Group A Strep Calculator Questions and Answers


Q: What defines “invasive” group A streptococcal disease?

A: A case of invasive disease is confirmed when group A Streptococcus (GAS) is isolated from a normally sterile site, such as blood. When GAS is cultured from a nonsterile wound from a patient with necrotizing fasciitis or streptococcal toxic shock syndrome, this is also considered evidence of invasive GAS disease.

Q: What if my investigation spans more than a calendar month? For example, what should I enter if I begin an investigation March 20th, and today is April 3rd?

A: Use any 30-day period of interest for the current month, and use the previous 30-day period as the previous month. Choose the calendar month with the most days in the 30-day period. You may also choose to run the model twice choosing different months or defining the 30-day period differently.

Q: How do you define population in a region?

A: If your county has more than 250,000 people, use the county population. If your county has less than 250,000 population, combine neighboring counties with similar characteristics (e.g., rural with rural) until the total population is more than 250,000. Make sure to include any invasive GAS cases in all counties in the region. Because invasive GAS infections are uncommon, the region population must be at least 250,000 to produce a valid prediction.

Q: Why do I need to input the percentage of population that is African American if the cases in this cluster are not African American? What if I don’t know the racial breakdown of my population?

A: African Americans have higher rates of invasive GAS disease than Caucasians for unknown reasons. Counties or regions that have higher percentages of African American population therefore have higher expected disease rates. You can run the model twice using both a high and a low estimate of the percent African American population in a region.

Q: How do I estimate my region’s size?

A: Land area in square miles is available from U.S. censusExternal. Add together the area of all counties in a region to get a total.

Q: Why do I need to input the percentage of invasive GAS cases in my region that occur in persons with HIV-infection or who inject drugs? Are these necessary? What is meant by “historically”?

A: Invasive GAS infections are more common in persons with HIV infection and in those who inject drugs. Including these percentages improves the model, however, these percentages are not required. “Historically” refers to cases in your region over an extended period of time, such as the past 12 months. If there are no cases related to HIV or injection drug use in your region, enter zero. If both variables are left blank, the model will calculate a prediction without these two variables.

Q: Where do I find the mean temperature for my region? What should I enter if the mean temperature of my region is different than that of the state?

A: The National Climatic Data CenterExternal provides the mean temperature of the entire state during the month. The model uses these values to account for climatic differences in different parts of the country. You may wish to enter the mean temperature of your county or region, if that is available.

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Page last reviewed: December 10, 2007