A study was conducted to assess the association between L-tryptophan use and the development of Eosinophilia-Myalgia Syndrome (EMS). Patients with EMS were asked about the use of products containing L-tryptophan during the last 6 months. At the same time, people without EMS were randomly selected from the same population where the patients came from, and asked about their experience with L-tryptophan containing products within the last 6 months. The study showed that the use of L-tryptophan is significantly associated with EMS. Which of the following measures of association are the investigators most likely to report?
The above case describes a typical case-control study design. Patients with the disease of interest (cases) and people without the disease (controls) are asked about previous exposure to the variable being studied (L-tryptophan use). The main measure of association is the exposure odds ratio, in which the exposure of people with the disease (cases) is compared to the exposure of those without the disease (controls).
(Choices A and D) Incidence measures (e.g., relative risk or relative rate) cannot be directly measured in case-control studies because the people being studied are those who have already developed the disease. Relative risk and relative rate are calculated in cohort studies, where people are followed over time for the occurrence of the disease.
(Choice B) Median survival is calculated in cohort studies or clinical trials, and is usually used to compare the median survival times in two or more groups of patients (e.g., receiving a new treatment or placebo).
(Choice E) Prevalence odds ratio is calculated in cross-sectional studies to compare the prevalence of a disease between different populations.
Educational Objective:
A case-control study is used to compare the exposure of people with the disease (cases) to the exposure of the people without the disease (controls). The main measure of association is the exposure odds ratio.