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Difficulty: MediumMeasures of Disease Frequency

A regional health department is assessing the burden of a chronic autoimmune myopathy in a stable regional population of 200,000200,000 adults. Active epidemiological surveillance over the past decade demonstrates a steady incidence rate of 4040 new cases per 100,000100,000 persons per year. A breakthrough biologic therapy, introduced uniformly across the region several years ago, does not cure the condition but has successfully increased the average post-diagnosis survival time from 33 years to 55 years. Assuming steady-state epidemiological conditions have been reached, what is the expected total number of individuals currently living with this disease in the region?

  1. A
    120120
  2. B
    200200
  3. C
    240240
  4. 400400Answer
  5. E
    800800

Answer

The expected total number of individuals currently living with the disease is 400400.
In a steady-state population, the point prevalence (the total number of existing cases at a specific time) of a rare disease can be estimated using the relationship: Prevalence≈Incidence×Duration\text{Prevalence} \approx \text{Incidence} \times \text{Duration}. First, the absolute incidence for the entire region must be calculated. The incidence rate is given as 4040 cases per 100,000100,000 persons per year. For a regional population of 200,000200,000, the absolute incidence is 40×2=8040 \times 2 = 80 new cases per year. The introduction of the new biologic therapy successfully increased the average disease duration (post-diagnosis survival) from 33 years to 55 years. Using the updated steady-state duration, the expected prevalence is 80 cases/year×5 years=40080 \text{ cases/year} \times 5 \text{ years} = 400 living prevalent cases.

Step-by-Step Solution

1
Calculate the absolute annual incidence for the region.
4040 cases per 100,000100,000 persons translates to 8080 new cases per year for the target population of 200,000200,000.
Epidemiological rates must be scaled to the actual population size to determine absolute case numbers.
2
Identify the current steady-state disease duration.
The new average survival time is 55 years.
The introduction of the biologic therapy has established a new steady state, replacing the historical 33-year survival metric.
3
Calculate the point prevalence.
Prevalence=Incidence×Duration=80×5=400\text{Prevalence} = \text{Incidence} \times \text{Duration} = 80 \times 5 = 400 prevalent cases.
In a steady-state population with a rare disease, the number of existing cases equals the annual influx of new cases multiplied by the average time individuals live with the disease.

Key Concept

The relationship between point prevalence, absolute incidence, and disease duration in steady-state epidemiology (P≈I×DP \approx I \times D).
Estimated Time:1m 30s
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Topics

Measures of Disease Frequency

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