A 58-year-old man with hypertension and stage 3a chronic kidney disease brings a pharmaceu… — USMLE Step 3

A 58-year-old man with hypertension and stage 3a chronic kidney disease brings a pharmaceutical advertisement to his primary care physician. The advertisement promotes a newly approved medication, "Renalox," highlighting that it "reduces the risk of progressing to end-stage renal disease (ESRD) by 44%!" The patient asks if this medication would be beneficial for him over the next several years.

The physician reviews the phase 3, randomized, double-blind, placebo-controlled trial cited in the advertisement. In this trial, patients with similar baseline characteristics to this patient were randomized 1:1 to receive either Renalox or a placebo. Because participants were followed for variable durations, the study reported event incidence utilizing person-years of follow-up.

The trial results are summarized in the table below:

Outcome	Placebo Arm (N = 2000)	Renalox Arm (N = 2000)
Total Follow-up	8,000 person-years	8,000 person-years
Progression to ESRD	360 events	200 events
All-Cause Mortality	160 events	120 events

Assuming the incidence rate of ESRD remains constant over time, what is the number of patients who need to be treated with Renalox for exactly 5 years to prevent one additional case of progression to ESRD?

Answer: 10 patients

Answer

To calculate the Number Needed to Treat (NNT) over a specific timeframe from person-year data, you must first determine the annual absolute rate reduction. The placebo group incidence is

360 \text{ events} / 8000 \text{ person-years} = 0.045 \text{ events/person-year}

. The treatment group incidence is

200 \text{ events} / 8000 \text{ person-years} = 0.025 \text{ events/person-year}

. The absolute difference is

0.020 \text{ events/person-year}

. Since the physician wants to know the NNT for a 5-year period, the cumulative absolute risk reduction over 5 years is

0.020 \times 5 = 0.10

. The NNT is the reciprocal of the absolute risk reduction:

1 / 0.10 = 10

. Therefore, 10 patients must be treated for 5 years to prevent one case of ESRD.

Step-by-Step Solution

Select the correct data row for the targeted outcome.

Progression to ESRD: Placebo = 360 events / 8,000 person-years; Renalox = 200 events / 8,000 person-years.

The question specifically asks for the NNT to prevent ESRD, making the 'All-Cause Mortality' row a distractor.

Calculate the annual incidence rate for both groups.

Placebo rate = 360 / 8,000 = 0.045 events/person-year. Renalox rate = 200 / 8,000 = 0.025 events/person-year.

Person-year data must be converted into a standardized annual incidence rate to calculate absolute differences.

Determine the Absolute Rate Reduction (ARR) per year.

ARR per year = 0.045 - 0.025 = 0.020 events/person-year.

The absolute rate reduction represents the fraction of an event prevented by treating one patient for one year.

Scale the ARR to the specific time horizon requested (5 years).

5-year ARR = 0.020 * 5 = 0.10 (or 10%).

Because the clinical question asks for the NNT over a 5-year treatment period, the cumulative absolute risk reduction over that exact timeframe must be determined.

Calculate the Number Needed to Treat (NNT).

NNT = 1 / 0.10 = 10.

NNT is the inverse of the absolute risk reduction. Treating 10 patients for 5 years will prevent exactly one progression to ESRD.

Key Concept

Converting person-time incidence rates to time-adjusted absolute risk reductions for Number Needed to Treat (NNT) calculations.

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic