Measures of Disease Frequency

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 change prevalence 
    • 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