University:
American Career College
Course:
NURS115 | Intermediate Medical
Academic year:

2020
Views:

468

Pages:

24
Author:

Obo

PRINCIPLE OF EPIDEMIOLOGY NATURAL HISTORY OF DISEASE AND PROGNOSIS 2 five different ways of describing the natural history of disease      case-fatality, 5-year survival, observed survival, Median survival time, relative survival The natural history of disease 3 Prognosis 4     deaths from the disease survivors with the disease. Because death is inevitable, we are not talking about dying versus not dying, but rather about extending the interval until death occurs following diagnosis. end points      interval from diagnosis to recurrence of disease Interval from diagnosis to the time of functional impairment disease-specific complication, disability changes in the patient’s quality of life, prognosis 5    affected by the invasiveness of the available treatment When the treatment was initiated the extent to which some of the symptoms can be relieved even if the patient’s life span cannot be extended. Case-Fatality 6      the number of people who die of a disease divided by the number of people who have the disease. It is a proportion and time is expressed implicitly generally used for acute diseases in which death, if it occurs, occurs relatively soon after diagnosis. if the usual natural history of the disease is known, the term case-fatality refers to the period after diagnosis during which death might be expected to occur. In chronic diseases, in which death may occur many years after diagnosis and the possibility of death from other causes becomes more likely, case-fatality becomes a less useful measure. Person-Years 7    A useful way of expressing mortality is in terms of the number of deaths divided by the person-years over which a group is observed. Because individuals are often observed for different periods of time, the unit used for counting observation time is the person-year. It is assumed that risk is the same in any personyear observed. 10 person-years 8 Five-Year Survival 9      frequently used in clinical medicine, particularly in evaluating treatments for cancer. the percentage of patients who are alive 5 years after treatment begins or 5 years after diagnosis. it is a proportion. most deaths from cancer typically occurred during this period after diagnosis when it was first used in the 1950s, Some literature on chronic diseases, such as cardiovascular diseases, use 10-year survival instead of 5-year survival. 5-year survival and better screening programs 10 lead time bias 11    When the interval between diagnosis (and treatment) and death is increased through earlier diagnosis, but there was no delay in the time of death, the interval between the earlier diagnosis made possible by the screening test, and the later usual time of diagnosis is called the lead time. no change in the natural history of the disease has occurred the only change that has taken place is that when the diagnosis was made 3 years earlier, the patient received medical care for breast cancer, with all its attendant difficulties, for an additional 3 years survival 12 13 approaches for calculating observed survival over time   the life table approach the Kaplan-Meier method 14 life table 15   the exact event time is not known the interval within which the event took place is known. 16 CALCULATING A LIFE TABLE 17 The Kaplan-Meier Method 18 Kaplan-Meier plot 19 Kaplan-Meier 20  When information on the exact time of death is available, the Kaplan-Meier method clearly makes fullest use of this information because the data are used to define the intervals, instead of predetermined arbitrary intervals used in the life tables method. 21 Assumptions Made in Using Life Tables and Kaplan-Meier Method  there has been no secular (temporal) change in the effectiveness of treatment or in survivorship over calendar time.  survivorship in one calendar year of the study is the same as in another calendar year of the study.  if a study is conducted over many years, this assumption may not be valid because, fortunately, therapies improve  the survival experience of people who are lost to follow-up is the same as the who are followed up. Comparison of the groups 22 Median Survival Time 23   the length of time that half (50%) of the study population survives. Median survival offers two advantages over mean survival.  it is less affected by extremes  if we used mean survival, we would have to observe all of the deaths in the study population before the mean could be calculated Relative Survival 24  the ratio of the observed survival to the expected survival