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Difficulty: MediumMeasures of Association and Impact

A public health department is investigating an increased incidence of chronic kidney disease of unknown etiology (CKDu) in a specific rural region. Investigators conduct a 10-year cohort study to evaluate the association between exposure to a specific agrochemical mixture and the development of CKDu. The study includes agricultural workers with documented daily exposure to the mixture and non-agricultural workers from the same region without exposure. The findings are summarized in the following table:

Exposure StatusCKDu DevelopedTotal Subjects
Exposed160800
Unexposed601,200

Based on these study results, what percentage of the CKDu cases among the exposed agricultural workers can be attributed to the agrochemical exposure?

  1. A
    15%
  2. B
    20%
  3. C
    25%
  4. 75%Answer
  5. E
    300%

Answer

75%
The correct answer is 75%75\%. The question asks for the percentage of disease cases in the exposed group that can be attributed to the exposure, which is the Attributable Risk Percent (ARP), also known as the etiologic fraction. ARP is calculated using the formula: ARP=Incidence in exposed−Incidence in unexposedIncidence in exposedARP = \frac{\text{Incidence in exposed} - \text{Incidence in unexposed}}{\text{Incidence in exposed}}. First, find the incidence in the exposed group (160/800=0.20160 / 800 = 0.20 or 20%20\%). Next, find the incidence in the unexposed group (60/1,200=0.0560 / 1,200 = 0.05 or 5%5\%). Finally, apply the formula: 0.20−0.050.20=0.150.20=0.75\frac{0.20 - 0.05}{0.20} = \frac{0.15}{0.20} = 0.75, which is 75%75\%. This means 75%75\% of the CKDu cases among these agricultural workers are due to the agrochemical exposure.

Step-by-Step Solution

1
Calculate the incidence of CKDu in the exposed group (IeI_e).
Ie=160800=0.20I_e = \frac{160}{800} = 0.20 (20%20\%).
This establishes the total disease risk among those exposed to the agrochemicals.
2
Calculate the incidence of CKDu in the unexposed group (IuI_u).
Iu=601,200=0.05I_u = \frac{60}{1,200} = 0.05 (5%5\%).
This establishes the baseline risk of the disease in the population without the exposure.
3
Calculate the Attributable Risk Percent (ARP).
ARP=Ie−IuIe=0.20−0.050.20=0.150.20=0.75ARP = \frac{I_e - I_u}{I_e} = \frac{0.20 - 0.05}{0.20} = \frac{0.15}{0.20} = 0.75 (75%75\%).
ARP determines the proportion of the disease in the exposed group that is directly linked to the exposure, accounting for the baseline risk.

Key Concept

Attributable Risk Percent (ARP)
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Topics

Measures of Association and Impact

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