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Difficulty: MediumMeasures of Disease Frequency and Association

A prospective cohort study is conducted to evaluate the association between the use of a newly approved oral contraceptive and the incidence of deep vein thrombosis (DVT) in women aged 2020 to 3535. The study follows 10,00010,000 women using the new oral contraceptive and 20,00020,000 women using a non-hormonal copper intrauterine device (IUD) over a 55-year period. During the follow-up, 4040 women in the oral contraceptive group and 2020 women in the IUD group develop a DVT. Based on these findings, what percentage of DVT cases among the oral contraceptive users can be attributed to the use of the new medication?

  1. A
    0.3%0.3\%
  2. B
    25%25\%
  3. 75%75\%Answer
  4. D
    300%300\%
  5. E
    400%400\%

Answer

75%75\%
The question asks for the percentage of DVT cases among the oral contraceptive users that can be attributed to the new medication. This is the definition of Attributable Risk Percent (ARP). ARP is calculated using the formula: ARP=Risk in exposed−Risk in unexposedRisk in exposedARP = \frac{\text{Risk in exposed} - \text{Risk in unexposed}}{\text{Risk in exposed}}. First, we find the risk in the exposed group: 4040 cases per 10,00010,000 users =0.004= 0.004 (or 0.4%0.4\%). Next, we find the background risk in the unexposed group: 2020 cases per 20,00020,000 users =0.001= 0.001 (or 0.1%0.1\%). Subtracting the background risk from the exposed risk yields the absolute attributable risk: 0.004−0.001=0.0030.004 - 0.001 = 0.003. Dividing this absolute difference by the total risk in the exposed group gives the ARP: 0.003/0.004=0.750.003 / 0.004 = 0.75, or 75%75\%. This means that 75%75\% of the DVT cases in the exposed group were excess cases specifically caused by the medication.

Step-by-Step Solution

1
Calculate the incidence rate (risk) of DVT in the exposed group (oral contraceptive users).
Risk in exposed = 40/10,000=0.00440 / 10,000 = 0.004 (or 0.4%0.4\%).
This establishes the total risk of developing the outcome in individuals with the risk factor.
2
Calculate the incidence rate (risk) of DVT in the unexposed group (IUD users).
Risk in unexposed = 20/20,000=0.00120 / 20,000 = 0.001 (or 0.1%0.1\%).
This establishes the baseline (background) risk of developing the outcome without the risk factor.
3
Calculate the Attributable Risk Percent (ARP) using the formula: Risk in exposed−Risk in unexposedRisk in exposed×100%\frac{\text{Risk in exposed} - \text{Risk in unexposed}}{\text{Risk in exposed}} \times 100\%.
0.004−0.0010.004×100%=0.0030.004×100%=75%\frac{0.004 - 0.001}{0.004} \times 100\% = \frac{0.003}{0.004} \times 100\% = 75\%.
ARP represents the proportion of disease cases among exposed individuals that can be directly attributed to the exposure.

Key Concept

Attributable Risk Percent (ARP) is calculated as Risk in exposed−Risk in unexposedRisk in exposed\frac{\text{Risk in exposed} - \text{Risk in unexposed}}{\text{Risk in exposed}}, and it represents the fraction of cases among the exposed that are directly attributable to the exposure.
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Topics

Measures of Disease Frequency and Association

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