A case-control study evaluated the association between long-term use of a heartburn medication and bladder and colon cancer. The study enrolled 130 patients with bladder cancer, 130 patients with colon cancer, and 230 patients matched by age and sex with no history of cancer. All study participants were interviewed and their medical records reviewed for past long-term use of the heartburn medication. Long-term use of the heartburn medication was found in 30 (23.1%) patients with bladder cancer, in 28 (21.5%) patients with colon cancer, and in 30 (13.0%) patients with no history of cancer. Based on these results, which of the following is the closest interpretation of the odds ratio (OR) of long-term use of the heartburn medication in patients with bladder cancer relative to patients with no history of cancer?

Question

A case-control study evaluated the association between long-term use of a heartburn medication and bladder and colon cancer.  The study enrolled 130 patients with bladder cancer, 130 patients with colon cancer, and 230 patients matched by age and sex with no history of cancer.  All study participants were interviewed and their medical records reviewed for past long-term use of the heartburn medication.  Long-term use of the heartburn medication was found in 30 (23.1%) patients with bladder cancer, in 28 (21.5%) patients with colon cancer, and in 30 (13.0%) patients with no history of cancer.  Based on these results, which of the following is the closest interpretation of the odds ratio (OR) of long-term use of the heartburn medication in patients with bladder cancer relative to patients with no history of cancer?

Quizplus · Accepted Answer

Explanation: The odds of an event are related to the probability (P) of that event, with odds equal to P / (1 − P). For example, the probability of rolling a die and getting a 1 is 1/6, but the odds of rolling a 1 are 1/5. The odds ratio (OR) is a measure of association used in case-control studies to compare the odds of exposure in cases (ie, individuals with disease) relative to controls (ie, individuals without disease): rolling a die OR = (odds of exposure in cases) / (odds of exposure in controls) A shortcut to calculate the OR when a contingency table has a standard format (ie, exposure of interest in the first row of exposure status and cases in the first column of disease status) is the following: standard format OR = (a/c) / (b/d) = ad / bc In this study, cases are patients with bladder cancer and controls are patients with no history of cancer. The exposure of interest is long-term use of a heartburn medication, so the reference category of exposure is no long-term use of the heartburn medication. 30 of 130 cases (ie, patients with bladder cancer) included long-term use of the heartburn medication, so 130 − 30 = 100 of cases had no long-term use of the heartburn medication. 30 of 230 controls (ie, patients with no history of cancer) included long-term use of the heartburn medication, so 230 − 30 = 200 controls had no long-term use of the heartburn medication. Based on these data, the standard contingency table is as follows: The odds of long-term use of medication in cases and controls are calculated as follows: Odds of exposure in cases: a/c = 30/100 = 0.30 Odds of exposure in controls: b/d = 30/200 = 0.15 Therefore, the OR of long-term use of the heartburn medication in patients with bladder cancer relative to patients with no history of cancer is: OR = (a/c) / (b/d) = ad / bc = 0.30/0.15 = 2.0 The estimated OR = 2.0 is >1 (ie, null value for the OR); therefore, there are higher odds of long-term use of the heartburn medication among patients with history of bladder cancer, with OR = 2.0. Educational objective: The odds ratio (OR) is a measure of association calculated as: OR = (odds of exposure in cases) / (odds of exposure in controls) For a contingency table in the standard format: OR = ad / bc

A Case-Control Study Evaluated the Association Between Long-Term Use of a Heartburn