Background: The Centor and McIsaac scores guide testing and treatment for group A streptococcal (GAS) pharyngitis in patients presenting with a sore throat, but they were derived on relatively small samples. We perform a national-scale validation of the prediction models on a large, geographically diverse population.

Methods: We analyzed data collected from 206,870 patients 3 years or older who presented with a painful throat to a United States national retail health chain from September 1, 2006, to December 1, 2008. Main outcome measures were the proportions of patients testing positive for GAS pharyngitis according to the Centor and McIsaac scores (both scales, 0-4).

Results: For patients 15 years or older, 23% (95% CI, 22%-23%) tested positive for GAS, including 7% (95% CI, 7%-8%) of those with a Centor score of 0; 12% (95% CI, 11%-12%) of those with a Centor score of 1; 21% (95% CI, 21%-22%) of those with a Centor score of 2; 38% (95% CI, 38%-39%) of those with a Centor score of 3; and 57% (95% CI, 56%-58%) of those with a Centor score of 4. For patients 3 years or older, 27% (95% CI, 27%-27%) tested positive for GAS, including 8% (95% CI, 8%-9%) of those testing positive with aMcIsaac score of 0; 14% (95% CI, 13%-14%) of those with a McIsaac score of 1; 23% (95% CI, 23%-23%) of those with a McIsaac score of 2; 37% (95% CI, 37%-37%) of those with a McIsaac score of 3; and 55% (95% CI, 55%-56%) of those with a McIsaac score of 4. The 95% CIs overlapped between our retail health chain–derived probabilities and the prior reports.

Conclusion: Our study validates the Centor and McIsaac scores and more precisely classifies risk of GAS infection among patients presenting with a painful throat to a retail health chain.