Disease Frequency Measure

Disease Frequency Measure A. Disease Frequency Measure Measures of Disease Frequency reflect the incidence of disease (morbidity) or death due to disease (mortality) in a population. Disease frequency measures are usually measured as a rate or proportion. B. Incidence Incidence will reflect the number of new (incident) cases that develop over a period of time among the population at risk. A new case is a change in status from healthy to sick. The time period is the amount of time observed from healthy to sick. 1. Cumulative Incidence a. Other names: Risk, proportion of incidence b. Cumulative incidence = Risk = Proportion of Incidence c. Means the average risk of an individual getting the disease d. People in the denominator must be free of disease at the beginning of the period (observation or follow-up). e. Cumulative incidence = Risk = Proportion of Incidence f. This method is only feasible when there are few or no cases that escape observation due to death, not long at risk, missing from observation. g. Probability of an at-risk individual developing the disease in a given period of time h. States the individual did not die from any other cause during that period i. Dimensionless, scored from zero to one j. Refers to the individual k. Has a well-defined time reference period l. Cumulative incidence requires that all non-cases are observed during the entire observation period. 2. Incidence Density a. Other names: Person - time incidence, Incidence rate. b. Incidence density = Person-time incidence. c. Means the average rate for the population at risk during the specified time. d. Since the denominator is measured in person-time, it is not necessary that all individuals are observed for the same period. e. Incidence density expresses a number of new cases per person-time. f. For example: 9.1 cases/100 person-years The person-year units in the above example are: 1 x 100 x person-years = 4 x 25 person-years This number of person-years can be accumulated by observing 100 people for 1 year, 25 people for 4 years, 10 people for 10 years. C. Prevalent Prevalence reflects the number of existing cases (old and new cases) in a population at a given time or period of time. Prevalence reflects the probability that an individual becomes a case (or becomes ill) in a given time or period of time. Low prevalence can occur due to low incidence short duration of illness good treatment increased virulence of the disease resulting in quick death of the patient or both. For example, in cases that recover easily or in cases that die quickly. Since incidence depends on the emergence of new cases, a decrease in incidence can be due to an increase in the resistance of members of the population to the disease, a change in the etiology of the disease, and effective prevention. 1. Point of Prevalence a. Other names: prevalence, prevalence proportion probability that an individual becomes a case (or becomes ill) at a point in time b. Has no dimension c. Varies between zero and one d. Measures of disease frequency e. Prevalence formula = point prevalence = proportion of prevalence = f. Point prevalence 2. Period of Prevalence a. Annual of prevalence b. Lifetime of prevalence Period of Prevalence = Prevalence Period D. Incidence It is a measurement tool for research into the etiology of both acute and chronic diseases. It is a good indicator for estimating "risk" because incidence directly measures the chance that a healthy person will become ill. A high incidence rate of a disease indicates a high risk of getting the disease. Incidence provides information on the effectiveness of a disease prevention or intervention. A high prevalence rate of a disease does not necessarily indicate a high risk of getting the disease, as it could be due to: 1. Increased survival rate 2. Improved medical care A low prevalence rate of a disease may reflect the following conditions: 1. Rapid fatal process 2. Rapid healing process Data from prevalence measurements cannot be used to investigate disease etiology and measure risk. Data from prevalence measurements can be used to : estimate the need for health personnel and facilities, as well as to estimate the burden of a disease on the health care system.