Population Based Screening Flashcards
Define diagnosis
The definitive indenitification of a suspected disease or defect by application of tests, examinations, or other procedures (which can be extensive) to definitively label people as either having the disease or not having a disease
What does the process of diagnosis include?
Presentation, followed by history, examination, and tests, which then leads to the conclusion of disease or no disease
What will follow a diagnosis?
Treatment
What must the patient be prepared to accept regarding the treatment following a diagnosis?
The (reasonable) risks/side-effects associated with the treatment, in order to get well
What are the ways of detecting diseases?
- Spontaneous presentation
- Opportunistic case finding
- Screening
What happens in spontaneous presentation?
The patient self-defines themselves as a patient, and presents with symptoms
Who may a self-defined patient spontaneously present to?
- GP
- A&E
- Other services
What happens in an opportunistic case finding?
Person presents with symptoms related to a disease/problem, and the health professional takes the opportunity to check for other potential conditions
What kind of investigations may a HCP do that leads to a opportunistic case finding?
- BP measurement
- Urine dipstick
How does an opportunistic case finding differ from a spontaneous presentation?
The patient is still the one to intitate contact
What is screening?
A systematic attempt to detect unrecognised condition by the application of tests, examinations, or other procedures, which can be applied rapidly and cheaply to distinguish between apparently well persons who probably have a disease (or its precursor) and those who probably don’t
Does screening provide a definitive diagnosis?
No
What does the screening process involve?
Screening with a rapid/cheap test, leading to a positive or negative screen. People who screen positive are deemed to be at high risk, and so undergo diagnostic tests to determine wether they have the disease or not
How does the National Screening Committee define screening?
The process of identifying healthy people who may be at increased risk of a disease or condition. The screening provider then offers information, further tests, and treatment. This is to reduce associated risks and complications.
What is the purpose of screening?
To give a better outcome compared with finding something in the usual way (having symptoms and self-reporting to health services)
When is there no point in screening?
If treatment can wait until there are symptoms, only useful if better outcomes if you find/intervene earlier
What screening programmes are currently in place in the UK?
- Abdominal aortic aneurysm
- Bowel cancer screening
- Breast screening
- Cervical screening
- Diabetic eye screening
- Fetal anomaly screening programme
- Infectious diseases in pregnancy screening
- Newborn and infant physical examination
- Newborn blood spot
- Newborn hearing screening program
- Sickle cell and thalassaemia
What screening programmes are currently not in place nationally in the UK?
- Prostate cancer
- Breast cancer screening with mammography for women under the age of 50 except those in a pilot study
- Cervical cancer for women under the age of 25
What are five areas of criteria for screening programmes?
- Condition
- Test
- Intervention
- Screening programme
- Implementation
What are the ‘condition’ criteria for a screening programme?
- Must be an important health problem with good understanding
- All the cost-effective primary prevention interventions should have been implemented as far as practicable
- In the case of genetic screening, if the carriers of a mutation are identified as a result of screening, the natural history of people with the status should be understood
On what factors is the importance of a health problem measured when considering screening?
- Frequency
- Severity
What factors must be understood about a health problem when considering screening?
- Epidemiology
- Incidence
- Prevalence
- Natural history
Must know how a condition behaves, and what would happen if we didn’t intervene
What are the ‘test’ criteria for screening programmes?
- Simple, safe, precise, and validated screening test
- Distribution of test values in the population must be known
- Acceptable to target population
- Agreed policy on further diagnostic investigations to those who test positive, and choices available to them
- If the test is for a particular, or set of, genetic variants, the method for their selection, and the means through which these will be kept under review in the programme should be clearly set out
What are the ‘intervention’ criteria for screening tests?
- There must be effective intervention for patients identified through screening, with the evidence that intervention at a pre-symptomatic phase leads to better outcomes for the screened individual compared to usual care
- Should be agreed evidence based policies. covering which individuals should be offered interventions, and the appropriate intervention to be offered
What are the ‘screening programme’ criteria for screening programmes?
- Must have proven effectiveness in reducing mortality and morbidity
- Evidence that complete screening programme is acceptable to health professionals and the public
- Benefit gained by individuals should outweigh any harms
- Opportunity cost of screening programme should be economically balanced in relation to the expenditure on medial care as a whole
What is needed to prove the effectiveness of a screening programme at reducing mortality or morbidity?
High quality RCT data
In what respects should a screening programme be acceptable to health professionals and public?
- Clinically
- Socially
- Ethically
What harms could arise from screening programmes?
- Overdiagnosis
- Overtreatment
- False positives
- False reassurances
- Uncertain findings
- Complications
What are the ‘implementation’ criteria of screening programmes?
- Clinical management and patient outcomes should be optimised in all healthcare providers
- All other options for managing the condition should have been considered
- Management and monitoring programme should undergo quality assurance
- Must be adequate staffing and facilties for programme
- Must be evidence-based information available to potential participants
- Public pressure should be anticipated
Why must there be evidence-based information about a screening programme available to potential participants?
Must be able to make informed choice
What is important when anticipating public pressure?
Decisions should be scientifically justifiable to public
What types of error is any screening test going to make?
- Going to refer well people for further investigations - false positives
- Going to fail to refer people who actually do have an early form of the disease - false negative
Why is any screening test going to make errors?
Because it is not gold standard diagnostic
What is the problem with false positives?
- Puts patient through stress, anxiety, and inconvenience
- Direct costs
- Opportunity costs
What are the direct costs of false positives?
Services for further investigations, staff etc
What are the opportunity costs of false positives?
Money is no longer available to spend elsewhere
What is the problem with false negatives?
- Inappropriate reassurance
- Possibly delay presentation of symptoms, because they think they’ve been screened and so are fine
What are the features of test validity?
- Sensitivity (detection rate)
- Specificity
- Positive predictive value
Negative predictive value
What is sensitivity, regarding screening?
The proportion of people with the disease who test positive
How is sensivitity calculated?
A / (A+C)
What does a high sensitivity mean in screening tests?
The test is very good at correctly identifying people with the disease that you are screening for.
A high sensitivity is ideal, although not always possible
What is specificity, regarding screening tests?
The proportion of people without the disease who test negative
How is specificity calculated?
D / (B+D)
What does a high specificity mean in a screening test?
The test is very good at correctly identifying people without the disease as not having the disease
A high sensitivity is ideal, although not always possible
What kind of measures are specificity and sensitivity in screening tests?
They are a function of the characteristics of the test
What happens, regarding sensitivity and specificity, if the same test is applied in the asme way in different populations?
It will have the same sensitivity and specificity
What is the positive predictive value?
The probability that someone who has tested positive actually has the disease
What is the positive predictive value strongly influenced by?
The prevalence of the disease
How is PPV calculated?
A / (A+B)
What is the negative predictive value?
The proportion of people who test negative who don’t actually have the disease
How is the NPV calculated?
NPV = D / (C+D)
What is a false positive?
When the test indicates a patient may have the disease, when in fact they do not
What is the problem with false positives?
- They will be offered invasive diagnostic testing, with all its attendant anxieties and risks, for a condition they don’t actually have - they will be turned into a patient when they are not actually ill
- May lead to lower uptake of screening in the future, and greater risk of interval cancer
What happens if the PPV is low?
There will be a lot of people with false positive results who undergo stress and unneccessary procedures
What is a false negative?
When the screening test indicates that they do not have the disease when they in fact do
What is the problem with false negatives?
- They will not be offered diagnostic testing, when in fact they may have benefitted from it
- Their disease, although present, will not be diagnosed
- They will be falsely reassured, and may present late with symptoms as a consequence
What questions does screening raise?
- Is the natural history always understood?
- How many abnormalities would regress spontaneously, or never be problematic?
- Are the ‘right’ people being screened?
- Has screening caused any observed reduction in mortality?
- Over-diagnosis and over-treatment?
- Psychological impact?
What are the difficulties with evaluating screening programmes?
- Lead time bias
- Length time bias
- Selection bias
What is lead time bias?
Where early diagnosis falsely appears to prolong survival.
Screening patients appear to survive longer because they were diagnosed earlier, but actually live the same length of time but longer knowing they have the disease
What is length-time bias?
Screening programmes can be better at picking up slow growing, unthreatening cases than agressive, fast-growing ones, and so diseases that are detectable through screening are more likely to have a favourable prognosis, and would never have caused a problem, leading to the false conclusion that screening is beneficial in lengthening the lives of those found positive
What is selection bias?
The studies of screening are often skewed by the healthy volunteer effect, as those who have regular screening are likely to also do other things that protect them from the disease
Give an example of a screening programme that there is great pressure to start?
Prostate cancer
What is the problem with implementing a prostate cancer screening programe?
- No robust evidence that earlier detection improves outcome
- Screening could cause harm, in the form of unnecessary treatment and unwanted side effects
What is there an increasing emphasis on, regarding screening?
Promoting informed choice about screening
Why is promoting an informed choice about screening challenging?
The HCP has to have a thorough grasp on the current state of evidence for that particular intervention, and be able to clearly convey what is and is not known, what is certain, and the degree of uncertainty.
Why is it important that people are able to make an informed choice about screening?
Need to be able to make an individualised trade-off of potential benefits and harms in order to determine their own level of risk
What are the sociological critiques of screening?
- Victim blaming/individualising pathology
- Individuals and populations increasingly subject to surveillance
- Moral obligations
- Feminist critiques
