05/12 Flashcards
Is screening for people with or without symptoms of a disease?
For people without symptoms of a disease
Is screening optional or mandatory?
Always optional. (though sometimes there are consequences e.g. a screening is required for a certain job position)
Why is screening ‘rare’ in the UK?
Identification through to treatment all take part in the NHS. In some countries, screening is done routinely through a separate organisation to the health.
What does screening do?
Identifies people who appear well that probably have a disease - or precursors or susceptibility and those who are not.
Purpose of screening?
Secondary prevention - detect early disease in order to alter the course of a disease
Reduce risk of developing disease, provide treatment and information.
What is primary prevention?
Prevent occurrence of disease to begin with.
Define sensitivity of screening tests
the proportion of people with the disease who are correctly identified by the screening test (left column of table)
Define specificity of screening tests
the proportion of people without the disease who are correctly excluded by the screening test (right column of table)
Positive predictive value in screening tests?
The proportion of people with a positive test result that actually have the disease (see top row of table)
Negative predictive value in screening tests?
The proportion of people with a negative test result who do not have the disease (see bottom row of table)
How do specificity and sensitivity change as prevalence of a disease changes?
Specificity and sensitivity are the same.
As prevalance of a disease changes how do the predictive values change?
They become a lot less accurate- they become shifted. The test accuracy doesn’t change but as decreases to a VERY small number of the population, so a lot less likely to actually find someone with the condition to screen.
What are the Wilson and Jungner criteria for screening? SPLIT IT UP
- condition should be an important health problem
- the natural history of the disease must be well understood
- must have an easily detectable early stage
- there should be an accepted preexisting treatment for the disease
- facilities for diagnosis and treatment available
- adequate health service provision made for extra workload from screening
- suitable test devised from early stage
- test should be acceptable - want patients to want to be there so not too invasive
- intervals for repeating the test should be determined
-agreed policy on who to treat
- costs
- and risks should be balanced against the benefits
how is screening evaluated?
Ideally by RCT - doesn’t happen.
BUT if it fits the criteria it is generally rolled out then observed if it works well or not.
BUT
selection bias
lead-time bias
length-time bias
Selection bias in screening?
People who choose to participate are likely to be different.
e.g. More likely to have a family history of the disease.
