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Difficulty: HardEpidemiologic Study Designs

A pharmacovigilance team is investigating a potential link between a newly introduced fluoroquinolone antibiotic and the development of Achilles tendon rupture. The investigators access a large, national commercial insurance database containing continuous enrollment and claims data. They first identify 850 patients in the database who had a diagnostic code for Achilles tendon rupture between 2020 and 2025. Next, they select 3,400 patients from the same database who did not experience this injury, matching them to the initial group by age, sex, and geographic region. Finally, the investigators review the historical pharmacy claims data for all 4,250 selected individuals to determine whether they were prescribed the fluoroquinolone in the 6 months prior to the index date of tendon rupture or the equivalent matched date. Based on the methodology described, which of the following is the most appropriate primary measure of association to calculate for this investigation?

  1. Odds ratioAnswer
  2. B
    Relative risk
  3. C
    Hazard ratio
  4. D
    Prevalence ratio
  5. E
    Incidence rate ratio

Answer

Odds ratio
This study methodology describes a case-control study. The defining characteristic of a case-control study is that participants are selected based on their disease status (cases with Achilles tendon rupture and matched controls without the rupture). Once categorized by outcome, the investigators look backward in time to determine the frequency of a prior exposure (fluoroquinolone prescription) in both groups. Because case-control studies do not track a known population at risk over time, they cannot measure true disease incidence. Consequently, relative risk cannot be directly calculated. The appropriate measure of association for a case-control study is the odds ratio, which compares the odds of exposure among cases to the odds of exposure among controls.

Step-by-Step Solution

1
Analyze how the study population was selected.
Participants were selected based on the presence (850 patients) or absence (3,400 patients) of Achilles tendon rupture.
Determining the basis of group selection (outcome vs. exposure) is the critical first step in identifying the study design.
2
Determine the temporal direction of the data collection.
After group selection, investigators looked backward in time at pharmacy claims to assess prior fluoroquinolone exposure.
Selecting by outcome and looking retrospectively at exposure defines a case-control study.
3
Identify the appropriate statistical measure for this study design.
Because case-control studies do not track an at-risk population over time, true incidence cannot be determined. Therefore, the odds ratio must be used.
Relative measures like relative risk require incidence data, leaving the odds ratio as the mathematically valid measure of association for case-control designs.

Key Concept

Identifying case-control vs. retrospective cohort study designs based on participant selection methodology to determine the correct measure of association.
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Epidemiologic Study Designs

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