A prospective cohort study is conducted to evaluate the association between the initiation of a new selective serotonin reuptake inhibitor (SSRI) and the incidence of gastrointestinal (GI) bleeding in older adults. The researchers calculate an unadjusted (crude) Relative Risk (RR) of ( CI: , ).
To further investigate the relationship, the investigators stratify the analysis based on the concurrent use of nonsteroidal anti-inflammatory drugs (NSAIDs). The stratum-specific results are presented below:
| Stratum | Relative Risk (RR) | 95% Confidence Interval |
|---|---|---|
| Concurrent NSAID use | ||
| No concurrent NSAID use |
Based on these findings, which of the following best describes the role of concurrent NSAID use in this study?
- AIt acts as a confounding variable
- It acts as an effect modifierAnswer
- CIt indicates that an incorrect inferential statistical test was utilized
- DIt demonstrates a pharmacokinetic interaction via cytochrome P450 inhibition
- EIt alters the positive predictive value of the outcome by changing the prevalence
Answer
It acts as an effect modifier
Effect modification (or interaction) occurs when the magnitude of the association between an exposure and an outcome differs depending on the level of a third variable. In this scenario, the crude Relative Risk (RR) for GI bleeding is . However, upon stratification by NSAID use, the stratum-specific RRs differ significantly from each other ( vs. ). This indicates that concurrent NSAID use modifies the effect of the SSRI on GI bleeding risk. Biologically, this represents a pharmacodynamic synergism, as both SSRIs (by depleting platelet serotonin) and NSAIDs (by inhibiting COX and altering mucosal integrity) independently increase bleeding risk.
Step-by-Step Solution
Key Concept
Effect modification occurs when the magnitude of an effect (e.g., Relative Risk) differs depending on the level of a third variable. It is identified when stratum-specific measures of association are significantly different from one another.