Lack of reliability of primary grouping of beta-hemolytic streptococci by culture of throat swabs with streptocult supplemented with bacitracin disks in general practice

Abstract

Fifty-eight general practitioners took throat swabs from 434 patients with sore throats. Office cultures were performed on Streptocult supplemented with bacitracin disks in an attempt to carry out primary grouping of beta-hemolytic streptococci (BHS). In 424 cases the findings were compared with those obtained in a microbiological laboratory. Streptocult showed a sensitivity of 75% and a specificity of 84% in the detection of BHS. The office-performed grouping procedure of the correctly detected BHS, however, only had a sensitivity of 65% and a specificity of 87%. Overall, as many as 45% of the patients with BHS group A were misdiagnosed. The unsatisfactory results obtained with primary grouping of BHS may be due partly to incorrect determinations of the diameter of the inhibition zone around the bacitracin disks and partly to an inappropriate choice of breakpoint. It is concluded that cultures of throat swabs on Streptocult in general practice should not be accompanied by attempts to carry out primary grouping with bacitracin disks. A laboratory investigation showed that incubation at room temperature for 48 h and at 35 degrees C for 24 h gave identical BHS positivity rates.