Updated: 1/8/2021

Measures of Disease Frequency

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Introduction
  • Overview
    • incidence and prevalence are methods of measuring disease frequency in a population with respect to time
  • Incidence
    • describes the amount of new disease cases in at-risk people over a certain time period
    • at risk means those who are capable of developing the disease of interest 
      • do not already have the disease
      • are not vaccinated against the disease
      • have at-risk anatomy (patients without a prostate cannot get prostate cancer)
    • incidence can be specified 2 ways
      • # of new cases in a population/# of at risk people in the population per unit time  
        • e.g., 9 cases of Kawasaki disease per 10,000 children per year
      • # of new cases in a population/time spent at risk (person-time) 
        • person-time = number of people at risk x time spent at risk
          • e.g., 9 cases of Kawasaki disease/10,000 child-years 
            • if we watched 10,000 at-risk people (children) for 1 year, 5,000 children for 2 years, or 1,000 children for 10 years, etc., we would see 9 new cases during that observation period
  • Prevalence
    • proportion of population that has a disease or risk factor at a specified point or period in time 
    • existing cases/total number of people in specific population at a particular time
      • e.g., percent of the U.S. population with diabetes in 2018
    • indicates overall disease burden of population
    • helpful for resource allocation
    • prevalence = incidence x duration 
    • factors that increase prevalence include
      • increase in new cases (increased incidence)
      • improved quality of care → decreased mortality  longer duration
      • improved diagnostic ability → higher incidence
      • in-migration of cases or susceptible people
      • out-migration of healthy people
    • factors that decrease prevalence include
      • high case-fatality rate → shorter duration (aggressive cancers)
      • decrease in new cases (decreased incidence)
        • could result from preventative efforts such as vaccination 
        • in-migration of healthy people
        • out-migration of cases
        • improved recovery/cure rate 

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