An epidemiological study is conducted to learn about the relationship between celiac disease and colon cancer. Suppose there are 77 cases of colon cancer in 68,000 person-years in persons with celiac disease and 54 cases of colon cancer in 215,000 person-years in those without celiac disease. (The overall rate in both groups combined = 131 cases in 283,000 person-years overall.) Use this information to answer questions 1–3.
1. Calculate the rate of colon cancer in the celiac group (R1), in the no celiac group (R0), and overall (R). Express all rates in “per 100,000 person-years.”
2. Calculate and interpret the relative risk of colon cancer associated with celiac disease.
3. Calculate and interpret the attributable risk of colon cancer associated with celiac disease.
To answer these questions, we’ll calculate the rates and risks based on the provided information.
- Calculating the rates:
- Rate of colon cancer in the celiac group (R1): R1 = (Number of cases in celiac group / Person-years in celiac group) * 100,000 = (77 / 68,000) * 100,000 = 113.24 per 100,000 person-years
- Rate of colon cancer in the no celiac group (R0): R0 = (Number of cases in no celiac group / Person-years in no celiac group) * 100,000 = (54 / 215,000) * 100,000 = 25.12 per 100,000 person-years
- Overall rate of colon cancer (R): R = (Number of cases overall / Person-years overall) * 100,000 = (131 / 283,000) * 100,000 = 46.23 per 100,000 person-years
- Calculating the relative risk: Relative Risk (RR) = (Rate of colon cancer in the celiac group / Rate of colon cancer in the no celiac group) = R1 / R0 = 113.24 / 25.12 = 4.51
Interpretation: Individuals with celiac disease have a relative risk of 4.51 for developing colon cancer compared to those without celiac disease. This suggests that celiac disease may be associated with an increased risk of colon cancer.
- Calculating the attributable risk: Attributable Risk (AR) = (Rate in exposed group – Rate in unexposed group) / Rate in exposed group = (R1 – R0) / R1 = (113.24 – 25.12) / 113.24 = 0.778
Attributable Risk Percentage (AR%) = Attributable Risk * 100 = 0.778 * 100 = 77.8%
Interpretation: The attributable risk of colon cancer associated with celiac disease is 77.8%. This means that approximately 77.8% of the cases of colon cancer in individuals with celiac disease can be attributed to the presence of celiac disease.
Please note that these calculations assume that the person-years were correctly reported and that there were no other confounding factors influencing the relationship between celiac disease and colon cancer.