Lecture 6.1: Introduction to Screening Flashcards
What are the 3 Methods of Disease Detection?
- Spontaneous presentation
- Opportunistic case finding
- Screening
What is Screening?
Systematic attempt to distinguish between apparently well persons who are considered likely to have the disease in question, from those considered not likely to
How is Screening Achieved (3)
- Application of tests
- Examinations
- Other procedures
Why is Screening important?
Used to provide more favourable outcomes for patients when compared with spontaneous presentation and opportunistic case finding
Wilson and Jungner Criteria for Classic Screening Criteria (10)
1) Condition sought should be an important health
problem
2) There should be an accepted treatment for patients
with recognised disease
3) Facilities for diagnosis and treatment should be
available
4) There should be a recognisable latent or early
symptomatic stage
5) There should be a suitable test or examination
6) The test should be acceptable to the population
7) The nature of the condition should be adequately
understood
8) There should be an agreed policy on who warrants
treatment
9) Cost of case-finding, diagnosis and treatment should
be economically balanced
10) Case finding should be an ongoing process
How many NHS population screening programmes are there?
11
Who developed the NHS Population Screening Programmes?
Developed by the UK National Screening Committee (UK NSC) who advise the NHS about all aspects of population screening
NHS Population Screening Programmes
1) Abdominal Aortic Aneurysm (AAA)
2) Bowel cancer
3) Breast cancer
4) Cervical cancer
5) Diabetic retinopathy
6) Antenatal & Newborn Screening (there are six in total)
What are the 6 Antenatal & Newborn Screening Programmes
- NHS Fetal Anomaly Screening Programme (FASP)
- NHS Infectious Diseases in Pregnancy Screening (IDPS)
Programme - NHS Sickle Cell and Thalassaemia (SCT) Screening
Programme - NHS Newborn and Infant Physical Examination (NIPE)
Programme - NHS Newborn Hearing Screening Programme (NHSP)
- NHS Newborn Blood Spot (NBS) Screening Programme
What does ‘Screen Positive’ mean?
- It does not necessarily mean that the patient has the
disease - It means that the patient is considered sufficiently
likely to have the disease to warrant further
investigation
What is a False Positive result?
A false positive result means that the patient has tested positive despite being disease free
What is a False Negative result?
A false negative result means that the patient has tested negative despite having the disease in question
What are the consequences of a False Positive result?
Such individuals may be subject to unnecessary investigations or interventions which can cause them physical and/or psychological harm
What are the consequences of a False Negative result?
Such individuals may be denied necessary interventions, which may impact on their long-term quality of life and/or survival
What is Test Validity?
- Test validity represents the extent to which a test
measures what it intends to measure - Can be thought of as the ‘accuracy’ of the test
- Measured by sensitivity and specificity
What is Sensitivity?
- The ability of a test to correctly classify an individual as
having a disease - Expressed as a percentage of true positives relative to
a population of individuals with a disease
How to measure Sensitivity?
Sensitivity = true positives / (true positives + false negatives)
What is Specificity?
- The ability of a test to correctly classify an individual as
‘disease-free’. - Expressed as a percentage of true negatives in relation
to a population of ‘disease-free’ individuals
How to measure Specificity?
Specificity = true negatives / (true negatives + false positives)
Sensitive & Negative = ….
SNOUT = rule OUT
Specific & Positive = ….
SPIN = rule IN
What is the Positive Predictive Value (PPV)?
- The proportion test-positive individuals who actually
have the disease - A measure of how accurate a positive test result is
likely to be - For any given test, as disease prevalence decreases,
the test’s PPV for that population decreases - This is because there will be more false positives for
every true positive
How is the PPV calculated?
PPV = true positives / (true positives + false positives)
What is the Negative Predictive Value (NPV)?
- The proportion test-negative individuals who really do
not have the disease - A measure of how accurate a negative test result is
likely to be - For any given test, as disease prevalence decreases,
the test’s NPV for that population increases - This is because there will be more true negatives for
every false negative
How is the NPV calculated?
NPV = true negatives / (true negatives + false negatives)
What is Lead Time Bias?
Where early diagnosis falsely appears to prolong survival
What is Length Time Bias?
- Where a screening programme is better at picking up
less harmful diseases - Diseases that are detectable through screening are
more likely to have a favourable prognosis
What is a false belief associated with Length Time Bias?
Could lead to the false conclusion that screening is beneficial in lengthening the lives of those found positive
