A cohort of patients with myotonic dystrophy was retrospectively analyzed from 1993 to 2010 to identify incident cases of cancer. The researchers identified 2 cases of thyroid cancer and reported a standardized incidence ratio of 7.4 (P-value = 0.02). This ratio was derived using which of the following formulas?
A)Expected number of cases divided by the population at risk
B)Observed incidence of cancer minus expected incidence of cancer
C)Observed number of cases divided by expected number of cases
D)Observed number of cases divided by population at risk
E)Observed number of cases plus projected number of cases

Question

A cohort of patients with myotonic dystrophy was retrospectively analyzed from 1993 to 2010 to identify incident cases of cancer.  The researchers identified 2 cases of thyroid cancer and reported a standardized incidence ratio of 7.4 (P-value = 0.02).  This ratio was derived using which of the following formulas?
A)Expected number of cases divided by the population at risk
B)Observed incidence of cancer minus expected incidence of cancer
C)Observed number of cases divided by expected number of cases
D)Observed number of cases divided by population at risk
E)Observed number of cases plus projected number of cases

Quizplus · Accepted Answer

Explanation: The standardized incidence ratio (SIR) is a measure used to determine if the occurrence of cancer in a small population is high or low relative to an expected value derived from a larger comparison population. It is calculated by dividing observed cases (O_C) by the expected cases (E_C); the formula is SIR = O_C / E_C. E_C is calculated based on age/sex-adjusted incidence rates of disease in a control population that is usually much larger (eg, population of the state). SIR = 1 implies that the observed incidence of cancer is equal to the expected value; SIR >1 implies a higher observed incidence than expected (eg, SIR = 1.5 means a 50% higher observed incidence). _C _C _C _C _C In this example, the 2 cases of thyroid cancer in the patients with myotonic dystrophy resulted in a SIR of 7.4; this value is >1 so the incidence of thyroid cancer is higher than expected. Because SIR = O_C / E_C, then E_C = O_C / SIR = 2 / 7.4 = 0.27, meaning 0.27 cases would be expected in this population. The increased incidence was statistically significant (P-value = 0.02). _C _C _C _C (Choice A) The expected number of cases divided by the population at risk gives an idea of the expected incidence of disease in the population at risk. However, this study deals with actual observed cases of disease. (Choice B) The difference between observed and expected incidence of cancer is not the SIR. (Choice D) The observed number of cases divided by the population at risk gives the incidence of disease in the population at risk. It does not provide a measure of how the incidence in the study group compares to the population as a whole. (Choice E) This value would provide a projection of the number of cases likely to develop in a group, which is not the intended use of SIR. Educational objective: The standardized incidence ratio (SIR) is used to determine if the occurrence of cancer in a small population is high or low relative to an expected value derived from a larger comparison population. It is calculated by dividing the observed cases by the expected cases.

A Cohort of Patients with Myotonic Dystrophy Was Retrospectively Analyzed