Measures of Morbidity

PRINCIPLE OF EPIDEMIOLOGY MEASURES OF MORBIDITY Natural History of Disease 2        An individual is healthy, and at some point, biologic onset of a disease occurs. The person is often unaware of that point of time Later, symptoms develop The patient seeks medical attention. Hospitalization may be required, either for diagnosis or for treatment, or for both. A diagnosis is made and treatment may then follow. Outcomes:     cure control of the disease Disability death 3 progression of disease in a population 4  deaths occur in all of these rectangles but the death rate is proportionately greater in groups with more severe illness such as in those hospitalized. Which sources of data can be used to obtain information? 5      For the period of the illness that necessitates hospitalization, medical and hospital records are useful If hospitalization is not required, primary care providers’ records may be the best source. If we want information about the illness before medical care was sought, we may obtain this information from the patient using a questionnaire or an interview. If the patient cannot provide this information, we may obtain it from a family member The records of health insurers can provide very useful information. data source and rate 6   when we see rates for the frequency of occurrence of a certain disease, we must identify the sources of the cases and determine how the cases were identified. When we interpret the rates and compare them to rates reported in other populations and at other times, we must take into consideration the characteristics of the data source INCIDENCE RATE 7  The incidence rate of a disease is defined as the number of new cases of a disease that occur during a specified time and place Incidence rate 8 Incidence rate per 1, 000 No. of new cases of a disease in the population during a specified period of time ------------------------------------------ ×1,000 No. of persons who are at risk of developing the disease during that time Incidence rate 9    The critical element is NEW cases of disease. It is a measure of risk. This risk can be looked at in any population group  particular age group  among males or females  in an occupational group  In a group that has been exposed to radiation or a chemical toxin Trends of incidence of childhood thyroid cancer in Belarus, Ukraine, and Russia, 1986–94 10 Denominator 11    number of people who are at risk for developing the disease. Any individual who is included in the denominator must have the potential to become part of the group that is counted in the numerator. For the incidence of uterine cancer, the denominator must include only women with no history of hysterectomy. Types of denominator 12  Incidence measures can use two types of denominators:  people at risk who are observed throughout a defined time period;  when all people are not observed for the full time period, person-time Numerators 13 14 People at Risk Who Are Observed Throughout a Defined Time Period     we specify a period of time we must know that all of the individuals in the group represented by the denominator have been followed up for that entire period. The choice of time period is arbitrary The incidence is calculated using a period of time during which all of the individuals in the population are considered to be at risk for the outcome, also called the cumulative incidence proportion cumulative incidence proportion 15  1840 students are present in a dormitory. In the two months of follow up, 276 cases were identified.  Cumulative incidence: 276/1840 = 15%  When All People Are Not Observed for the Full Time Period 16  Often, every individual in the denominator cannot be followed for the full time specified loss to follow-up or  death from a cause other than that being studied    When different individuals are observed for different lengths of time, we calculate an incidence rate (also called an incidence density), in which the denominator consists of the sum of the units of time that each individual was at risk and was observed. This is called person-time and is often expressed in terms of person-months or person-years (py) of observation. When all are observed for the entire period 17 When people observed for different periods 18 incidence rate (incidence density) 19 Reported tuberculosis cases, United States, 1982–2015 20 Prevalence 21 the difference between incidence and prevalence 22  Prevalence can be viewed as a snapshot or a slice through the population at a point in time at which we determine who has the disease and who does not.     not determining when the disease developed. includes a mix of people with different durations of disease Prevalence is not have a measure of risk. If we wish to measure risk, we must use incidence, because in contrast to prevalence, it includes only new cases or events Types of prevalence 23   Point prevalence: Prevalence of the disease at a certain point in time Period prevalence: How many people have had the disease at any point during a certain time period 24 Question measure Do you currently have asthma? Point prevalence Have you had asthma during last years? Period prevalence Have you ever had asthma? Cumulative incidence example 25 26 SOME SOURCES OF MORBIDITY STATISTICS 27          Disease reporting—communicable diseases, cancer registries Data accumulated as a by-product of insurance care plans Tax-financed public assistance and medical care plans Hospitals and clinics Absenteeism records—industry and schools Pre-employment and periodic physical examinations in industry and schools Case-finding programs Records of military personnel Morbidity surveys on population samples numerator 28  Some diseases are difficult to diagnose  Estimate  is significantly affected by the set of criteria How do we find the cases?  regularly available data  conduct a study Denominators 29    selective undercounting of certain groups in the population Problems in classifying people by ethnic group different studies use different definitions LIMITATIONS OF HOSPITAL DATA 30  Hospital admissions are selective in relation to: a. Personal characteristics  b. Severity of disease  c. Associated conditions  d. Admission policies   Hospital records are not designed for research. a. Incomplete, illegible, or missing  b. Variable in diagnostic quality   Population(s) at risk (denominator) is (are) generally not defined RELATIONSHIP BETWEEN INCIDENCE AND PREVALENCE 31  Prevalence = Incidence * Duration of Disease Geographic Information System 32  geographic or spatial differences in the distribution of cases,  Incidence    or prevalence identifies clusters of diseases Helps policy makers in identifying and prioritizing health problems and resource allocation. greater-than-expected proximity of cases of a disease in time and space may be due only to chance 33