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Difficulty: HardBias and Confounding in Research

A multicenter prospective cohort study evaluates the safety of a newly approved oral Janus kinase (JAK) inhibitor compared to traditional disease-modifying antirheumatic drugs (DMARDs) for the treatment of severe rheumatoid arthritis. The primary safety outcome is the incidence of unprovoked venous thromboembolism (VTE) over a 5-year follow-up period.

Initial analysis of the data reveals a crude relative risk (RR) of 2.1 (95% CI: 1.6 - 2.8, p<0.01p < 0.01) for VTE in the JAK inhibitor group compared to the traditional DMARD group.

Concerned about baseline genetic differences, the investigators perform a stratified analysis based on the presence or absence of an inherited thrombophilia (e.g., Factor V Leiden mutation). The stratum-specific results are as follows:

- Among patients without inherited thrombophilia: RR = 1.2 (95% CI: 0.8 - 1.6, p=0.35p = 0.35)
- Among patients with inherited thrombophilia: RR = 6.4 (95% CI: 4.5 - 8.9, p<0.001p < 0.001)

Based on the results of the stratified analysis, which of the following epidemiological phenomena best explains the relationship between the JAK inhibitor, inherited thrombophilia, and VTE risk observed in this study?

  1. Statistical interactionAnswer
  2. B
    Confounding
  3. C
    Effect mediation
  4. D
    Selection bias
  5. E
    Information bias

Answer

Statistical interaction
The correct answer is statistical interaction (often used synonymously with effect modification in epidemiology). This phenomenon is identified during data analysis when the association between an exposure (JAK inhibitor) and an outcome (VTE) varies significantly depending on the level of a third variable (inherited thrombophilia). In this study, the medication poses no significant VTE risk to patients without thrombophilia (RR = 1.2), but synergistically increases the risk in patients who carry the genetic trait (RR = 6.4). Because the stratum-specific relative risks are meaningfully different from one another, the third variable is interacting with the exposure.

Step-by-Step Solution

1
Evaluate the crude relationship between the exposure and the outcome.
The crude RR is 2.1, indicating a statistically significant increased risk of VTE with JAK inhibitor use.
Establishing the baseline association is necessary before analyzing the impact of a third variable.
2
Analyze the stratum-specific relative risks.
The RR is 1.2 (not significant) in patients without thrombophilia and 6.4 (highly significant) in patients with thrombophilia.
Stratification isolates the effect of the exposure within specific subgroups of the suspected third variable.
3
Compare the stratum-specific risks to each other and to the crude risk.
The stratum-specific RRs (1.2 and 6.4) are distinctly different from each other.
When stratum-specific measures of association are significantly different from one another, the third variable is altering the biological effect of the exposure, confirming interaction.

Key Concept

Effect modification (statistical interaction) is identified when the magnitude of the association between an exposure and an outcome differs significantly across strata of a third variable.
Estimated Time:1m 30s
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Topics

Bias and Confounding in Research

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