Measuring diseases…

1. Liliana Iliescu, MD, PhD Measuring diseases…

2. MORBIDITY Liliana Iliescu, MD, PhD

3. Morbidity - definition • Morbidity is another term for illness • Means the burden of disease in a population, in a specific area and a specific period of time (usually an year)

4. Morbidity - definition • Morbidity has been defined as any departure, subjective or objective, from a state of physiological or psychological well-being. • In practice: disease, injury, and disability. • the term refers to the number of persons who are ill, it can also be used to describe the periods of illness that these persons experienced, or the duration of these illnesses.

5. Measuring morbidity • Incidence • Prevalence

6. INCIDENCE • incidence is a measure of disease that allows us to determine a person's probability of being diagnosed with a disease during a given period of time • incidence is the number of newly (for the first time!!) diagnosed cases of a disease.

7. INCIDENCE • Incidence refers to the occurrence of new cases of disease or injury in a population over a specified period of time. • Incidence proportion or risk • Incidence rate or person-time rate

8. Incidence proportion or risk • Incidence proportion is the proportion of an initially disease-free population that develops disease, becomes injured, or dies during a specified (usually limited) period of time. • Synonyms include attack rate, risk, probability of getting disease, and cumulative incidence. • Incidence proportion is a proportion because the persons in the numerator, those who develop disease, are all included in the denominator (the entire population).

9. Incidence proportion or risk New cases of disease/injury during specified period Size of population at start of period • it includes only new cases of disease in the numerator. The denominator is the number of persons in the population at the start of the observation period. Because all of the persons with new cases of disease (numerator) are also represented in the denominator, a risk is also a proportion.

10. Example • In an outbreak of gastroenteritis among attendees of a corporate picnic, 99 persons ate potato salad, 30 of whom developed gastroenteritis. Calculate the risk of illness among persons who ate potato salad. • Numerator = 30 persons who ate potato salad and developed gastroenteritis • Denominator = 99 persons who ate potato salad • Risk = “Food-specific attack rate” = (30 / 99) x 100 = 0.303 x 100 = 30.3%

11. Incidence rate or person-time rate • Incidence rate or person-time rate is a measure of incidence that incorporates time directly into the denominator. • Typically, each person is observed from an established starting time until one of four “end points” is reached: onset of disease, death, migration out of the study (“lost to follow-up”), or the end of the study.

12. Incidence rate or person-time rate • the numerator of the incidence rate is the number of new cases identified during the period of observation. • the denominator differs = the sum of the time each person was observedfor all persons. • denominator represents the total time the population was at risk and being watched for disease • the incidence rate is the ratio of the number of cases to the total time the population is at risk of disease.

13. Incidence rate or person-time rate No of new cases of disease / injury during specified period Time each person was observed, totaled for all persons • The denominator of the person-time rate is the sum of all of the person-years for each study participant. • An incidence rate describes how quickly disease occurs in a population.

14. • Finally, if you report the incidence rate of, say, the heart disease study as 2.5 per 1,000 person-years, epidemiologists might understand, but most others will not. Person-time is epidemiologic jargon. To convert this jargon to something understandable, simply replace “person-years” with “persons per year.” • Reporting the results as 2.5 new cases of heart disease per 1,000 persons per year. • It also conveys the sense of the incidence rate as a dynamic process, the speed at which new cases of disease occur in the population.

15. Incidence – how to calculate the incidence rate • An incidence rate is the number of new cases of a disease divided by the number of persons at risk for the disease • If, over the course of one year, 5 women are diagnosed with breast cancer, out of a total female study population of 200 (who do not have breast cancer at the beginning of the study period), then we would say the incidence of breast cancer in this population was 0.025. (or 2,500 per 100,000 women-years of study)

16. PREVALENCE • Prevalence is a measure of disease that allows us to determine a person's likelihood of having a disease. • Point prevalence • Period prevalence

17. PREVALENCE • Point prevalence - measured at a particular point in time. It is the proportion of persons with a particular disease or attribute on a particular date.

18. PREVALENCE • Period prevalence - measured over an interval of time. It is the proportion of persons with a particular disease or attribute at any time during the interval.

19. Prevalence– how to calculate the prevalence rate • the number of prevalent cases is the total number of cases of disease existing in a population. • A prevalence rate is the total number of cases of a disease existing in a population divided by the total population. • So, if a measurement of cancer is taken in a population of 40,000 people and 1,200 were recently diagnosed with cancer and 3,500 are living with cancer, then the prevalence of cancer is 0.118. (or 11,750 per 100,000 persons)

20. Calculation All new and pre-existing cases during a given time periodx 100 Population during the same time period • In a survey of 1,150 women who gave birth in Maine in 2000, a total of 468 reported taking a multivitamin at least 4 times a week during the month before becoming pregnant. Calculate the prevalence of frequent multivitamin use in this group. • Numerator = 468 multivitamin users • Denominator = 1,150 women • Prevalence = (468 / 1,150) x 100 = 0.407 x 100 = 40.7%

21. Incidence vs. prevalence • Incidence should not be confused with prevalence, which is a measure of the total number of cases of disease in a population, rather than the rate of occurrence of new cases. • Thus, incidence conveys information about the risk of contracting the disease, whereas prevalence indicates how widespread the disease is. • Prevalence is the ratio of the total number of cases in the total population. Prevalence can also be measured with respect to a relevant subgroup of a population.

22. Incidence vs. prevalence • While incidence depends only on the number of new cases during a certain time period, prevalence depends on two factors: the number of people who have been ill in the past (previous incidence) and the duration of illness.  • Diseases with a long duration will have a larger prevalence than incidence rate, and diseases with a short duration (because of recovery or death) will have a low prevalence. • It would appear intuitively that, if incidence reflects new cases and prevalence reflects existing cases, there should be some quantifiable relationship between them.  A rough estimate of this relationship follows: Prevalence = Incidence x Duration

23. Incidence studying causes of both acute and chronic disease, since they are direct indicators of disease risk.  Prevalence workload needs measuring the burden of chronic disease monitoring public health programs for chronic conditions. 

26. What for?....incidence and prevalence • The prevalence of chronic disease in a general population is growing: → both the higher incidence of chronic diseases (there are more cases each year than in the past) → the longer survival of patients with chronic disease (modern treatments). → If death rates drop, then the time that patients live with chronic disease grows and even a low incidence will produce a high prevalence. • Under specific assumptions, the inter-relationship between incidence and prevalence can be expressed by this formula: Prevalence = incidence x average duration. • Incidence is often used to describe the number of new cases of a particular disease, for example, lung cancer, breast cancer, heart disease. • Incidence is useful in understanding how commonly a disease or problem occurs over a period of time, which can be used to estimate one's risk of getting the disease.

27. What for?....incidence and prevalence http://www.painconsortium.nih.gov/symptomresearch/chapter_19/sec4/cihs4pg1.htm • Prevalence is a more relevant measure than incidence when assessing the impact of a problem within a community and to assess the subsequent needs. • For example, imagine you are planning health services in a community and you want to know how many services are need to care for patients with motor neuron disease. → The incidence is relatively low. But if only incidence was used to determine service needs, the need might be underestimated. → The mean survival for patients with motor neuron disease is three to five years. During many of these years patients experience increasing symptoms and problems. Thus prevalence is more useful to assess the need for services.

28. What for?....incidence and prevalence • Short-duration disease: A person who has a common cold for one day, would be added to the incidence statistics, but (theoretically anyway) shouldn't be on the prevalence list. • Newly diagnosed chronic disease: A person diagnosed with diabetes will be on the incidence numbers and prevalence numbers in that first year, but then only on the prevalence numbers for second or later years. • Deaths: A person who dies from a disease stops being on the prevalence data for both later years and also the current year (unless prevalence statistics include this time period). That person will be on the incidence numbers only for the year they were diagnosed, and not in the year they die if they had the disease more than a year. A death from a short disease like flu does get included in incidence, but not prevalence. A death after many years from a long-term disease like diabetes removes that person from prevalence numbers (and they should only have been on the incidence data their first diagnosis year). • http://www.rightdiagnosis.com/admin/preval.htm

30. Epidemiological studies

32. Observational studies • In an observational study, the researcher simply observes the exposure and disease status of each study participant. • The two most common types of observational studies are cohort studies and case-control studies; • A third type is cross-sectional studies.

34. COHORT STUDIES • In a cohort study the epidemiologist records whether each study participant is exposed or not, and then tracks the participants to see if they develop the disease of interest

36. Cohort study – risks calculation andinterpretation • relative risk (RR) • attributable risk (AR)

37. Cohort study – risks calculation andinterpretation

38. Cohort study – risks calculation andinterpretation • Risk for disease in exposed: p1 = a/a + b • Risk for disease in not-exposed: p0 = c/ c + d RR = p1 /p0

39. RELATIVE RISK (RR) • In statistics and mathematical epidemiology, relative risk (RR) is the risk of an event (or of developing a disease) relative to exposure. • Relative risk is a ratio of the probability of the event occurring in the exposed group versus a non-exposed group

40. Cohort study – risks calculation andinterpretation • In a simple comparison between an experimental group and a control group: • A relative risk of 1 means there is no difference in risk between the two groups. • An RR of < 1 means the event is less likely to occur in the experimental group than in the control group. • An RR of > 1 means the event is more likely to occur in the experimental group than in the control group.

41. Cohort study – risks calculation andinterpretation • attributable risk is the difference in rate of a condition between an exposed population and an unexposed population • Attributable risk is mostly calculated in cohort studies, where individuals are assembled on exposure status and followed over a period of time. Investigators count the occurrence of the diseases.

42. Cohort study – risks calculation andinterpretation • attributable risk AR =p1 - p0 • AR of 0 means there is no difference in risk between the two groups. • An AR of < 0 means the event is less likely to occur in the experimental group than in the control group. (protection factor) • An AR of > 0 means the event is more likely to occur in the experimental group than in the control group.

48. Cohort study - advantages • Allow complete information on the subject’s exposure, including quality control of data, and experience thereafter. • Provide a clear temporal sequence of exposure and disease. • Give an opportunity to study multiple outcomes related to a specific exposure. • Permit calculation of incidence rates (absolute risk) as well as relative risk. • Methodology and results are easily understood by non-epidemiologists. • Enable the study of relatively rare exposures.

49. Cohort study - disadvantages • Not suited for the study of rare diseases because a large number of subjects is required. • Not suited when the time between exposure and disease manifestation is very long, although this can be overcome in historical cohort studies. • Exposure patterns, for example the composition of oral contraceptives, may change during the course of the study and make the results irrelevant. • Maintaining high rates of follow-up can be difficult. • Expensive to carry out because a large number of subjects is usually required. • Baseline data may be sparse because the large number of subjects does not allow for long interviews.

50. Case control • A case-control study is an analytical study which compares individuals who have a specific disease ("cases") with a group of individuals without the disease ("controls"). • The proportion of each group having a history of a particular exposure or characteristic of interest is then compared. An association between the hypothesized exposure and the disease being studied will be reflected in a greater proportion of the cases being exposed.