Lecture 6.1: Introduction to Screening

Question 1

What are the 3 Methods of Disease Detection?

Answer 1

1. Spontaneous presentation
2. Opportunistic case finding
3. Screening

Question 2

What is Screening?

Answer 2

Systematic attempt to distinguish between apparently well persons who are considered likely to have the disease in question, from those considered not likely to

Question 3

How is Screening Achieved (3)

Answer 3

• Application of tests
• Examinations
• Other procedures

Question 4

Why is Screening important?

Answer 4

Used to provide more favourable outcomes for patients when compared with spontaneous presentation and opportunistic case finding

Question 5

Wilson and Jungner Criteria for Classic Screening Criteria (10)

Answer 5

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

Question 6

How many NHS population screening programmes are there?

Answer 6

11

Question 7

Who developed the NHS Population Screening Programmes?

Answer 7

Developed by the UK National Screening Committee (UK NSC) who advise the NHS about all aspects of population screening

Question 8

NHS Population Screening Programmes

Answer 8

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)

Question 9

What are the 6 Antenatal & Newborn Screening Programmes

Answer 9

• 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

Question 10

What does ‘Screen Positive’ mean?

Answer 10

• 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

Question 11

What is a False Positive result?

Answer 11

A false positive result means that the patient has tested positive despite being disease free

Question 12

What is a False Negative result?

Answer 12

A false negative result means that the patient has tested negative despite having the disease in question

Question 13

What are the consequences of a False Positive result?

Answer 13

Such individuals may be subject to unnecessary investigations or interventions which can cause them physical and/or psychological harm

Question 14

What are the consequences of a False Negative result?

Answer 14

Such individuals may be denied necessary interventions, which may impact on their long-term quality of life and/or survival

Question 15

What is Test Validity?

Answer 15

• 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

Question 16

What is Sensitivity?

Answer 16

• 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

Question 17

How to measure Sensitivity?

Answer 17

Sensitivity = true positives / (true positives + false negatives)

Question 18

What is Specificity?

Answer 18

• 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

Question 19

How to measure Specificity?

Answer 19

Specificity = true negatives / (true negatives + false positives)

Question 20

Sensitive & Negative = ….

Answer 20

SNOUT = rule OUT

Question 21

Specific & Positive = ….

Answer 21

SPIN = rule IN

Question 22

What is the Positive Predictive Value (PPV)?

Answer 22

• 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

Question 23

How is the PPV calculated?

Answer 23

PPV = true positives / (true positives + false positives)

Question 24

What is the Negative Predictive Value (NPV)?

Answer 24

• 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

Question 25

How is the NPV calculated?

Answer 25

NPV = true negatives / (true negatives + false negatives)

Question 26

What is Lead Time Bias?

Answer 26

Where early diagnosis falsely appears to prolong survival

Question 27

What is Length Time Bias?

Answer 27

• 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

Question 28

What is a false belief associated with Length Time Bias?

Answer 28

Could lead to the false conclusion that screening is beneficial in lengthening the lives of those found positive