Lecture 35: Screening Flashcards
Define screening
the widespread use of a simple test for a disease in an apparently healthy (asymptomatic) population
Define a screening programme
an organised system using a screening test among asymptomatic people in the population
What is the purpose of a screening programme?
to identify early cases of disease in order to improve outcomes
Define screening test
a test, usually relatively cheap and simple, used to test large number of apparently healthy people to identify individuals suspected of having early disease who will then go on to have further diagnostic tests to confirm the diagnosis.
How does a screening test differ from a diagnostic test?
in a screening test, there is a greater emphasis on cost and safety and less on a definitive diagnosis.
Why would we want to detect early?
to limit the consequences of disease through early diagnosis and treatment
Define lead time
the time between when the disease is detectable and when the symptoms begin to appear
What is the pre-clinical phase of a disease?
the time from the first biological onset of the disease to when the symptoms appear
What is the clinical phase?
The time from when the symptoms appear to when the outcome occurs (eg. death)
Case finding is the same as
opportunistic screening
What does screening aim to do?
it aims to improve outcomes, usually to improve mortality
Screening is a _________ not a _________
pathway
test
What is the difference between screening programmes and case finding?
screening programmes work on a population whereas case finding works on an individual level
Give an example of case finding
When you go into the doctor for a broken ankle and she checks your blood pressure and finds out that you have high blood pressure
Describe the process of a screening programme
- there is health promotion initiatives
- an invitation extended to the eligible population
- there is the screening procedure
- if this is negative, that person goes into the recall population that might get an invite again
- if this is positive, they get a diagnostic test
- if this is positive, they may get treatment
What is an example of a health promotion initiative?
ads to make people aware of the screening programme
Give an example of a diagnostic test:
an ultrasound
What underpins a screening programme and what guidelines does it have to follow?
quality control, monitoring and evaluation
When should we screen? (4)
- is the disease appropriate?
- is the test appropriate?
- would a programme be effective?
- is the benefits outweigh the harms of screening
What 4 things do we need to consider when thinking about whether the disease is appropriate?
- the seriousness of the disease
- the prevalence of the pre-clinical disease
- the ability to alter the course of the disease
- the lead time of the disease
Why do we need to consider the seriousness of the disease?
because screening is resource-intensive so it makes sense to screen for diseases with potentially severe consequences
Why do need to consider the prevalence of pre-clinical disease? What is an exception to this?
Because screening is resource intensive and so it is more efficient when there is a high prevalence of pre-clinical disease.
The exception to this is a very serious, rare disease if it is cheap and easy to administer (such as heel pricking in babies)
Why so we need to consider the lead time?
because a longer lead time means there is a greater chance of detecting disease early
What and when are the three critical points? During which of these do we screen?
- critical point 1: between the first biological onset of the disease to when the disease is detectable so at this point, it is not detectable
- critical point 2: between when the disease is detectable to when symptoms appear so this is when we do screening
- critical point 3: between when symptoms appear to when the outcome occurs so the disease is usually diagnosed anyway and there is no benefit in screening; treatment may not alter the course of the disease
What is meant by changing the course of disease?
We need to be able to change the course of the disease in order for screening to be effective
What is overdiagosis?
when you get a diagnosis for the disease and live with the knowledge that you have it, even though it may not have killed you
What is lead time bias?
when someone undergoes screening in the lead time and it appear as though they live longer than someone with no screening but they die at the same time
When considering whether a test is appropriate, what two things so we need to consider?
- is the test accurate
- is the test acceptable and safe (what are the risks?)
Describe an accurate test
when 100% of the people who test positive for a disease actually have it and when 100% of the people who test negative don’t have the disease
What is a false positive?
When someone tests positive for the disease but they don’t actually have it
What is a false negative?
when someone tests negative for a disease but they actually do have it
What is sensitivity? How can it be calculated?
the proportion of the people with the disease who test positive
a/(a+c)
What is specificity? How can it be calculated?
the proportion of people without the disease who test negative
d/(b+d)
Which is best, sensitivity or specificity?
we want to maximise both and we do this by improving the screening test and the choice of the disease threshold as therefore is a trade off between the two
How do we choose which (specificity or sensitivity) to maximise?
we need to consider consequences of missing cases (false negatives) verses false alarms (false positives)
What would the harm be of a false negative?
if you go in for a screening programme and you are told your test is negative, you may ignore symptoms that come up later which can delay seeking diagnosis which can have negative health outcomes in the future
What are the harms of a false negative?
if you get told that it is likely you have cancer as this is shown through the screening which is very worrying and weeks can go by before you get your result.
Sensitivity involves
detecting as many cases as possible which we do it the costs or risks of the next step are not too high
Sensitivity and specificity are __________ ________ of the test and they determine
intrinsic properties
determine what proportion of people with or without the disease the test correctly classifies
What do predictive values measure?
test performance in a particular population
ie. what proportion of people who test positive/negative do/don’t have the disease
What is a positive predictive value? How is this calculated?
this is the proportion of people who test positive and have the disease
a/(a+b)
ie. the proportion of people who have the disease given they tested positive
What is a negative predictive value? How is this calculated?
the proportion of people who test negative and don’t have the disease
d/(c+d)
ie. the proportion of people who do not have the disease given they have tested negtive
What are prevalence and predictive values influenced by?
prevalence in the population of interest, unlike sensitivity and specificity ie. they are population specific
When considering whether a test is appropriate, one of the things we need to consider is “is the test acceptable and safe?” What is meant by this?
we need to think about whether people will accept the test and if it is going to be safe enough to inflict on asymptomatic, healthier people, most of whom do not have the disease
We need to consider the effectiveness of the screening programme. What do we need to consider when thinking about this?
- are there resources to implement and cope with positive outcomes?
- is the programme actually effective in the population?
What two things do we need for a screening programme?
evidence from randomised control trials of benefit and onging evaluation of programmes once implemented
What are three benefits of screening programmes?
- potential for early detection and intervention (reduced mortality and/or morbidity and possibly less radical treatment required)
- reassurance if the result is a true negative
- possibility of improved health of the population
What are four harms of screening programmes?
- physical: from complications, invasive tests and/or treatments, especially if false positive, or from delayed presentation if falsely negative
- psychological: from anxiety from waiting, distress from invasive tests or procedures, knowing about serious diagnosis for longer, false negative or false positive results
- financial: to the individual or to the health service
- over-diagnosis and/or over-treatment may increase morbidity without reducing mortality
What is over-diagnosis and/or over-treatment associated with?
false positives - the period of stress and uncertainty until the diagnostic test. It mat diagnose a disease that would never have been apparent
Screening is biased towards
detecting slowly developing disease that may never have required treatment
What is length bias?
detecting slowly developing disease that would never have required treatment
When to screen (disease):
need to consider:
- the seriousness of the disease (normally only screen serious diseases)
- the prevalence of pre-clinical disease
- need an identifiable lead time where we are able to diagnose the disease
- need to be able to alter the course of the disease
When to screen (test)
- is the test accurate (sensitivity, specificity, PPV, NPV)
- is the test acceptable and safe
When to screen (effectiveness)
- there needs to be resources available to implement the entire screening programme
- the programme needs to actually be effective
When should we not screen?
we need to consider
- the missed cases of disease (false negatives)
- cost effectiveness
- over diagnosis
What is meant by screening having to change the course of the disease?
Screening has to be done at the right moment because there has to be some effective therapy or treatment available. You can not screen before the disease is detectable and you can not screen after symptoms appear because at that point, the outcome is determined and you can not alter the course of the disease
