Duration of positive throat cultures for group A streptococci after initiation of antibiotic therapy.

To determine if it is appropriate to recommend that patients with group A beta-hemolytic streptococcal pharyngitis, who are clinically well by the morning after starting antibiotic treatment, can return to school or day care, or if they should wait until they have completed 24 hours of antibiotics as recommended by the American Academy of Pediatrics Committee on Infectious Diseases. We examined the duration of positivity of the throat culture after antibiotics were begun as a means of assessing the potential risk of transmission to close school contacts. Forty-seven children (4 to 17 years of age) with pharyngitis and a positive throat culture for group A streptococci in an outpatient, staff model health maintenance organization clinic were enrolled and were randomly selected to receive therapy with either oral penicillin V, intramuscular benzathine penicillin G, or oral erythromycin estolate. Additional throat cultures were obtained and clinical findings were recorded for each child during three home visits in the 24 hours after their initial clinic visit. Acute and convalescent sera were obtained for determination of anti-streptolysin O and anti-DNase B titers. Seventeen (36.2%) of the 47 patients had a positive culture the morning after initiating antibiotic therapy. However, thirty-nine (83%) of the patients became "culture negative" within the first 24 hours. Neither the time interval to the first negative culture nor the presence or absence of group A streptococcal organisms on any single convalescent culture could be predicted by clinical findings. Six of the eight children who failed to convert to a "negative" throat culture within 24 hours of initiating therapy were receiving erythromycin. We could detect no difference in either time to conversion to a negative culture or the presence of a positive culture 24 hours after starting antibiotics between those who demonstrated a significant antibody increase and those who did not. The data from this study strongly suggest that children with group A beta-hemolytic streptococcal pharyngitis should complete a full 24 hours of antibiotics before returning to school or daycare.