screening Flashcards
define screening
the widespread use of a simple test for a disease in an apparently healthy population.
whats a screening programme
an organised system using a screening test among asymptomatic people in the population to identify early cases of a disease in order to improve health outcomes
define screening test
a test that is usually cheap and simple, that is used to test large numbers of apparently healthy people to identify individuals suspected of having disease. these people will then go on to have diagnostic test to test the diagnosis
what prevention type is screening an example of and why
example of secondary prevention as it aims to limit the consequences of disease through early diagnosis and treatment
4 considerations of when we should screen
is the disease appropriate
is the test appropriate
would a screening program be effective
consider benefits vs harms
4 considerations when determining if disease is appropriate to screen
seriousness,
ability to alter the course of the disease
lead time
prevalence of pre-clinical disease
why consider disease seriousness
screening is resource intensive, so it makes sense to screen for diseases with potentially severe consequences
why is prevence of pre-clinical disease an important consideration
screening is resource intensive, its more efficient when there’s a high prevalence of pre-clinical disease.
unless disease is rather serious and the test is easy and cheap
why is lead time an important consideration
long lead time means greater chance of detecting disease early
what are the three critical points
between first biological onset and disease detectable is critical point one, at this point no disease is detectable.
between DD and symptoms appear is CP 2, screening may be beneficial here
between SA and outcome is CP3 at this point disease is usually diagnosed, so no benefit.
what is length time bias
how screening is biased towards detecting slowly developing disease that may not have required treatment, therefore we get the impression of a better prognosis
what is lead time bias
when those who are screened earlier for a disease appear to have survived with the disease longer, simply due to an earlier diagnosis date.
what are the two intrinsic properties of a test
sensitivity and specificty, there is usually a trade off between these two factors
what is sensitivity
proportion of people with the disease who test positive
what is specifity
proportion of people without the disease who test negative
when do we chose to maximise specificity in a screening test
when there is high cost or high risk involved in the next step
when do we choose a screening test which maximises sensitivity
when detecting as many cases as possible is important. costs or risks of the following step is no too high
what are predicted values
these are test performance measures in a particular population. these are no intrinsic to the test type
whats positive predicted value
the proportion of people who test positive and actually have the disease
what is negative predicted value
proportion of people who test negative who don’t have the disease
what are PPV and NPV influenced by
these are influenced by the prevalence of the disease in the population, unlike sensitivity and specificity
when looking at screening programme effectiveness what do we need to consider
will it be effective?
are there resources to implement and cope with positives,
is the programme actually effective
important considerations when looking if there are resources to implement and cope with positives
Facilities and systems
Manage participation
Cost and accessibility
Quality control and monitoring
Treatment
Capacity to treat true positives
Cost effectiveness
Many people over long period
Cost vs benefit
what must we consider if we are looking at if the screening test will actually be effective
is there Evidence from Randomized Controlled Trials of benefit (reduced mortality and/or morbidity) prior to initiation
Ongoing evaluation of programmes once implemented