1. Point prevalence survey epidemiology
Facilitator notes:
Epidemiological methods for point prevalence surveys of healthcare-associated infections and antimicrobial use in acute care hospitals
Point prevalence survey epidemiology
Version 2017
Revision: 2018

2. Objectives Specific objectives of this session:
Describe the principles and approaches to PPS epidemiology inclusive of strengths and limitations
Understand the concept of reliability in the context of the ECDC PPS
Understand reporting output from the ECDC PPS
These are similar to the general course objectives
Facilitator notes:

3. Outline This session consists of the following elements
Prevalence and incidence
What are prevalence and incidence?
Relationship between prevalence and incidence
Prevalence or incidence?
Numerators and denominators
Using local PPS data to inform practice
Also on ward-level
4. Validity and reliability
What are validity and reliability?
ECDC PPS methods for testing validity and reliability
Facilitator notes:

4. Prevalence and Incidence

5. What is Prevalence?
Number of affected individuals present in a population at a specific point in time.
Presented as a proportion or percentage of all individuals in the population at that point in time.
Example:
Prevalence of HAI
Survey population = 100
For each patient: Does the patient currently have an HAI?
If 5 patients have an HAI at the time of survey, prevalence = 5/100= 5%
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6. What is Prevalence? Prevalence of antimicrobial prescribing
Survey population = 500
For each patient: Is the patient currently receiving an antimicrobial?
If 150 patients are receiving an antimicrobial at the time of survey, prevalence = 150/500= 30%
Prevalence of invasive device use
Survey population = 1000
For each patient: Does the patient currently have a device in situ?
If 30 of these patients have a central vascular catheter (CVC) in situ at the time of survey, what would the prevalence of CVC use be calculated as in the survey population?
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7. What is Incidence?
The number of new cases occurring in a population at risk over a defined period of time.
Presented as the percentage or proportion of those “at risk” that go on to develop the outcome.
Requires patients without the outcome (e.g. HAI) to be followed over time to determine whether they develop the outcome.
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8. Examples of Incidence Incidence of surgical site infection (SSI)
There were 45 new cases of SSI following 1000 caesarean section procedures during 2008.
Incidence of SSI = 45/1000= 4.5% during 2008
Incidence of catheter-associated urinary tract infection (CAUTI)
There were 14 new cases of CAUTI in a population of 100 patients catheterised between January to March 2010.
What is the incidence of CAUTI between January and March 2010?
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9. Prevalence or Incidence
Quick and relatively easy to perform
Can measure the burden of all HAI types (not only those included in incidence surveillance)
ONLY represents the situation at the time of survey (sample of one of many survey days)
Cannot measure risk of HAI, monitor trends in HAI or identify HAI outbreaks
Repeated prevalence surveys can also enable to interpret even weak trends
Incidence
Resource and labour intensive
Can measure risk of HAI
Can monitor trends and identify outbreaks
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10. Excercises

11. Prevalence and Incidence
Jan 2017
Dec 2017
May
Sept
Case 7
Case 6
Case 5
Case 4
Case 3
Case 2
Case 1
Notes to trainer: This diagram represents cases of infection during the period January 2009 to December You should demonstrate the course of infection using the following examples…
Case 1 developed before January 2009 and was ongoing until just after September 2009.
Case 5 developed in April 2009 and was ongoing until July 2009.
Now stop the presentation and ask the participants to work in small groups to answer the questions on the slides.
Solutions-
Question 1- Any cases occurring after January 2009= 5 Incidence= 5/100=5%
Question 2- four existing cases of infection present in May Prevalence= 4/30= 13.3%
Question 3- four existing cases of infection present in September Prevalence= 4/36=11.1%
PPS performed
How many new cases developed between January and December 2017?
If there were a total of 100 “at risk” patients between January and December 2017, what is the incidence of infection?
How many existing cases of infection were prevalent at the May PPS?
If 30 patients were surveyed in May, what was the prevalence of infection?
How many existing cases of infection were prevalent at the September PPS?
If 36 patients were surveyed in September, what was the prevalence of infection?

12. Prevalence and Incidence
HAI Case
Recovery or death or discharge
Incidence
No HAI
Prevalence
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13. Numerators and denominators for the ECDC PPS

14. Measuring prevalence Prevalence = numerator/denominator Numerator
Number of patients on the ward with active infection at the time of survey
Denominator
Number of patients on the ward at 8am on the morning of the survey
Data collector visits a ward where there were 27 patients present at 8:00 AM on the morning of the survey. After carrying out the survey, 4 patients were identified as having active infection.
Denominator is 27
Numerator is 4
Prevalence = 4/27 = 15%
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15. Using local PPS data to inform practice

16. Local PPS data
Opportunity with collection of local data to inform practice at a local level as well as national and European level.
Local PPS data can be used to
Compare the structure and process indicators to national and European level
Identify targets for quality improvement
Evaluate effect of interventions (in repeated PPS)
Requires caution due to small sample size and survey day being a sample of all possible survey days.
Especial caution when interpreting results for individual wards.
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17. Validity and reliability

18. Validity What is validity?
Measure of ability of a data collector to distinguish between who has a HAI and who does not.
Data collected are compared with „gold standard” data collection.
Validation exercises check whether the data collector is correctly applying the case definitions, for example:
Patient has had surgery 5 days ago, and has S. aureus isolated from an aseptically obtained fluid from the superficial incision.
Data collector decides “No surgical site infection (SSI)”, the gold standard decides “Superficial SSI”.
Different answers to the same question!
The data collector’s decision is not valid when compared with the gold standard data collector
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19. Reliability What is reliability?
Measure of repeatability of HAI diagnosis by data collectors
Measures the agreement of diagnosis between data collectors
Reliability exercises check that the data collectors are applying the definitions consistently each time, for example:
10 data collectors collect data from the same patient
9 data collectors decide “No urinary tract infection (UTI)”
1 data collector decides “UTI”
Different answers to the same question
The data collectors are not applying the case definitions consistently so the results are not reliable
Facilitator notes:

20. Validity and reliability
VALID AND RELIABLE
RELIABLE NOT VALID
NOT RELIABLE OR VALID
5 data collectors
All 5 diagnose SSI-S
Gold standard- SSI-S
7 data collectors
All diagnose SSI-S
Gold standard- SSI-D
11 data collectors
5 diagnose BRON
2 diagnose PN1
3 diagnose PN4
1 diagnose no HAI
Gold standard - No HAI
Notes to trainer: This diagram demonstrates validity and reliability. The data collectors all collect data on the same patient and their diagnosis of HAI is compared.
Explanation of the diagram
VALID and RELIABLE- the data collector is applying the definitions in a consistent and correct manner. Example: 5 data collectors- all consistently diagnose SSI agreeing with the gold standard
RELIABLE not VALID- the data collector is applying the definitions incorrectly in the same way every time. Example: 7 data collectors- all consistently diagnose superficial SSI but the gold standard data collector has diagnosed a deep SSI
NOT RELIABLE or VALID- the data collector is not applying the definitions in the same way or correctly each time. Example: 11 data collectors- there is no consistency in the diagnosis of HAI with several different respiratory infections being diagnosed. Only one data collector has reached the same decision as the gold standard data collector of NO HAI

21. ECDC PPS Validation
All Member States are expected to perform a validation study of their national or regional PPS:
at the same time as a national or regional PPS
by a national/regional PPS validation team
a minimum sample of 5 hospitals and 250 patients
a recommended sample of 25 hospitals and 750 patients
The current protocol is the ECDC PPS validation protocol v3.1
National PPS coordinating centre will select and further inform the selected hospitals
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22. ECDC PPS 2016-2017 Validation The objectives of the validation:
to assess the validity, reliability and inter-country comparability of the data
to assess the data accuracy of selected hospital process and structure indicators
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23. In summary List of learning points in this session:
1. Point prevalence surveys are an important methodology for:
Measuring the burden of HAI and antimicrobial use
Describing the prevalence of all HAI types
Informing priority setting at a local, national and European level
A consistent and robust approach is essential to ensuring the validity and reliability of the results
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24. Facilitator notes:
Acknowledgements The creation of this training material was commissioned in 2010 by ECDC to Health Protection Agency (UK) with the direct involvement of Dr. S. Hopkins, Prof. J. Reilly, S. Cairns, Dr. E. Sheridan, Dr. G. Hughes, Prof. B. Cookson, Dr. A. Charlett, G. Kafatos, B. Muller Pebody, F. Cowan, and Y. Sueiro. The revision and update of this training material was commissioned in 2017 by ECDC to Transmissible (NL) with the direct involvement of Dr. Arnold Bosman and Dr. Ágnes Hajdu