Investigators are conducting a prospective cohort study to evaluate the association betwee… — USMLE Step 3

Investigators are conducting a prospective cohort study to evaluate the association between a new sodium-glucose cotransporter-2 (SGLT2) inhibitor and the incidence of ischemic stroke in patients with type 2 diabetes. Initial analysis reveals a crude Relative Risk (RR) of $0.75$ ( $95\%$ CI: $0.61 - 0.92$ , $p = 0.01$ ) for stroke in the SGLT2 inhibitor group compared to patients on standard sulfonylurea therapy. The investigators suspect that concurrent use of antiplatelet therapy might influence the results, so they perform a stratified analysis based on baseline antiplatelet use:

- Stratum 1 (Concurrent antiplatelet use): RR = $0.45$ ( $95\%$ CI: $0.32 - 0.60$ )
- Stratum 2 (No concurrent antiplatelet use): RR = $1.05$ ( $95\%$ CI: $0.88 - 1.25$ )

Based on these statistical findings, which of the following is the most appropriate approach for the investigators to present their final data?

Report the stratum-specific relative risks separately, as the concurrent use of antiplatelet therapy modifies the treatment effect.Answer
B
Calculate and report a single pooled relative risk adjusted for antiplatelet use via multivariable logistic regression.
C
Utilize a Chi-square test for independence on the crude data, disregarding the strata, to determine the true predictive value.
D
Exclude the patients taking antiplatelet therapy from the final analysis to control for a pharmacokinetic drug interaction.
E
Conclude the medication has no cardiovascular benefit and halt the study, as the risk reduction is insignificant in non-users.

Answer

Report the stratum-specific relative risks separately, as the concurrent use of antiplatelet therapy modifies the treatment effect.

Effect modification (or statistical interaction) is present when the effect of an exposure on an outcome varies depending on the level of a third variable. In this scenario, the protective effect of the SGLT2 inhibitor against stroke is strong in patients taking antiplatelets (RR =

0.45

) but non-existent in those not taking antiplatelets (RR =

1.05

). Because the stratum-specific estimates differ significantly from one another, the third variable is an effect modifier. The standard epidemiological rule is that when effect modification is present, the investigator must report the stratum-specific estimates separately. Pooling the data into a single adjusted estimate would average out the effects and hide the fact that the drug only appears beneficial in a specific subgroup.

Step-by-Step Solution

Analyze the relationship between the crude Relative Risk (RR) and the stratum-specific RRs.

The crude RR is

0.75

. The stratified RRs (

0.45

and

1.05

) are significantly different from each other and from the crude RR.

Comparing crude and stratified estimates is the standard method to distinguish between confounding and effect modification.

Diagnose the epidemiological phenomenon present.

Because the magnitude of the drug's effect differs substantially depending on the presence of a third variable (antiplatelet use), this is effect modification.

If the stratified RRs were similar to each other but different from the crude RR, it would indicate confounding. Different stratum RRs indicate effect modification.

Determine the standard epidemiological reporting requirement for this phenomenon.

The correct approach is to report the stratum-specific results separately.

Adjusting or pooling data (as done for confounding) would hide the fact that the drug's efficacy heavily depends on concurrent antiplatelet use, which is critical clinical information.

Key Concept

Effect modification occurs when the magnitude of an effect differs depending on the level of a third variable, and it dictates that stratum-specific results must be reported separately rather than pooled.

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