A case-control study is conducted to evaluate the association between the consumption of a… — USMLE Step 2 CK

A case-control study is conducted to evaluate the association between the consumption of a widely used artificial sweetener and the incidence of transitional cell carcinoma of the bladder. The investigators enroll $500$ patients with newly diagnosed bladder cancer and $500$ age-matched controls without bladder cancer. An initial crude data analysis reveals an odds ratio (OR) of $2.8$ ( $95\%$ CI: $2.1-3.7$ , $p < 0.05$ ) for bladder cancer among artificial sweetener users compared to non-users.

During the study's review phase, an epidemiologist points out that cigarette smoking is a significant risk factor for bladder cancer and is also strongly associated with higher artificial sweetener consumption in this population. To address this, the investigators perform a stratified analysis based on the participants' smoking history. The results are presented below:

Stratum	Odds Ratio (OR)	$95\%$ Confidence Interval	p-value
Current or former smokers	$1.1$	$0.8-1.5$	$0.52$
Never-smokers	$1.0$	$0.7-1.4$	$0.65$

Based on these findings, which of the following statements is most accurate?

A
Cigarette smoking acts as an effect modifier, and the investigators should report the stratified odds ratios separately to highlight the differing risks.
Cigarette smoking acts as a confounding variable, and the investigators should report an adjusted odds ratio to account for this factor.Answer
C
The crude findings are inherently flawed because a case-control design was chosen instead of a retrospective cohort study, which is required to assess past exposures.
D
The differing odds ratios indicate that the prevalence of smoking in the study population has altered the positive predictive value of artificial sweetener use.
E
The significant drop in the odds ratio after stratification indicates a systematic data collection error, requiring a root cause analysis to identify the responsible researcher.

Answer

Cigarette smoking acts as a confounding variable, and the investigators should report an adjusted odds ratio to account for this factor.

The correct answer identifies cigarette smoking as a confounding variable. A confounder is an extraneous variable that is associated with both the exposure (artificial sweetener) and the outcome (bladder cancer) but is not on the causal pathway between them. In epidemiological studies, stratification is a primary method used to identify confounding. When data is stratified by a confounder, the resulting stratum-specific measures of association (e.g.,

\text{OR} = 1.1

and

1.0

) will be similar to each other but significantly different from the crude, unadjusted measure (

\text{OR} = 2.8

). Because the initial crude association was artificially inflated by the confounding effect of smoking, the investigators must use analytical techniques (such as Mantel-Haenszel pooling or multivariable logistic regression) to calculate and report an adjusted odds ratio. This adjusted value provides an accurate estimate of the true relationship by controlling for the confounder.

Step-by-Step Solution

Analyze the initial crude relationship.

The unadjusted data shows a statistically significant association between artificial sweetener use and bladder cancer with an OR of

2.8

Establishing the baseline finding is necessary before evaluating the impact of potential external variables.

Evaluate the data after stratification by smoking status.

The OR drops to

1.1

in smokers and

1.0

in non-smokers, and both values lose statistical significance.

Stratification separates the study population by the suspected third variable to observe the true exposure-disease relationship within each subgroup.

Compare the crude and stratum-specific results to identify the epidemiological phenomenon.

The stratum-specific ORs (

1.1

and

1.0

) are similar to each other but markedly different from the crude OR (

2.8

When stratum-specific measures are similar to one another but differ significantly from the crude measure, it indicates that the stratifying variable is a confounder rather than an effect modifier.

Determine the appropriate analytical next step.

The investigators must use analytical techniques to calculate and report an adjusted OR that controls for the confounding effect of smoking.

Because the crude OR was artificially inflated by confounding, reporting an adjusted OR provides an accurate estimate of the true relationship.

Key Concept

Distinguishing confounding from effect modification using stratified analysis

Estimated Time:1m 30s

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