Class 5 - Epidemiology Flashcards
What is epidemiology
the science of understanding the causes and distribution of population health so that we may intervene to prevent disease and promote health
Epidemiology study populations in order to…
- clarify clinical and demographic characteristics
- identify who is at risk for illness
- provide clues to causes of disease
- guides preventative measures and interventions
- monitor health of population
- identify DOH and disease in communities
- investigate and evaluate interventions to prevent disease and maintain health
What are the 2 sides of epidemiology
- descriptive
- analytical
What is the outline for descriptive epi studies
the who, what, when, and where of the health event
- what: disease, injury, death (clinical - case definition)
- who: age, gender, sociodemographic, occupation
- when: time, seasonality, secular trends
- where: place, neighborhood, city, county, census tract
- distribution: frequency of the event and pattern of the frequency
What is the outline for analytical epi studies
determinants of disease
- understand factors that influence the occurrence of health-related event (causes, risk factors, modes of transmission)
- understand the “how” and “why” aspects of the event
Epi Descriptive - describing ‘what’
Alerts to new problems:
- look for other cases
- develop new surveillance system
- propose hypothesis
Epi Descriptive - describing ‘what’ clinical info includes
- symptoms/signs (case definition)
- lab results
- hospitalization
- lived or died
Epi Descriptive - describing ‘who’ demographic info includes
age, sex, marital status
Epi Descriptive - describing ‘who’ socioeconomic info includes
- education
- occupation, income
- place of work or residence (postal codes)
Epi Descriptive - describing ‘where’ outline
- where illness began (home, vacation site)
- where exposure occurred (education, occupation, income, place of work or residence)
- source of contamination (farm, unwashed produce)
Epi Descriptive - describing ‘when’ includes
- trends (change over time, long-term trends/secular)
- patterns (cyclical trends seen over several years/seasonal)
- epidemics
What is an endemic
rate of a disease, injury, or other condition is the rate of occurrence that is usual in a population
What is an epidemic
occurs when the rate of a disease, injury, or other condition clearly exceeds the usual level for that condition
What is a pandemic
rate of a disease, injury, or other condition is the rate of its occurrence in the geographically widespread population
What is our goal with an epidemiological triad of disease
break at least 1 side of the triangle to stop the continuation of disease
What are descriptive epi studies used for
- program planning
- generating hypotheses
- suggesting ideas for further study
What are analytical epi studies used for
to test for hypotheses about the cause of disease and studying how exposure relates to disease
Types of epi studies
descriptive and analytic
Types of descriptive studies
- case report: individual case data
- case series: individual data on a group of cases
- incidence study: new cases during a specific time
Types of analytic studies
experimental (exposure is randomly assigned) and observational
Analytic studies, types of experimental studies
- clinical trial: randomization at the individual level
- community trial: randomization at the community level
Analytic studies, types of observational studies
- cohort
- case-control
- cross-sectional
- ecological
What is a cohort study
Observe groups of people who do and don’t experience an exposure, to compare how many in each group have the disease
What is a case-control study
Compares a group of people who have a disease to a similar group who don’t have a disease, and the investigator records who had exposures of interest
What is a cross-sectional study
Snapshot of a specific period in time, with the aim of finding the same kind of relationships as experimental studies; describes exposure and disease in a population at some point in time
What is an ecological study
Compares populations rather than individual, association at the group level do not always mean association at the individual level
Data for epidemiological studies commonly come from three categories:
- routinely collected data (census data, vital records, surveillance data)
- data collected for other purposes (medical, health department, insurance records)
- original data collected for specific epi studies
Prevalence =
Incidence X Duration of Disease
relationship between prevalence and incidence
incidence (new cases) increase prevalence above baseline. and vice versa
Incidence Proportion =
of new cases of disease occurring during a specific period / # of persons at risk of developing the disease during that same period
Prevalence Proportion =
of cases of disease occurring during a specific period / # of persons in the population at that same period
What does incidence proportion capture
disease onset
What does prevalence proportion capture
disease status (present or absent)
What increases prevalence
- increase incidence
- increase duration of disease
- influx of cases
- emigration of healthy people
- influx of susceptible people
- changes in diagnostic criteria
- better reporting
What decreases prevalence
- decreased incidence
- decreased duration
- high case fatality rate
- influx of healthy people
- out-migration of cases
- poorer case detection
- influx of immune people
What are screenings
process of classifying people as to whether they are likely to have a disease
what are the 3 levels of prevention to consider
primary, secondary, tertiary
what is primary prevention
seeks to prevent new cases of a disease from developing in the population
examples of primary prevention
- no smoking campaigns
- sunscreen
- PrEP
what is secondary prevention
seeks to reduce the number of existing cases of a disease
examples of secondary prevention
Cancer Screenings
- mammography
- colonoscopy
what is tertiary prevention
seeks to limit the disability resulting from disease and improve functioning
examples of tertiary prevention
- cardiac rehab
- PT, OT
What is the natural history of disease
refers to the progression of the disease process from onset to recovery:
- pre-clinical (susceptibility to disease)
- clinical (from the preclinical stage to death, disability, or recovery)
When are screening tests appropriate
(BEFORE symptoms develop)
- disease is an important cause of morbidity and mortality
- tx is available
- impact of disease can be minimized before symptoms develop
- prevalence of preclinical disease is high
What makes a good screening test
- easy to admin
- results can be easily available (automated lab reporting)
- test is inexpensive
- test imposes minimal discomfort to the screenee
screening tests vs diagnostic tests
Screening
- done on asymptomatic, healthy individuals who are encouraged to get screened
- the benefit that early tx conveys in terms of prognosis should outweigh the net costs of screening
Diagnostic
- done on sick people who present with symptoms
- pre-test probability of disease is high (high disease prevalence)
- often performed after a +ve screening test to establish a definitive diagnosis
Define the validity of a test
its ability to distinguish between who has a disease and who does not
What are the two components of validity
sensitivity and specificity
What is test sensitivity
- The ability to correctly identify individuals who have the disease
- is it to identify true positive
- a test with high sensitivity will have few false negatives
what is test specificity
- the ability to correctly identify individuals who do not have the disease
- to call a true negative ‘negative’
- a test with high specificity has few false positives
true positive =
- have disease
- positive test
false positive =
- no disease
- positive test
true negative =
- no disease
- negative test
false negative =
- have disease
- negative test
Sensitivity =
TP/ TP + FN
Specificity =
TN / TN + FP
What are sensitive tests good for?
ruling disease OUT
What are specific tests good for?
ruling disease IN
What is a positive predictive value?
the percent chance that a positive test result I a true positive (i.e. that a person w/ a positive result actually has the condition)
PPV =
TP/ (TP+FP)
= TP / (all that tested positive)
What is a negative predictive value?
the percent chance that a negative test result is a true negative (i.e. that person w/ a negative result is actually disease free)
NPV =
TN/ (TN+FN)
= TN / (all that tested negative)
why are we concerned about false positive results?
- additional testing, perhaps more invasive
- anxiety and distress associated with positive test results
- impose additional costs on the individual and the health care system
why are we concerned about false negative results?
- may delay tx and lead to poorer prognosis
- public health concerns if the disease is transmissible
When do we want a test with high sensitivity
if missing a case leads to high penalty, aim is usually to minimize false negatives
When do we want a test with high specificity
if subsequent diagnostic testing is particularly expensive or invasive, the aim may be to minimize false positives
