Streptococcal pharyngitis: diagnosis by gram stain.

Group A beta-hemolytic streptococci were isolated from 49 (10.4%) of 472 patients with pharyngitis. Throat culture results, interpreted by five observers of varying experience, showed the mean sensitivity, specificity, and predictive value of a positive Gram-stained smear of pharyngeal secretions as 73%, 96%, and 71%. Assignment to a high-risk group by clinical algorithm gave sensitivity, specificity, and predictive value of only 45%, 83%, and 23%. The Gram-stained smear is the most accurate method of early diagnosis of streptococcal pharyngitis.