An internal medicine specialist is analyzing the published results of a multi-center rando… — USMLE Step 3

An internal medicine specialist is analyzing the published results of a multi-center randomized controlled trial to decide if a new antithrombotic drug should be added to the hospital formulary. The study compared the new drug against standard care over a $3$ -year period.

The following event frequencies were observed in the trial:

Treatment Arm	Patients Enrolled ( $N$ )	Ischemic Strokes	Major Bleeding Episodes
Standard Care	$5000$	$300$	$100$
New Antithrombotic	$5000$	$30\%$ Relative Risk Reduction*	$40\%$ Relative Risk Increase*

*Note: Relative risk changes are calculated against the standard care baseline.

To assess the overall clinical utility, the specialist evaluates a composite outcome of "total major clinical events," defined as the sum of ischemic strokes and major bleeding episodes.

Calculate the net number of total major clinical events PREVENTED for every $10,000$ patients treated with the new antithrombotic instead of standard care over the $3$ -year period.

Answer: 100 events

Answer

100 net major clinical events prevented

To find the net clinical benefit, you must translate relative risk changes into absolute numbers. First, project the standard care rates to the requested 10,000 patient scale: 600 strokes and 200 bleeds. A 30% Relative Risk Reduction in strokes means 180 fewer strokes (600 * 0.30). A 40% Relative Risk Increase in bleeding means 80 additional bleeds (200 * 0.40). The net number of major events prevented is the strokes prevented minus the excess bleeds caused: 180 - 80 = 100 net events prevented.

Step-by-Step Solution

Scale the control group events from N=5000 to the requested population size of N=10000.

Standard Care group would experience 600 ischemic strokes and 200 major bleeding episodes per 10,000 patients.

Establishes the baseline Absolute Risk (Control Event Rate) over the specified population size required for the final answer.

Multiply the baseline stroke rate by the Relative Risk Reduction (RRR) to find strokes prevented.

600 \times 0.30 = 180

ischemic strokes prevented.

Converts the reported Relative Risk Reduction into the Absolute Risk Reduction (number of beneficial events prevented) for this population.

Multiply the baseline bleeding rate by the Relative Risk Increase (RRI) to find excess bleeds caused.

200 \times 0.40 = 80

excess major bleeding episodes.

Evaluates the absolute harm introduced by the new therapy by converting Relative Risk Increase into an Absolute Risk Increase.

Subtract the excess harms from the prevented target events.

180

(prevented strokes) -

80

(excess bleeds) =

100

net events prevented.

Determines the overall clinical impact by weighing the absolute harms against the absolute benefits.

Key Concept

Integrating Relative Risk Reduction (RRR) and Relative Risk Increase (RRI) with baseline incidence to determine the net absolute clinical impact (Absolute Risk Reduction minus Absolute Risk Increase).

Alternative Method

Calculate the total Absolute Risk (AR) for both events combined. Standard Care AR = 6.0% (stroke) + 2.0% (bleed) = 8.0%. New Drug AR = (6.0% * 0.70) + (2.0% * 1.40) = 4.2% + 2.8% = 7.0%. Net Absolute Risk Reduction = 8.0% - 7.0% = 1.0%. Apply this to the requested population: 1.0% of 10,000 = 100 events prevented.

Estimated Time:2m 0s

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Alternative Method

Topics

More questions from this topic