to determine the validity of the rapid antigen test rat osom strepa genzyme for the diagnosis of acute pharyngitis caused by group a beta haemolytic strep gabhs diagnostic techniques survey urban primary care centre spain all patients over 14 years old seen in 6 surgeries with sore throat and 2 or more centor criteria pharyngotonsillar exudate tender laterocervical nodes absence of coughing and or history or presence of fever pharyngeal swabs were taken from all the patients one for rat and another to send for culture in the microbiology department a total of 182 patients were evaluable with a mean age of 30 6 12 1 years of which 116 were women 63 7 63 patients had 2 centor criteria 83 had 3 and 36 the 4 criteria the culture was positive in 102 patients 56 with gabhs showing infection in forty 22 95 confidence interval ci 21 2 22 8 group c streptococcus was isolated in 26 patients 14 3 gabhs was higher among patients with four centor criteria 38 9 vs 25 3 observed among those with 3 criteria and 7 9 with 2 criteria p 001 sensitivity of rat was 95 with a specificity of 93 a positive predictive value of 79 2 and a negative predictive value of 98 5 these results show the usefulness of rat for diagnosing streptococcal pharyngitis its use should be spread to all primary care practices.