University:
Western Sydney University
Course:
MEDI 2003 | Clinical Sciences 2
Academic year:

2024
Views:

114

Pages:

2
Author:

Mariam A. A.

Measure of disease occurrence • Objective of epidemiology: obtain valid and precise estimates of the effect of a potential cause on the occurrence of disease • Often a binary outcome (case/non-case; dead/alive) • To achieve this we need to measure disease frequency • Main measures of disease frequency: o Incidence rate o Incidence proportion o Prevalence Incidence rate (aka person-time rate, incidence density, hazard rate, disease intensity) • Rate at which new events occur in a population: (number of new cases of disease in specified period/total amount of person-time at risk during period) • Example: A follow-up study of MI in WA men: 170 men had a first MI during 27,825 personyears follow-up: o IR = 170 / 27,825 = 0.0061 per person-year ▪ = 6.1 per 1,000 person years ▪ = 610 per 100,000 person-years Cumulative incidence • Within a specified time period, the incidence number of cases can also be expressed in relation to the size of the population at risk 🡪 IP = (Number of new cases of disease in a specified period/ Number of people at risk of disease at start of period) • Example: follow-up survey of 482 women using oral contraceptives, 27 developed a urinary tract infection during the 3 year follow-up period o IP = 27 / 482 = 0.056 = 5.6% in 3 years or 1.9% in 1 year • IP = measure of risk/average risk: probability that disease develops in a person within specified time interval • The IP is a proportion not a rate. It has a range of 0-1 • A relevant time period must always be specified • Is more accurate over short time periods or for relatively rare diseases • Assumes no losses over specified time Incidence rate vs. incidence proportion • Incidence Proportion (cumulative incidence) measures risk of getting disease • Incidence Rate measures rate at which cases occur Prevalence • Prevalence focusses on burden of disease, not new events • Prevalence (point prevalence) measures proportion of a population with disease at a specific point in time Number of cases of disease Number of people in population • E.g. Among a sample of 2,382 Australian adults on 31/12/2014, there were 432 cases of depression. The prevalence of depression among this group was = 432 / 2382 = 0.1813 = 18.1% OR 181 per 1,000 ‘Period’ prevalence • Period prevalence measures the proportion of the population with disease during a specified period of time • E.g: “Did your child have diarrhoea during any of the last 7 days?” 🡪 often used for short duration condition, allows smaller sample size, prone to recall bias • Extreme example of period prevalence is lifetime prevalence or “Have you ever in your life …….?” o Very prone to recall bias 35 Prevalence and incidence 🡪 prevalence ≈ Incidence x Duration • Only if: o Stationary population o Mortality rate is low, recovery rate is low and no big changes in incidence over time o People sometimes use prevalence in place of incidence to measure disease occurrence, but it is rare that these conditions are met Note • Clear definitions of numerator (cases) and denominator (population at risk) are essential • Numerator and denominator must refer to the same time period • Everyone in the numerator must also be in the denominator (persons or person-time) • All individuals in the denominator must be ‘at risk’ of moving to the numerator • When comparing measures check that they refer to the same number of people (e.g. per 1,000)