A clinical research group conducts a prospective cohort study to evaluate the efficacy of … — USMLE Step 3

A clinical research group conducts a prospective cohort study to evaluate the efficacy of a newly approved monoclonal antibody targeting interleukin-5 (IL-5) compared to standard inhaled corticosteroid therapy in preventing severe asthma exacerbations. The primary outcome is the incidence of hospital admissions for asthma over a $12$ -month period.

The initial crude analysis reveals a relative risk (RR) of $0.65$ ( $95\%$ CI: $0.50-0.85$ ) for hospital admission in patients receiving the monoclonal antibody versus standard therapy.

To further investigate the data, the researchers perform a stratified analysis based on the patients' baseline absolute peripheral blood eosinophil counts. The results of the stratified analysis are as follows:

Patient Subgroup	Relative Risk	$95\%$ CI
Eosinophil count $< 300$ cells/µL	$0.98$	$0.82 - 1.15$
Eosinophil count $\ge 300$ cells/µL	$0.32$	$0.20 - 0.45$

Based on these statistical findings, which of the following best describes the epidemiological role of the baseline absolute eosinophil count in this study?

Effect modifierAnswer
B
Confounding variable
C
Dependent variable requiring analysis of variance
D
Determinant of negative predictive value
E
Incidental finding lacking clinical significance

Answer

Effect modifier

The correct answer is effect modifier. Effect modification (or interaction) occurs when the magnitude of an effect (such as relative risk or odds ratio) differs significantly between different strata of a third variable. In this scenario, the biologic therapy's efficacy is drastically different depending on the patient's baseline eosinophil count: there is no significant effect in patients with

< 300

cells/µL (as the

95\%

confidence interval crosses

1.0

), but there is a strong protective effect in those with

\ge 300

cells/µL. This biological interaction demonstrates that the baseline eosinophil count modifies the treatment effect of the drug.

Step-by-Step Solution

Identify the initial statistical finding from the crude analysis.

The crude relative risk (RR =

0.65

) suggests an overall protective effect of the monoclonal antibody.

Establishing the baseline overall effect is necessary before evaluating the impact of stratification.

Evaluate the results of the stratified analysis.

The RR in the low eosinophil group is

0.98

(no significant protective effect, as the CI crosses

1.0

), while the RR in the high eosinophil group is

0.32

(a very strong protective effect).

Comparing stratum-specific measures of association is the standard method for identifying interaction or confounding.

Apply the epidemiological definitions of confounding and effect modification.

Because the measure of association differs significantly between the two strata, the stratifying variable (eosinophil count) represents effect modification.

Effect modification describes a biological or physical interaction where the exposure's effect on the outcome genuinely differs across levels of a third variable.

Key Concept

Effect modification is present when the magnitude of an association between an exposure and an outcome differs significantly according to the level of a third variable (the effect modifier).

Question

Answer

Step-by-Step Solution

Key Concept

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