A multi-center randomized controlled trial evaluates a novel PCSK9 inhibitor for the preve… — USMLE Step 2 CK

A multi-center randomized controlled trial evaluates a novel PCSK9 inhibitor for the prevention of major adverse cardiovascular events (MACE) in patients with familial hypercholesterolemia. The trial enrolls $10,000$ patients, randomizing them evenly into an intervention group receiving the drug and a control group receiving a placebo. Over the 5-year follow-up period, $400$ patients in the intervention group and $600$ patients in the control group experience a MACE. Safety analysis reveals that severe myopathy occurred in $150$ patients receiving the intervention and $50$ patients receiving the placebo.

Match each epidemiological metric described below with its correct calculated value based on the trial data.

The absolute decrease in the probability of experiencing a major adverse cardiovascular event due to the intervention $4\%$
The proportion of the baseline risk for major adverse cardiovascular events that is eliminated by the therapy $33.3\%$
The ratio comparing the probability of major adverse cardiovascular events in the treated group to that in the control group $0.67$
The number of individuals who must be treated with the intervention to prevent one major adverse cardiovascular event $25$
The number of individuals who must be treated with the intervention to cause one additional case of severe myopathy $50$

Cevap

Absolute risk decrease (ARR) is 4%, baseline risk proportion eliminated (RRR) is 33.3%, relative probability ratio (RR) is 0.67, Number Needed to Treat (NNT) is 25, and Number Needed to Harm (NNH) is 50.

Each narrative description fundamentally corresponds to a specific epidemiological calculation. By first determining the event rates for the intervention group (

8\%

for MACE,

3\%

for myopathy) and the control group (

12\%

for MACE,

1\%

for myopathy), each metric can be derived using its standard equation, yielding the precise numerical values provided on the right.

Adım Adım Çözüm

Calculate the event risks (incidence) for both outcomes in each trial arm.

MACE: Control = 12%, Intervention = 8%. Myopathy: Control = 1%, Intervention = 3%.

Raw event rates are the foundational variables required for all subsequent epidemiological calculations.

Derive the association metrics for the primary efficacy outcome (MACE).

ARR = 12% - 8% = 4%. RRR = 4% / 12% = 33.3%. RR = 8% / 12% = 0.67.

These calculations address the first three descriptive items requiring absolute and relative treatment effects.

Calculate the Number Needed to Treat (NNT) for MACE.

NNT = 1 / 0.04 = 25.

NNT translates the absolute risk reduction into a clinical volume metric, answering how many patients require treatment to avert one event.

Calculate the Number Needed to Harm (NNH) for severe myopathy.

ARI = 3% - 1% = 2%. NNH = 1 / 0.02 = 50.

NNH translates the absolute risk increase of adverse events into a population impact metric.

Anahtar Kavram

Derivation and conceptual interpretation of Absolute Risk Reduction, Relative Risk Reduction, Relative Risk, Number Needed to Treat, and Number Needed to Harm from a randomized controlled trial.

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