Measures of Disease Frequency - Epidemiology | PDF

Measures of Disease Frequency Edric D. Estrella, PhD, MPH, RPh Principles of Epidemiology Diploma in International Health Program Faculty of Management and Development Studies University of the Philippines Open University  Recall:...

Measures of Disease Frequency Edric D. Estrella, PhD, MPH, RPh Principles of Epidemiology Diploma in International Health Program Faculty of Management and Development Studies University of the Philippines Open University  Recall: Descriptive Epidemiology (generation of hypothesis) person Description of the Distribution of place disease occurrence Epidemiology Identification of disease time determinants Analytical Epidemiology (test the hypothesis)  Count basic measure of disease frequency absolute number of persons who possess a certain characteristic of interest very limited utility example: ???  Hypothetical data Location New cases of Reporting period Population size dengue District A 179 2011 35,400 District B 96 2011-2012 7,000 Annual rate of Occurrence of dengue: District A: 179/35,400/ 1 year = 506 x 100,000 per year District B: 96/7,000/ 2 years = 1371 x 100,000 per 2 years = 686 x 100,000 per year  Counts To allow direct comparisons, it is imperative to: – know the size of the source population where such count is derived; – time period of data collection  Ratio obtained by simply dividing one quantity by another without implying any specific relationship between the numerator and denominator (i.e., numerator is not part of the denominator) expressed in the form: A:B or A/B x (k) where: A and B are the 2 quantities k is a factor (usually a multiple of 10) example: ???  Proportion a special type of ratio where the numerator is part of the denominator expressed as: A/(A+ B) example:???  Rate Strictly defined, is a ratio in which there is a distinct relationship between the numerator and denominator and most essentially, a measure of time is an intrinsic part of the denominator. expressed as: A/B x F Contains the following elements: – Disease frequency – numerator – Unit size of the population – denominator – Time period during which the event occurs (beginning and end) example: ???  Measures of Disease Frequency/ Occurrence or Morbidity 1. Prevalence – Point prevalence – Period prevalence 2. Incidence – Incidence proportion/Cumulative incidence – Incidence rate/Incidence density  Prevalence proportion of individuals in a population who have a particular disease or condition at a specified point in time cases = old + new point in time = calendar time or fixed point in the course of events that varies in real time from person to person  Prevalence still picture of the disease experience cases are counted in prevalence until they recover or die appropriate for stable chronic conditions such as congenital defects, non-lethal degenerative diseases with no clear onset, mental disease  Prevalence At a given time: number of cases of a disease (old + new) P= total population examined usually expressed as percentage  Prevalence Point prevalence – measures the frequency of an existing outcome at one point in time Period prevalence – measures the frequency of an existing outcome during a given period  What is the point prevalence of Disease X in 2002? Subject 1999 2000 2001 2002 2003 2004 2005 2006 A B C D E The prevalence of Disease X in 2002 is 2/5 or 40%.  Incidence indicates the development of new cases of a disease in a population during a specified period of time gives information about the speed of development of a disease condition estimates the risk of developing a disease in the population during the specified time period  Incidence denominator includes all people who are “at risk” at the start of the time period of interest appropriate for acute illnesses  Incidence measure of choice in determining etiologic factors 2 types of incidence measures – Cumulative Incidence/ Incidence proportion – Incidence density/ Incidence rate  Risk the probability that a person will develop the disease within a specified period of time difficult to ascertain an individual’s risk epidemiology focuses on the average risk of a group of individuals  Types of Population Fixed – membership is defined by a life event and is permanent Dynamic or open – membership is defined by a changeable state or condition and is transitory  Cumulative Incidence proportion of people who become diseased during a specified period of time average risk of developing the “disease” appropriate when the population at risk is “fixed” assumes that all people in the population contributed the same amount of time to the measure unitless value ranges from 0 to 1  Cumulative Incidence no. of new cases of a particular disease in a given period of time 𝐶. 𝐼. = Total population at risk of developing the disease in the same time period  Attack rate not a rate but a proportion special type of incidence proportion used during an infectious disease outbreak or epidemic useful when the development of the outcome rapidly follows the exposure during a time period that is fixed because of the nature of the disease process expressed for the entire epidemic period, from the first to the last case  Incidence density instantaneous rate of development of disease in a population force of morbidity or mortality appropriate when the population at risk is dynamic or open  Incidence density denominator is the total “person-time at risk” – length of time that an individual stays at risk of the disease of interest (person remained under observation and free from disease) Total person-time at risk is equal to the: – sum of each individual’s time at risk, or – product of the average population size and length of follow-up ( n x t )  Incidence density true rate unit: 1/time or time-1 may take any value from 0 to infinity  Incidence density no. of new cases of a disease developing in a period of time I.D. = population at risk of developing the disease during the same period of time (Total Person Time) 1st Possible Scenario: Enter at the same time but some lost to ff up A B C D E J F M A M J J A S O N D 2nd Possible Scenario: Enter the study at different points over the study period but common termination points A B C D E J F M A M J J A S O N D 3rd Possible Scenario: Enter at different points, terminate at different points A B C D E J F M A M J J A S O N D  Calculation of Person Time (PT) Case 1: Case 2: PT= sum of each If individual observation individual’s time at risk period not known: or the sum of the time that each person Estimated PT= product remained under of the average size of observation the population and the length of the follow-up period  Case 1 (known individual observation period) 90 91 92 93 94 95 96 97 98 99 00 Time at risk A 6.0 B X 6.0 C 10.0 D 8.5 E X 5.0 Total years at risk 35.5 --- time followed X disease onset  Incidence Rate = 2/35.5 person years = 0.056 cases/person year = 5.6 cases / 100 person years = 56 cases / 1000 person years Case 2: Unknown individual observation period PT= average population x length of follow up No. at beginning of the observation period = 700 people No. at the end of the period = 520 people No. of people who developed the disease = 400 people Length of follow-up = 4 years Person-time = (700 + 520) x 4 2 = 2,440 Incidence Rate = 400 2,440 = 16.3 per 100 person years  Example During the period Jan. 1, 2000 - Dec. 31, 2001, there were 870 cases of bacterial meningitis within a geographical area where the average population size during the two year period was 7,250,000. Compute for the incidence. 34  Solution 𝐼. 𝐷. 870 = × 100,000 (7,250,000 × 2 𝑦𝑒𝑎𝑟𝑠) = 6/100,000 person-years 35  Assumptions for Incidence: a. The population is stable. (no drastic changes in the size and age structure of the population) b.The rate is more or less constant. Interrelationship Between Incidence & Prevalence P≈IxD Assumptions: a. disease is in a steady state incidence constant duration constant b. Prevalence is low ( < 0.1 )