Screening Pt 1 Flashcards
What are the three methods of detecting a disease?
Spontaneous presentation
Opportunistic case finding
Screening
What is spontaneous presentation in terms of detecting a disease?
Person presents with symptoms
- Self-defined as a ‘patient’
Occurs at
- GP
- A&E
- Other services
A diagnosis is made
What is opportunistic case finding in terms of detecting a disease?
Person presents with symptoms related to a disease/problem
GP takes the opportunity to check for other diseases
- BP measurement
- Urine dipstick
What is a diagnosis?
The DEFINITIVE identification of a SUSPECTED disease or defect by application of tests, examination or other procedures (which can be extensive) to DEFINITELY LABEL people as HAVING a disease or NOT HAVING a disease
The diagnosis is made following tests
Treatment will follow
The ‘patient’ will be prepared to accept the risk (side effects) associated with the treatment in order to get well
What is the definition of screening?
A systematic attempt to detect an UNRECOGNISED condition by the application of tests, examinations, and 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 DO NOT
What is screening? (not the definition, just more info)
A process that labels people as screen positive or screen negative
Following screening if a person is labelled as ‘screen positive’ - this does not mean that they DEFINITELY have the disease
Further test tests are required before the diagnosis of disease is made
Treatment will only follow once a definitive diagnosis is made
What is the purpose of screening?
To give a better outcome compared with finding something in the usual way (having symptoms self-reporting to the health services)
If treatment can wait until there is symptoms, there is no point in screening
What are some NHS population screening programs?
Abdominal Aortic Aneurysm (AAA) Bowel cancer Breast cancer Cervical cancer Diabetic Retinopathy Down's syndrome Foetal anomalies PKU Sickle Cell and Thalassaemia Inherited metabolic diseases
What are the benefits and risks of AAA screening?
All men are offered a single ultrasound screening scan when they are 64
Benefits
- Reduces risk of death from rupture by about half
Risks
- 238 men needed to be invited to screening to
prevent one ruptured AAA in next ten years
- Approximately 1 in 10,000 will attend for screening,
have a large AAA and then undergo treatment which
is complicated by death
- Quality of life is impaired (at least short term) for
those who have treatment
- Some cannot tolerate anxiety of knowing they have a
small AAA
What are the four criteria for deciding on whether to screen?
Disease/Condition
Test
Treatment
Programme
Describe the criteria of disease/condition for deciding on whether to screen or not
Must be an important health problem
Epidemiology and natural history must be well known
Must have an early detectable stage
Cost-effective primary prevention interventions must have been considered and where possible implemented
Describe the criteria of the test for deciding on whether to screen or not
Simple & safe (screening healthy people)
Precise & valid (‘tells the truth’)
Acceptable to the population
Distribution of test values in the population must be known i.e. the proportion who test positive and negative
An agreed cut-off level must be defined (who do we count as testing positive)
There must be an agreed policy on whom to investigate further (i.e. the test positives)
What are the two types of errors any screening programme makes?
It will refer well people for further investigation
- Put them through stress, anxiety and inconvenience
- Direct costs
- Opportunity costs
- These are known as false positives
It will fail to refer people who actually do have an early form of the disease
- Inappropriate reassurance
- Possibly delay presentation with symptoms
- These are known as false negatives
What are the features of test validity?
Sensitivity (detection rate)
Specificity
Positive predictive value (PPV)
Negative predictive value (NPV)
What are the four different types of result you can get for a person who has been screened?
True positives - Disease present and a positive test
True negatives - Disease absent and negative test
False positives - Disease absent and positive test
False negatives - Disease present and negative test
What is the sensitivity of a test?
It is the proportion of the people with the disease who are test positive
The proportion of the people who really have the disease and are detected by the test as having the disease
Sensitivity is the probability a case will test positive
Sensitivity = (True +ves) / ((True +ves) + (False -ves))
If the sensitivity of the test is high then the test is very good at correctly identifying people with the disease
A high sensitivity is ideal.
What is the specificity of a test?
It is the proportion of the people without the disease who are negative test
Probability a non-case will test negative
Specificity = (True -ves) / ((False +ves) + (True -ves)
What are the features of sensitivity and specificity?
They are a function of the characteristics of the test
When the same test is applied to different population the test will have the same sensitivity and specificity
What is the positive predictive value (PPV) of a test
Probability that someone who has tested positive actually has the disease
This value is strongly influenced by the prevalence of the disease
PPV = (True +ves) / ((True +ves) + (False +ves)
A lower prevalence condition will have a lower PPV than a higher prevalence condition.
What is negative predictive value (NPV) of a test?
It is the proportion of the people who are test negative who actually do not have the disease.
NPV = (True -ves) / ((False -ves) + (True -ves))
What are the implications of false positive results?
The test indicates they might have a disease when in fact they do not
They will be offered (invasive) diagnostic testing with all its attendant anxieties and risks - for a condition they actually do not have.
They will be turned into “patients” when they are not actually ill
May lead to a lower uptake of screening in the future and greater risk of interval cancer
If the PPV is low there will be a lot of people with false positive results who undergo stress and unnecessary procedures
Implications of false negative results
The screening test indicates that they do not have the disease when in fact they do
They will not be offered (invasive) diagnostic testing when in fact they may have benefited from it. Their disease, although present will not be diagnosed
They will be falsely reassured - may present late with symptoms as a consequence
Describe the criteria of treatment for deciding on whether to screen or not
Effective evidence based treatment must be available
Early treatment must be advantageous - you must not just bring forward the date of diagnosis
Agreed policy on whom to treat
Clinical management of the condition and patient outcomes should be optimised in health care providers before participation in screening programmes
Describe the criteria of the programme for deciding on whether to screen or not
Proven effectiveness (preferably with RCT data)
Quality assurance for the whole programme not just the test
Facilities for counselling
Facilities for diagnosis and treatment
Other options should be considered e.g. improving treatment
Think about opportunity costs
Decisions about parameters should be scientifically justifiable to the public
Benefit should outweigh physical and psychological harm (test, diagnostic procedures or treatment)
