1. Prevalence & Incidence
25th June 2012
Prevalence & Incidence
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Presenter: Mari Morgan & Wendy Harrison

2. Prevalence & Incidence
Two different measures of the distribution of a disease in a population
Related
Can be situations of high prevalence but low incidence of a disease and vice versa
When to use prevalence or incidence depends on what you want to know, sometimes you need both to fully understand issue
Information for Action 2012

3. Prevalence
Measure of how much of a disease or condition that there is in a population at a particular time point
Everyone with the disease of interest at that time point is counted, regardless of how long they have had the disease and where they got it from
Calculated by dividing number of persons with the disease at a particular time point by the number of individuals examined
Often expressed as a percentage
Information for Action 2012

4. Prevalence
Indicates how widespread the disease is
Used to measure the burden of disease on a population and to assess subsequent needs
Caution with using it for associations between variables:
difficult to distinguish the effects of factors affecting occurrence of disease from effects of factors that increase duration of the problem
Information for Action 2012

5. Incidence Measures the rate of occurrence of NEW cases of a disease
Calculated by dividing the number of new cases that occur during a specified time period by the number of individuals in the specified population (initially disease free)
Expressed as a rate
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6. Incidence Allows risk of contracting the disease to be assessed
Useful for understanding disease aetiology – if incidence rate increases can look for risk factor that promotes incidence
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7. Relationship between Prevalence & Incidence
In order for a case to exist, it must first occur
Can only be prevalence if there has been incidence (when?) - relationship
In general, greater incidence greater prevalence, but not always
Prevalence also dependent on duration of disease – role of cures, deaths, immigration and emigration of cases
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8. Relationship between Prevalence & Incidence
Incidence: Rain
Prevalence: Water in the puddle at one time point
Size of puddle at time point dependent on how much water already there (old cases), the amount of rainfall (new cases) and also on water draining away into the soil or evaporating (cures, deaths, migration)
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9. Relationship between Prevalence & Incidence
High Prevalence & Low Incidence
Low number of cases per year, cases survive for a long time but are not cured
High Incidence & Low Prevalence
High number of cases per year, cases are cured rapidly or cases survive only a short period
Information for Action 2012

10. HCAI Surveillance
Surveillance to assess both the prevalence and incidence of HCAI are undertaken
Most HCAI surveillance is incidence surveillance targeted at specific infections, locations or patient groups
Information for Action 2012

11. Incidence Surveys Advantages
Can collect larger data set on specific HCAI
Easier to detect HCAI (can wait for further results)
Can determine risk of infection
Can establish relationship between risk factors and infection
Disadvantages
Time-consuming and expensive
Not feasible to do all HCAI continuously
Information for Action 2012

12. Prevalence Surveys for HCAI
Reasons for Doing a Prevalence Survey
To assess the burden of HCAI in a population i.e. hospital/ward/specialty and to assess the need for resources (treatment, interventions, further targeted surveillance)
To compare the prevalence of HCAI in different populations i.e. between hospitals/wards/specialties
To examine trends in HCAI prevalence over time (measure efficacy of interventions)
To examine prevalence of known risk factors
Information for Action 2012

13. Prevalence Surveys Advantages Relatively quick and inexpensive
Provides an overall picture of all HCAI
Disadvantages
Snapshot – may not be representative of other time periods
Can’t establish relationship between risk factors and HCAI
Biased towards longer staying inpatients and infections of longer duration
Reliant on all documentation being available on the day
Information for Action 2012