Session 7 Lecture 1&2 - Screening Flashcards
what is screening?
the process of identifying healthy people who may have an increased chance of a disease or condition
Determinants of Screening
- the condition
- the test
- the intervention
- screening programme
- implementation criteria
“The condition”- criteria
-an important health problem with understood epidemiology, incidence, prevalence and natural history
“The test”-criteria
- simple. safe, precise and validated screening test
- accepted to target pop.
- distribution of test values in the pop. must be known and an agreed cut-off level must be agreed upon
4 ways in which the effectiveness of a screen test is assessed
- sensitivity
- specificity
- positive predictive value
- negative predictive value
Definition of Sensitivity
the proportion of cases which the test correctly detects
Definition of Specificity
The proportion of non-cases which the test correctly detects
Definition of Positive Predictive Value
the proportion of positive tests who are cases
Definition of Negative predictive value
the proportion of negative tests who are non-cases
How to calculate sensitivity
a/a+c
How to calculate specificity
d/b+d
How to calculate Postive predictive value
a/a+b
How to calculate Negative predictive value
d/c+d
“The intervention” - criteria
-evidence that intervention at a presymptomatic phase leads to better outcomes compared with usual care
“The screening programme”- criteria
- proven effectiveness in reducing mortality or morbidity through high-quality research
- evidence that the complete screening programme is clinically, socially and ethically acceptable to health professionals and public
- the opportunity cost of the screening programme should be economically balanced in relation to expenditure on medical care as a whole
“Implementation”- criteria
- adequate staff and facilities available
- managemnet and monitoring programmes
- evidence based info available to potential participants to make informed choices
state the difficulties for evaluation of screening programmes
- lead time bias
- length time bias
- selection bias
What is Lead time bias?
- early diagnosis falsely appears to prolong survival
- screening patients appear to survive longer but only because they were diagnosed earlier
- patients actually live the same length of time, but longer knowing they have the disease
What is length time bias?
- screening programmes better at picking up slowing growing unthreatening cases than aggressive fast-growing ones
- could lead to a false conclusion that screening is beneficial in lengthening the lives of those found positive
What is selection bias?
- studies of screening are often skewed by “healthy volunteer” effect
- those who have regular screening likely to also do other things that protect them from disease
