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Difficulty: Very hardMeasures of Disease Frequency and Association

An industrial city's health department is planning an intervention to completely eliminate occupational exposure to a specific aerosolized solvent. Historical data show that 30% of the city's adult residents are exposed to this solvent at their workplaces. Extensive cohort studies have demonstrated that the relative risk of developing a severe demyelinating neuropathy in exposed residents compared to unexposed residents is 3.03.0. The baseline annual incidence of this neuropathy in the unexposed population is 55 cases per 1,0001,000 people.

Assuming the intervention successfully reduces exposure to zero with an immediate effect on risk, and the population size remains stable, how many incident cases of this neuropathy will be prevented per 100,000100,000 residents in the first year?

Answer: 300 cases

Answer

300
The number of cases prevented in the entire population by eliminating the exposure is determined by the Population Attributable Risk (PAR). PAR is most directly calculated by determining the Attributable Risk (AR) in the exposed group (15−5=1015 - 5 = 10 per 1,0001,000) and multiplying it by the prevalence of the exposure (30%30\%). This yields a PAR of 33 per 1,0001,000, which scales to 300300 cases per 100,000100,000 population.

Step-by-Step Solution

1
Calculate the incidence in the exposed population (IeI_e).
Ie=15I_e = 15 cases per 1,0001,000 people
The relative risk (RR) is 3.03.0 and the baseline incidence in the unexposed (IuI_u) is 55 per 1,0001,000. Therefore, Ie=RR×Iu=3.0×5=15I_e = RR \times I_u = 3.0 \times 5 = 15.
2
Calculate the Attributable Risk (AR).
AR=10AR = 10 cases per 1,0001,000 people
AR is the excess incidence among the exposed individuals compared to the unexposed. AR=Ie−Iu=15−5=10AR = I_e - I_u = 15 - 5 = 10.
3
Calculate the Population Attributable Risk (PAR).
PAR=3PAR = 3 cases per 1,0001,000 people
PAR represents the excess incidence in the total population that is attributable to the exposure. It is calculated by multiplying the AR by the prevalence of exposure (PP). PAR=AR×P=10×0.30=3PAR = AR \times P = 10 \times 0.30 = 3.
4
Scale the PAR to the requested population size (100,000100,000).
300300 cases prevented
The question asks for the number of prevented cases per 100,000100,000 people. Since the PAR is 33 per 1,0001,000, we multiply by 100100 to scale the denominator: 3×100=3003 \times 100 = 300.

Key Concept

Population Attributable Risk (PAR) represents the excess incidence of a disease in a total population that is attributable to a specific exposure, and it indicates the absolute number of cases that could be prevented if the exposure were completely eliminated.
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Topics

Measures of Disease Frequency and Association

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