Measures of Disease Occurrence and Frequency PDF

Measures of disease occurrence and frequency Dr. Mais Alkhalili Community Medicine Specialist  Tuberculosis in New York City ▪ Tuberculosis is a reportable condition ▪ All diagnosed cases must be reported to the department of health ▪ In 2011, there were 689 ne...

Measures of disease occurrence and frequency Dr. Mais Alkhalili Community Medicine Specialist  Tuberculosis in New York City ▪ Tuberculosis is a reportable condition ▪ All diagnosed cases must be reported to the department of health ▪ In 2011, there were 689 new cases of tuberculosis in New York City Is this information useful? Epidemiology matters - Chapter 5 2  Counts ▪ Provide an absolute number of the burden of disease ▪ However counts has limited utility for two reasons ▪ The burden of disease in the population is very different if the population size is 100,000 versus 1,000,000 ▪ Some people are not at risk for developing a new onset of tuberculosis in 2011 (due to pre-existing infection), thus we need to know not only the size of the total population, but the size of the total population at risk Epidemiology matters - Chapter 5 3  Incidence and prevalence ▪ Two measures overcome many of the limitations of a simple count of cases - incidence and prevalence ▪ Prevalence tells us about the proportion of cases among the total population at any given time ▪ Incidence tells us the probability of a new onset of disease among those at risk for developing the illness Epidemiology matters - Chapter 5 4  Prevalence The proportion of current people who have the disease (preexisting cases plus new cases) over the total population for a given time period Estimates the magnitude of a health problem (burden). Epidemiology matters - Chapter 5 5 Disease occurrence in a sample of population over time Year 1, 5 individuals developed the outcome Year 2, an additional 7 people developed the outcome Year 3, an additional 4 people developed the outcome What is the prevalence of disease in Year 2? ▪ What is the numerator? 5 cases in Year 1 + 7 cases in Year 2 = 12 ▪ What is the denominator? Total sample size = 30 ▪ Prevalence = 12/30 = 0.4 The prevalence of disease in Year 2 is 40% Epidemiology matters - Chapter 5 7 What is the prevalence of disease in Year 3? ▪ What is the numerator? 5 cases in Year 1 + 7 cases in Year 2 + 4 cases in Year 3 = 16 ▪ What is the denominator? Total sample size = 30 ▪ Prevalence = 16/30 = 0.533 ▪ The prevalence of disease in Year 3 is 53.3% Epidemiology matters - Chapter 5 8  Summary: Prevalence ▪ For prevalence, we need a numerator (number of existing cases), and denominator (total sample size), and a time period of interest ▪ The time period should be specified as much as possible Epidemiology matters - Chapter 5 9  Incidence ▪ Perhaps the most widely used tool in epidemiology ▪ Goes by many names - most common alternative name is “risk,” and less commonly, “incidence proportion”, cumulative incidence. ▪ Numerator = number of new cases ▪ Denominator = population at risk of becoming a new case at start of time interval ▪ Specified over a specific time period Epidemiology matters - Chapter 5 10 What is the incidence of disease in Year 2? ▪ What is the numerator? 7 new cases in Year 2 ▪ What is the denominator? 25 people at risk (5 people already developed the disease in Year 1 and are thus not at risk) ▪ Incidence = 7/25 = 0.28 The incidence (risk) of disease in Year 2 is 28% Epidemiology matters - Chapter 5 11 What is the incidence of disease in Years 2 and 3? ▪ What is the numerator? 7 new cases in Year 2 + 4 new cases in Year 3 = 11 ▪ What is the denominator? ▪ 25 people at risk (5 people already developed the disease in Year 1 and are thus not at risk) ▪ Incidence = 11/25 = 0.44 The incidence (risk) of disease in Years 2 and 3 is 44% Epidemiology matters - Chapter 5 12  Summary: Incidence ▪ For incidence, we need a numerator (number of new cases), and denominator (total sample size at risk), and a time period of interest ▪ The time period should again be specified as much as possible Epidemiology matters - Chapter 5 13  Understanding incidence and prevalence: the bathtub example Cure or death Epidemiology matters - Chapter 5 14  The relation between incidence and prevalence The following formula can express the relationship between prevalence, incidence, and duration of a disease : Prevalence = Incidence × Duration of Disease If the incidence is high and the disease lasts for a long time, prevalence will be high. If the incidence is low or the disease has a short duration (e.g., it leads quickly to recovery or death), prevalence will be low. ❖ This relationship assumes a stable population with no significant migration, births, or deaths affecting the prevalence other than those caused by the disease itself. Epidemiology matters - Chapter 5 15 Examples of the relation between incidence and prevalence ▪ High incidence, steady prevalence [ P = I * D ] ▪ Example: a highly contagious infectious disease with a very short duration or a high case fatality ▪ Low incidence, high prevalence [ P = I * D ] ▪ Examples: diseases with long duration such as arthritis, diabetes, Crohn’s disease, and other chronic illnesses Epidemiology matters - Chapter 5 16 Examples of the relation between incidence and prevalence Impact of a new treatment that prolongs life with the disease but does not cure it People Living with HIV New HIV Infections Epidemiology matters - Chapter 5 17  Summary: incidence, prevalence ▪ Prevalence is affected by incidence and duration ▪ If a disease has a short duration, [ P = I * D ] ▪ Prevalence ~= incidence* ▪ If a disease has a long duration, in general, ▪ Prevalence > incidence * Assumes that incidence is constant over time Epidemiology matters - Chapter 5 18