Screening Flashcards
Screening definition
Presumptive identification of unrecognised disease or defect by conducting test/examination
Process of screening (2)
- Identifies high risk patients that test positive (go for further diagnostic tests)
- Low risk patients (no further tests)
Purpose of screening
Give a better outcome compared with finding something in the usual way
Five areas of criteria - screening
Condition
Test
Intervention
Screening programme
Implementation
Condition (3)
Must be important health problem (frequency and severity) with known epidemiology, incidence, prevalence, and history
All cost-effective primary prevention has been implemented
Understanding of psychological implications of mutations/carriers
Test (3) - screening
Simple, safe, precise, and valid
Cut off value defined and agreed
Agreed policy on further diagnostic investigation
Intervention (2) - screening
Evidence that intervention at pre-symptomatic phase leads to better outcomes
Agreed evidence based policies covering which individuals are offered which interventions
Screening programme (4)
Proven effectiveness in reducing mortality/morbidity
Evidence that it is clinically, socially and ethically acceptable to health
professionals and public
Benefits outweight harm (e.g. overdiagnosis, false positives etc)
Cost of screening economically balanced
Implementation (3) - screening
All other options for managing condition should have been considered
Adequate staffing and facilities for screening
Patients make informed choice (adequate evidence and information)
Why is it important to evaluate the screening programmes?
Programmes must be based on good quality evidence
Difficulties in evaluation (3) - screening
Lead time bias
Length time bias
Selection bias
Lead time bias
Early diagnosis falsely appears to prolong survival
Actually - they live longer knowing they have the disease - same as a patient who is not diagnosed early
Length time bias
Better at picking up slow growing and unthreatening cases that if had gone undetected may have never caused a problem
Could lead to false conclusion that screening is beneficial in lengthening lives of those
found positive (curing people that didn’t need curing)
Seleciton bias - screening
Screening studies are scewed by healthy volunteers
Those who attend regular screening, are likely to attend other screening programmes and
do other things to protect them from disease
Informed consent is required
What is this?
communicating benefits harms and risk of preventative interventions
Types of testing error (2)
False positive
False negative
Test validity features (2)
Sensitivity
Specificity
Sensitivity
proportion of people with the disease who test positive (true positive/test positive)
Specificity
proportion of people without the disease who test negative (true negative/tested negative)
Positive predictive value
probability that someone tests positive has the disease (true positive/is positive)
Negative predictive value
probability that someone tests negative actually does not have the disease (true negative/is negative)
Examples of UK screening programmes
Abdominal aortic aneurysm (men 65 years) Cervical cancer (women 25-64 years)
Diabetic eye screening (diabetic patients > 12 years)
Bowel cancer screening
Breast cancer screening
Factors that affect screening uptake
Acceptability of the test (non-invasive)
Awareness of benefits of screening and corresponding risks
Convenience (time/location)
Accessibility (information)
Reminders and endorsements
Inequalities in screening
Demographic factors
Level of affluence
Deprivation
Ethnic diversity
