A prospective cohort study evaluates the efficacy of a newly introduced targeted therapy, … — USMLE Step 2 CK

A prospective cohort study evaluates the efficacy of a newly introduced targeted therapy, a selective JAK inhibitor, in achieving clinical remission in adults with moderate-to-severe rheumatoid arthritis (RA). The primary outcome is clinical remission at $24$ weeks. The initial analysis reveals a crude relative risk (RR) of $1.6$ ( $95\%$ CI: $1.3 - 1.9$ ) for achieving clinical remission with the new therapy compared to standard care.

During peer review, concerns are raised that the patient's baseline autoantibody status (rheumatoid factor or anti-CCP positivity) might be influencing the results. The investigators perform a stratified analysis based on baseline serologic status. The results show:
- Seropositive patients: $RR = 2.1$ ( $95\%$ CI: $1.7 - 2.6$ )
- Seronegative patients: $RR = 1.0$ ( $95\%$ CI: $0.8 - 1.2$ )

Which of the following best describes the role of baseline serologic status in this study?

A
Confounding variable
Effect modifierAnswer
C
Information bias
D
Mediator variable
E
Selection bias

Answer

Effect modifier

The correct answer is effect modification. Effect modification occurs when the true magnitude of an association between an exposure and an outcome differs depending on the level of a third variable. In this scenario, the relative risk of achieving clinical remission with the new therapy varies significantly based on baseline serologic status (a relative risk of

2.1

in seropositive patients versus

1.0

in seronegative patients). Unlike confounding, which is a distortion of an association that researchers attempt to eliminate, effect modification represents a biological interaction and a natural phenomenon that should be described and reported.

Step-by-Step Solution

Identify the initial study findings.

The initial analysis shows a crude relative risk of

1.6

, indicating a statistically significant positive association between the JAK inhibitor and clinical remission.

This establishes the baseline association before accounting for any third variables.

Analyze the results after stratification.

The data is divided based on serologic status, revealing a relative risk of

2.1

in seropositive patients and

1.0

in seronegative patients.

Stratification is the standard epidemiological method used to distinguish between confounding and effect modification.

Compare the stratified results to each other.

The stratified relative risks (

2.1

and

1.0

) are significantly different from each other.

If the variable were a confounder, the stratified relative risks would be similar to each other. Because they differ from each other, this demonstrates effect modification.

Key Concept

Distinguishing between confounding and effect modification using stratified analysis.

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic