16. Epidemiological indicators I: indicators of disease frequency and population impact of a disease

From greek.doctor
  • Maths
    • A fraction = number of cases / population
    • A ratio = a number in relation to another one
    • A proportion = the ratio of a part of the population to the total
    • A rate = a proportion over time
  • Indicators of disease frequency
    • Number of cases
      • Simplest measure, but doesn’t take into account the size of the population
      • Useful for allocating health resources
    • Incidence rate
      • New cases of a disease in the population over a period of time
      • Good for diseases with a short duration
      • Important when studying the etiology of the disorder
    • Cumulative incidence rate
      • Proportion of new cases in the population over a period of time
      • Number of new cases over a period of time / population at risk over that period of time
      • For example: out of 5500 sex workers at risk for HIV in a 4-year period, 40 contracted HIV -> 40/5500 = 0,007 = 7%
      • Easy to interpret
    • Incidence density
      • Number of new cases in the population / the number of time the population has spent at risk
      • The unit of the incidence density is “cases of disease/patient-year”
      • For example: 194 cases of breast cancer per 100 000 patient-years
      • Difficult to interpret
      • Gives a better estimate of true risk than the incidence
    • Prevalence rate (often called just prevalence)
      • Measures all cases in the population, not just new cases
      • Period prevalence = number of all cases over a period of time / population at risk over that period of time
      • Point prevalence = number of all cases at a certain point in time / population at risk at that point in time
      • Lifetime prevalence = number of people who ever had the disease during their lifetime / population at risk at the beginning of the period
      • Good for permanent diseases, conditions, or risk factors
      • Gives an estimate of societal burden of the disorder
    • Relationship between incidence and prevalence
      • A common disease with short duration, like the common cold, has a high incidence but low prevalence
      • A less common disease with a long duration, like diabetes type 2, has a low incidence but high prevalence
    • Morbidity rate – when we measure cases of disease
    • Mortality rate – when we measure death caused by disease
    • Lethality (case fatality rate)
      • = number of people who died because of disease / number of people who had the disease
  • Indicators of population impact of a disease
    • These are all measures of disease burden
    • Attributable risk (AR)
      • Shows the proportion of cases in exposed individuals which can be attributed to the exposure
      • The difference between the risk of disease in the population exposed to the risk factor, and the risk of disease in the non-exposed population
      • AR = incidence of exposed population – incidence of non-exposed population
      • Used in cohort studies
    • Population attributable risk (PAR)
      • Shows how much of the incidence of a disease would decrease if the risk factor was eliminated
      • The difference between the risk of disease in the population as a whole, and the risk of disease in the population not exposed to the risk factor
      • PAR = incidence of whole population – incidence of non-exposed population
    • Years of potential life lost (YPLL)
      • How many years a young person would live if they didn’t die prematurely
    • Quality-adjusted life year (QALY)
      • Used to evaluate the value of public health interventions
    • Disability-adjusted life year (DALY)
      • Includes both YPLL and QALY into one measurement
      • DALY = years lived with disability + years of life lost
    • Numbers needed to treat