In primary care settings, the Centor criteria are used to determine the likelihood of group A beta-hemolytic streptococcus (GABHS) infection in an acute tonsillitis and the need of antibiotics for tonsillitis treatment. However, the Centor criteria have their weaknesses in making precise diagnosis for adults. The Centor criteria are also ineffective in diagnosis for tonsillitis in children and in secondary care settings (hospitals). A modified version of the Centor criteria, which modified the original Centor criteria in 1998, is often used to aid in diagnosis. The original Centor criteria had four major criteria but the modified Centor criteria have five. The five major criteria of the modified Centor score are:

Presence of tonsillar exudate
Painful neck lymph nodes
History of fever
Age between five and fifteen years
Absence of coughThe possibility of GABHS infection increases with increasing score. The probability for getting GABHS is 2 to 23% for the score of 1, and 25 to 85% for the score of 4. The diagnosis of GABHS tonsillitis can be confirmed by culture of samples obtained by swabbing the throat and plating them on blood agar medium. This small percentage of false-negative results are part of the characteristics of the tests used but are also possible if the person has received antibiotics prior to testing. Identification requires 24 to 48 hours by culture but rapid screening tests (10–60 minutes), which have a sensitivity of 85–90%, are available. In 40% of the people without any symptoms, the throat culture can be positive.
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