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Difficulty: MediumMeasures of Association and Impact

A prospective cohort study is conducted to evaluate the association between the regular use of a novel synthetic oral glucocorticoid and the development of avascular necrosis (AVN) of the femoral head. The study follows 4,0004,000 patients prescribed the new glucocorticoid and 6,0006,000 patients prescribed standard nonsteroidal anti-inflammatory drugs (NSAIDs) for similar rheumatologic conditions. Over a 1010-year follow-up period, 160160 patients in the glucocorticoid group and 6060 patients in the NSAID group develop AVN.

Based on these findings, what percentage of the AVN cases in the glucocorticoid group is theoretically attributable to the use of the new drug?

Answer: 75 %

Answer

75
The correct answer is 7575. The Attributable Risk Percent (ARP) represents the fraction of disease in the exposed group that is directly attributable to the exposure. It is calculated as Incidence in exposed−Incidence in unexposedIncidence in exposed\frac{\text{Incidence in exposed} - \text{Incidence in unexposed}}{\text{Incidence in exposed}}. In this study, the incidence in the exposed (glucocorticoid) group is 160/4,000=4%160 / 4,000 = 4\%. The incidence in the unexposed (NSAID) group is 60/6,000=1%60 / 6,000 = 1\%. Therefore, ARP=4%−1%4%=3%4%=75%\text{ARP} = \frac{4\% - 1\%}{4\%} = \frac{3\%}{4\%} = 75\%. Alternatively, this can be calculated using Relative Risk (RR). RR=4%/1%=4RR = 4\% / 1\% = 4. ARP=RR−1RR=4−14=75%\text{ARP} = \frac{RR - 1}{RR} = \frac{4 - 1}{4} = 75\%.

Step-by-Step Solution

1
Calculate the incidence of the outcome in the exposed group.
Ie=1604,000=0.04I_e = \frac{160}{4,000} = 0.04 (or 4%4\%)
This establishes the total absolute risk of developing the disease when the exposure is present.
2
Calculate the incidence of the outcome in the unexposed group.
Iu=606,000=0.01I_u = \frac{60}{6,000} = 0.01 (or 1%1\%)
This establishes the baseline risk of the disease occurring without the exposure.
3
Calculate the Attributable Risk Percent (ARP).
ARP=Ie−IuIe×100%=0.04−0.010.04×100%=75%\text{ARP} = \frac{I_e - I_u}{I_e} \times 100\% = \frac{0.04 - 0.01}{0.04} \times 100\% = 75\%
This formula determines the specific proportion of cases in the exposed group that are a direct result of the exposure.

Key Concept

Attributable Risk Percent (ARP) measures the proportion of disease in an exposed group that is directly attributable to the exposure, calculated as (Ie−Iu)/Ie(I_e - I_u) / I_e or (RR−1)/RR(RR - 1) / RR.
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Topics

Measures of Association and Impact

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