients with a positive culture of GAS did not receive antibiotics. If antibiotics had been prescribed to only those patients who had more than three clinical features of the Centor criteria in 360 patients, four patients with positive throat swab cultures for GAS would not have received antibiotics. If the QuickVue Dipstick Strep A test had been performed on all 360 patients and antibiotics had been prescribed to only those patients with positive test results, five patients with positive throat swab cultures for GAS would not have received antibiotics. If antibiotics had been prescribed to only those patients who had more than three clinical features of the Centor criteria in 246 patients, four patients with positive throat swab cultures for GAS would not have received antibiotics. If Sofia Strep A+ FIA had been performed on 246 patients and antibiotics had been prescribed to only those patients who had positive test results, five patients with positive throat swab cultures for GAS would not have received antibiotics.

Conclusion: The prevalence of GAS in the throat swabs of ambulatory adult patients with a sore throat associated with acute respiratory infections was very low. The sensitivity of the QuickVue Dipstick Strep A test and Sofia Strep A+ FIA for diagnosis of GAS infection in this population was modest. In this study population, using the rapid antigen detection tests to diagnose a GAS infection and to guide the prescribing of antibiotics for adult patients with a sore throat associated with acute respiratory infections seemed to offer no significant additional clinical benefits, and it was not cost-effective.

Keywords: Rapid antigen detection test, Group A beta-hemolytic Streptococcus, Sore throat, Acute respiratory infections