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Difficulty: MediumBias and Confounding in Research

An investigator is evaluating a new transdermal hormonal contraceptive for its association with venous thromboembolism (VTE) in women of reproductive age. Initial overall analysis reveals that women using the transdermal patch have a relative risk (RR) of 4.2 for developing VTE compared to non-users. The research team suspects that an underlying genetic predisposition might alter this relationship. They subsequently divide the cohort based on the presence or absence of the prothrombin G20210A mutation. Among participants who carry the mutation, the relative risk of VTE in transdermal patch users compared to non-users is 15.5. Among participants who do not carry the mutation, the relative risk is 2.1. Both stratified findings are statistically significant. Which of the following epidemiological concepts best describes the role of the prothrombin G20210A mutation in this scenario?

  1. Effect modificationAnswer
  2. B
    Confounding
  3. C
    Selection bias
  4. D
    Recall bias
  5. E
    Lead-time bias

Answer

Effect modification
The scenario describes effect modification (also known as interaction). This epidemiological concept is present when the magnitude or direction of an association between an exposure and an outcome varies according to the level of a third variable (the effect modifier). In this study, the relative risk of venous thromboembolism associated with the transdermal patch is markedly different depending on whether the patient has the prothrombin G20210A mutation (RR = 15.5) or does not have it (RR = 2.1). Stratified analysis is the definitive way to identify effect modification. Unlike confounding, effect modification is a true biological finding indicating synergy rather than a flaw in study design.

Step-by-Step Solution

1
Analyze the initial study findings.
The crude relative risk (RR) for developing VTE with transdermal patch use is 4.2.
Establishing the baseline crude association is necessary before evaluating the impact of a third variable.
2
Evaluate the effect of stratification by the third variable (prothrombin G20210A mutation).
Stratification reveals two significantly different relative risks: 15.5 for mutation carriers and 2.1 for non-carriers.
Stratified analysis is the primary method used to distinguish between confounding and effect modification.
3
Interpret the stratified results to classify the epidemiological phenomenon.
Because the stratified RRs (15.5 and 2.1) are significantly different from each other, the third variable is demonstrating effect modification.
If the variable were a confounder, the stratified RRs would be similar to each other but different from the crude RR.

Key Concept

Effect modification occurs when the effect of an exposure on an outcome is different across levels of a third variable, which is identified when stratified analysis reveals significantly different measures of association.
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Topics

Bias and Confounding in Research

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