L17 Screening : a prevention strategy Flashcards
What is screening in health?
Identifying risk factors for disease or unrecognised disease or complication of a disease by applying tests on a large scale to a population
At what levels does screening function as a prevention strategy?
Primary - before biological onset (identification of risk factors)
Secondary - before diagnosis (early diagnosis)
Tertiary - after diagnosis (identifying further complication)
Explain how a screening programme is designed
A large population who are deemed high risk to a certain disease and show no symptoms are defined as the eligible population
All members of this population undergo screening
Those who test positive undergo a diagnostic test (gold standard test) and those who test positive from this are treated
Those who test negative in both the diagnostic test and the screening test are re-screened after a period of time
What is the objective of screening?
Improve health outcome - reduce morbidity, mortality, disability
What are the criterias to consider when designing an appropriate screening programme?
Suitable disease
Suitable test
Suitable treatment
Suitable screening programme
What is a suitable disease for screening?
- Important public health problem
- A relatively common disease
- A relative uncommon disease in which if detected early can lead to better outcomes - Knowledge of the natural history of the disease
- Detectable early
- Long duration of pre-clinical phase (higher chance that screening test will help detect the disease)
What is a suitable test for screening?
Reliable - consistent results Safe Simple Affordable Acceptable Accuracy - sensitivity and specificity
What are the two measures of accuracy of a screening test?
Sensitivity and specificity
What is a diagnostic test accuracy study?
A test designed to determine the accuracy of a screening test by comparing it to a diagnostic test (gold standard test)
What is sensitivity?
The likelihood a person who actually has the disease shows a positive result from the screening test
True positive / All from study with disease (determined by diagnostic test)
What is specificity?
The likelihood a person who does not have the disease shows a negative result from the screening test
What is the positive predictive value PPV?
The proportion of people who actually have the disease from all those who show a positive result from the screening test
i.e. the probability of having the disease if the test shows a positive result
What is the negative predictive value NPV?
The proportion of people who actually do not have the disease from all the people in the study who show a negative result from the screening test
i.e. the probability that the person actually does not have the disease when they test negative in the screening test
What is the difference between fixed and non fixed characteristics of diagnostic test accuracy studies?
Fixed - sensitivity and specificity
Non-fixed - PPV and NPV
PPV and NPV values reflect both the test accuracy and the prevalence of the disease
What is suitable treatment for a screening programme?
Evidence that early treatment will lead to better outcomes
Effective, acceptable and accesible
Evidence-based policies identifying who should be offered treatment and what treatment should be offered