Earlier this week, the Centers for Disease Control and Prevention (CDC) released their latest report on diabetes in the United States. According to the report, new cases of diabetes declined by 20 percent between 2008 and 2014. However, the number of people diagnosed with diabetes in the United States is still at an all-time sugar high. Since the 1980s, diabetes rates have more than quadrupled and approximately 9.3 percent of the population has been diagnosed with diabetes. So how can one statistic be declining (new cases) and the other be rising?
The answer to that lies in the science of epidemiology, “the study of the distribution and determinants of health related states and events in populations and the application of this study to control health problems”. When the CDC reports “new cases” it’s talking about diabetes incidence; when they report the “number of people diagnosed with diabetes” they’re talking about diabetes prevalence. But what’s the difference between these two and how are they related?
In epidemiology, incidence and prevalence are the two basic measures of calculating disease frequency. Incidence is the number of new cases during a certain time frame and is measured using the following equation:
Incidence= Number of new diabetes cases _
Average population at risk * time interval
Prevalence is partly based on incidence and is calculated based on the proportion of the population that has been diagnosed:
Prevalence = N
umber of existing (and new) diabetes cases
Average population at risk
The two equations above show that prevalence depends on incidence: an increase in incidence will increase prevalence (assuming there isn’t a high level of mortality due to the disease). However, a decrease in incidence may not necessarily lead to a decrease in prevalence. One of the easiest ways to think about the relationship between incidence and prevalence is illustrated by Ann Aschengrau and George Seage’s sink model:
Diabetes disease incidence is represented by the water inflow from the faucet (each drop represents a newly diagnosed diabetes case). Diabetes prevalence is represented by the water level in the sink (it includes new and existing diabetes cases). So while the inflow of water from the faucet may slow down, that doesn’t necessarily mean that the water level in the sink will change.
We can and should celebrate about the decline of new diabetes cases over the past five years, but we still need to focus on addressing diabetes prevalence. As Ann Albright, director of the CDC’s Division of Diabetes Translation, said during her interview about the new report with NPR yesterday : “we still have a long, long way to go.”
Author’s note: While I was an undergraduate student at the University of North Carolina’s Gillings School of Global Public Health, I took Professor Victor Schoenbach’s course on “The Principles of Epidemiology for Public Health.” At that time, I did not appreciate the educational foundation that was being built for me, but in recent years I am forever grateful to use these foundations as a sounding board for my public health explorations.