Match each clinical research scenario with its accurately calculated epidemiological measu… — USMLE Step 2 CK

Match each clinical research scenario with its accurately calculated epidemiological measure.

A longitudinal study follows $1,000$ smokers and $1,000$ non-smokers. Over $5$ years, $150$ smokers and $50$ non-smokers are diagnosed with chronic bronchitis. Calculate the proportion of chronic bronchitis cases among smokers that is directly attributable to their smoking habit. $66.7\%$
A randomized trial evaluating a new prophylactic medication for migraines finds that severe monthly attacks occur in $20\%$ of the placebo group and $15\%$ of the intervention group. Calculate the number of patients who must be treated to prevent one additional severe attack. $20$
A case-control investigation of a rare neurologic condition identifies prior chemical exposure in $160$ out of $200$ cases, and in $200$ out of $400$ matched controls. Calculate the most appropriate primary measure of association for this study design. $4.0$
Post-marketing data for a novel biologic therapy shows that $12\%$ of patients develop a systemic infection, compared to $2\%$ of patients on standard therapy. Calculate the number of patients needed to be treated to cause one additional systemic infection. $10$

Answer

The longitudinal study matches with 66.7% (Attributable Risk Percent). The migraine trial matches with 20 (Number Needed to Treat). The case-control investigation matches with 4.0 (Odds Ratio). The post-marketing biologic study matches with 10 (Number Needed to Harm).

Each clinical scenario corresponds to a unique epidemiological calculation. The longitudinal study's Attributable Risk Percent (ARP) evaluates the preventable fraction of disease in the exposed group:

\frac{0.15 - 0.05}{0.15} = 66.7\%

. The migraine trial's Number Needed to Treat (NNT) equals

\frac{1}{0.05} = 20

. The case-control study relies on the Odds Ratio (OR) due to its retrospective design, yielding

\frac{160 \times 200}{40 \times 200} = 4.0

. Finally, the post-marketing study's Number Needed to Harm (NNH) equals

\frac{1}{0.10} = 10

Step-by-Step Solution

Identify the required epidemiological metric for the longitudinal cohort study and calculate it.

Requires Attributable Risk Percent (ARP). ARP =

(0.15 - 0.05) / 0.15 = 66.7\%

The phrase 'proportion of disease... directly attributable' refers to the ARP, representing the fraction of disease in the exposed group preventable by eliminating the exposure.

Determine the measures of clinical effect for the interventional and post-marketing trials.

NNT for the migraine trial is

20

. NNH for the biologic therapy is

10

Number Needed to Treat (NNT) is the inverse of Absolute Risk Reduction (

1 / 0.05

). Number Needed to Harm (NNH) is the inverse of Absolute Risk Increase (

1 / 0.10

Select and calculate the correct measure of association for the case-control study.

Odds Ratio (OR) =

4.0

Case-control studies cannot directly measure incidence; thus, Relative Risk is invalid. The Odds Ratio is the requisite primary measure of association.

Key Concept

Measures of Disease Frequency and Association (Attributable Risk Percent, NNT, NNH, and Odds Ratio)

Estimated Time:2m 30s

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