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Difficulty: MediumBias and Confounding in Research

A prospective cohort study is conducted to evaluate the risk of acute kidney injury (AKI) associated with a newly approved intravenous antifungal medication in hospitalized patients. The initial unadjusted analysis yields a relative risk (RR) of 3.2. The investigators suspect that concurrent use of a known nephrotoxic immunosuppressant medication might be influencing these results. To investigate this, they re-analyze the data by stratifying the patients based on concurrent immunosuppressant use. In the stratum of patients taking the immunosuppressant, the RR for AKI with the new antifungal is 5.8. In the stratum of patients not taking the immunosuppressant, the RR is 1.1. Which of the following epidemiologic principles best describes the role of concurrent immunosuppressant use in this study?

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

Answer

Effect modification
The correct answer is effect modification. The hallmark of effect modification is that the measure of association (in this case, relative risk) differs significantly across the strata of a third variable. Here, the risk of AKI from the antifungal is much higher in patients taking the immunosuppressant (RR=5.8) compared to those not taking it (RR=1.1). The immunosuppressant is actively modifying the effect of the antifungal.

Step-by-Step Solution

1
Identify the unadjusted (crude) measure of association.
The unadjusted relative risk (RR) is 3.2.
This establishes the baseline association between the exposure (antifungal) and outcome (AKI) before accounting for other variables.
2
Analyze the measure of association after stratification by the suspected third variable.
The stratified RRs are 5.8 (with immunosuppressant) and 1.1 (without immunosuppressant).
Stratification separates the study population into groups based on the third variable to see its specific impact on the exposure-outcome relationship.
3
Compare the stratified results to each other and to the crude result.
The stratified RRs (5.8 and 1.1) are significantly different from each other.
If stratified RRs are similar to each other but different from the crude RR, it indicates confounding. If the stratified RRs are significantly different from each other, it indicates effect modification.

Key Concept

Effect modification occurs when the magnitude of the association between an exposure and an outcome differs depending on the level of a third variable.
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Bias and Confounding in Research

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