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Difficulty: MediumBias and Confounding in Research

A cohort study is conducted to evaluate the safety profile of a novel calcitonin gene-related peptide (CGRP) inhibitor for migraine prophylaxis. The researchers investigate the association between the use of the new CGRP inhibitor and the incidence of severe constipation requiring medical intervention. In the initial unadjusted analysis, patients taking the CGRP inhibitor had a significantly higher risk of severe constipation compared to those taking standard prophylactic medications, with a relative risk (RR) of 3.2 (p < 0.01).

Suspecting that concurrent medication use might be influencing the results, the researchers perform a stratified analysis based on the concurrent use of tricyclic antidepressants (TCAs), which are also known to decrease gastrointestinal motility. The results of the stratified analysis are as follows:

Patient GroupRelative Risk (RR)p-value
Concurrent TCA use1.20.45
No concurrent TCA use1.30.38

Based on these findings, which of the following best describes the role of concurrent TCA use in the observed relationship between the CGRP inhibitor and severe constipation?

  1. Confounding variableAnswer
  2. B
    Effect modifier
  3. C
    Mediator variable
  4. D
    Selection bias
  5. E
    Information bias

Answer

Confounding variable
The scenario describes a classic case of a confounding variable. A confounder is a third variable that is associated with both the exposure (CGRP inhibitor use) and the outcome (constipation) but is not on the causal pathway. Stratification is an analytical method used to control for confounding. When a study is stratified by a true confounder, the strong association seen in the crude (unadjusted) analysis disappears or significantly diminishes across all strata. Because the crude relative risk was 3.2, but the stratum-specific risks were nearly identical to each other and much closer to the null value (1.2 and 1.3), it indicates that the initial observed association was artificially inflated by the uneven distribution of TCA use between the groups.

Step-by-Step Solution

1
Analyze the crude (unadjusted) relative risk.
The crude RR is 3.2, indicating a strong positive association between the CGRP inhibitor and constipation.
To establish the baseline association before accounting for potential third variables.
2
Examine the stratum-specific relative risks after adjusting for TCA use.
The RR for TCA users is 1.2, and the RR for non-users is 1.3. Both are close to 1.0 (null) and not statistically significant.
To determine how the third variable (TCA use) affects the relationship between the exposure and outcome.
3
Compare the crude RR to the stratified RRs.
The crude RR (3.2) is vastly different from the stratified RRs (1.2 and 1.3), while the stratified RRs are similar to each other.
This specific pattern (crude ≠ stratified, but stratum 1 ≈ stratum 2) is the hallmark definition of a confounding variable.

Key Concept

Distinguishing confounding from effect modification using stratified analysis.
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Topics

Bias and Confounding in Research

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