Classes #3-#7: Descriptive Epidemiology and Measures of Disease Frequency Flashcards
What are the 3 different surveillance systems used by epidemiologist?
- Passive Surveillance System
- Active Surveillance System
- Syndromic Surveillance System
- Dr. Segars also discussed Biosurveillance (human, plant, animal, and environment)
What is a passive surveillance system?
Relies on a healthcare system to follow regulations on required reportable diseases/conditions (e.g: a doctor reporting a disease in his patients).
*public health system passively waits for reports to come in, in order to track disease frequency/occurrence over time and within populations.
Describe what an active surveillance system is.
public health officials go out into the community to search for new disease/condition cases.
Define what a syndromic surveillance system is.
A system that looks for pre-defined signs/symptoms of patients, either being reported or evaluated.
What do you think is the most critical element that must be defined before any of the W’s of descriptive epidemiology can be accomplished?
“Case” Definition – The definition of disease. The epidemiologist has to know what they’re looking for.
This is called Diagnostic Criteria – In medicine this is called set criteria.
What is a “case” definition?
Is a set of uniform criteria used to define a disease/condition for public health surveillance.
>enable public health to classify and count cases consistently across reporting jurisdictions
>while the list of reportable conditions varies by state, the CSTE has recommended that state health departments report cases of selected diseases to NNDSS
>every year, case definitions are updated using CSTE’s Position Statements. They provide uniform criteria of nationally notifiable infectious and non-infectious conditions for reporting purposes.
>Epidemiologists and Healthcare providers must carefully and consistently define and execute exactly HOW we will detect (diagnose) what we want to, or have to, count; accurately! (Confirmed vs. Probable case definitions)
(The reported Criteria changes with knowledge of the disease, which is handle and carried out by the CSTE)
What is the definition of epidemic?
The occurrence of disease clearly in excess of normal expectancy.
>community/period clearly defined
>goal is to capture disease as early as possible
What is the definition of outbreak (cluster)?
An epidemic limited to a localized increase in the occurrence of disease. (sometimes interchanged with “cluster”)
What is the definition of endemic?
The constant presence of a disease within a given area or population in excess of normal levels in other areas. (e.g: HIV in Africa is an Endemic)
What is a Pandemic?
An epidemic occurring over a very wide area involving a large number of people.
>many times multi-region or multi-national
>when disease crosses country to country
What is “The Epidemic Curve”?
A visual depiction created during an outbreak/epidemic of the # of cases; by date of onset.
> Visually Depicts
What tool do epidemiologist construct/use to help form hypotheses on routes of transmission, probable exposure period, and incubation period (may help identify/eliminate causes)?
An Epidemic Curve
What type of epidemiology is used to determine if a site/location is experiencing disease occurrence MORE FREQUENTLY THAN USUAL (for the location)?
Descriptive Epidemiology
What are the 3 basic types of relative measures of disease frequencies utilized by epidemiologists (either within a group or between groups)?
1) Ratios
2) Proportions
3) Rates
How do you calculate a ratio, and what does it tell you?
A ratio is the division of 2 unrelated numbers.
>the numerator is not part of the denominator
> e.g: YES / NO
What is a proportion and how do you calculate it?
A proportion is the division of 2 related numbers.
>the numerator is a subset of the denominator
>e.g: YES / ALL
What is a rate and how do you calculate it?
A rate is the division of 2 numbers with TIME incorporated into the denominator.
>Rate is the easiest because it has time in the denominator. Time is important because we want to make sure we are comparing apples to apples.
What is infectivity and how do you calculate it?
Infectivity –> is the ability to invade a patient (host)
formula = #infected / # susceptible (at risk)
What are the 3 measure in infectious disease?
Infectivity – the ability to invade a patient (host)
Pathogenicity – the ability to cause clinical disease
Virulence – the ability to cause death
What is pathogenicity and how do you calculate it?
Pathogenicity is the ability to cause clinical disease.
formula = # with clinical disease / # infected
What is virulence and how is it calculated?
Virulence is the ability to cause death.
formula = # of deaths / # with infectious disease
What are the 3 key factors in comparing measures of disease frequency between groups?
1) # of people affected/impacted (frequency)
2) Size of the source population
3) Length of time the population (or individuals in the population) is/are followed.
The _____ and time period of evaluation MUST be equal to adequately and appropriately compare frequencies between groups.
Population Size
What is the INCIDENCE in measures of disease frequency?
The new cases of disease.
What is the PREVALENCE in the measure of disease frequency?
Is the existing cases of disease + new cases of disease.
How do you calculate incidence (risk, attack rate, cumulative incidence)?
incidence rate = # new cases of disease / # persons at risk for the disease
**remember to subtract out (from the starting population), those who already have the disease or are immune to the disease (not “at risk”).
**Not very precise for dynamic populations (fluctuations)
How do you calculate incidence density?
incidence density = # of new cases of a disease / total person-time of population at risk for the disease
** appropriate for dynamic populations and fluctuating “at risk” periods
What is the calculation for prevalence?
prevalence = # of existing cases of a disease/# of persons in population
- *Time frames for numerator/denominator must be the same
- *The denominator includes those with the disease and those at risk of getting the disease.
- *Point Prevalence – prevalence at a given point in time
- *Period Prevalence – prevalence over a given period of time
