A cohort study is conducted to evaluate the risk of hepatotoxicity associated with a novel… — USMLE Step 3

A cohort study is conducted to evaluate the risk of hepatotoxicity associated with a novel antifungal agent, MycoStop, compared to standard fluconazole therapy in adult patients. The study follows $5,000$ patients prescribed MycoStop and $5,000$ patients prescribed fluconazole over a 12-month period. At the end of the follow-up, $250$ patients in the MycoStop group and $50$ patients in the fluconazole group developed signs of hepatotoxicity.

During the data analysis, researchers perform a stratified analysis to account for concurrent daily use of acetaminophen. The relative risk (RR) of hepatotoxicity with MycoStop compared to fluconazole is found to be $6.2$ in patients taking acetaminophen daily, and $1.5$ in patients who do not take acetaminophen. The crude relative risk for the entire cohort is $5.0$ .

Based on these results, what is the Number Needed to Harm (NNH) for MycoStop compared to fluconazole, and how should concurrent acetaminophen use be classified?

NNH = $25$ ; concurrent acetaminophen use is an effect modifierCevap
B
NNH = $25$ ; concurrent acetaminophen use is a confounding variable
C
NNH = $20$ ; concurrent acetaminophen use is an effect modifier
D
NNH = $100$ ; concurrent acetaminophen use is a confounding variable
E
NNH = $100$ ; concurrent acetaminophen use is an effect modifier

Cevap

NNH =

25

; concurrent acetaminophen use is an effect modifier

The correct answer accurately calculates the Absolute Risk Increase as

0.04

(

0.05 - 0.01

), which results in a Number Needed to Harm (NNH) of

25

(

1/0.04

). Furthermore, because the stratum-specific relative risks (

6.2

and

1.5

) are substantially different from each other, concurrent acetaminophen use modifies the effect of MycoStop on hepatotoxicity, making it an effect modifier.

Adım Adım Çözüm

Calculate the incidence of hepatotoxicity in both the exposed and unexposed groups.

MycoStop incidence =

250 / 5,000 = 0.05

(

5\%

). Fluconazole incidence =

50 / 5,000 = 0.01

(

1\%

Baseline and exposed risk rates are required to determine the absolute impact of the new medication.

Calculate the Absolute Risk Increase (ARI) and Number Needed to Harm (NNH).

ARI =

0.05 - 0.01 = 0.04

. NNH =

1 / 0.04 = 25

NNH is defined as the inverse of the ARI, representing the number of patients treated to cause one additional adverse event.

Compare the crude relative risk (RR) with the stratum-specific RRs to determine the role of acetaminophen use.

The stratum-specific RRs (

6.2

and

1.5

) are significantly different from each other.

When stratum-specific measures of association differ substantially from one another, it indicates effect modification, meaning the third variable biologically modifies the exposure-outcome relationship.

Anahtar Kavram

The Number Needed to Harm (NNH) is calculated as

1 / \text{Absolute Risk Increase (ARI)}

. Effect modification is present when the measure of association differs significantly across strata of a third variable.

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