Population Screening

Question 1

What is the definition of screening?

Answer 1

A systematic attempt to detect as UNRECOGNISED condition by the application of tests, examination or other procedures.
Usually rapid, cheap, distinguishes between healthy ad non healthy people.

Question 2

What are 5 areas of criteria for implementing a screening programme?

Answer 2

Condition - an important, well understood health problem
Test
Intervention - evidence that intervention leads to better outcomes
Screening programme - ethical, benefits outweigh risks, cost
Implementation - quality assurance (managing and monitoring), adequate resources to carry out, informed choice, public pressure anticipated.

Question 3

What are some factors to consider when deciding the ‘test’ in population screening?

Answer 3

Simple, safe
Test values must be agreed - ‘cut off’
Acceptable to target audience
Agreed policy if result is positive

Question 4

What are two types of error in a screening test?

Answer 4

False negative - anxiety, costs

False positive - delay in treatment, inappropriate reassurance

Question 5

How do you calculate test sensitivity?

Answer 5

True positives / tries positives + false negatives

Question 6

What do we mean by test specificity in test validity?

Answer 6

Proportion of people without the disease who test negative.

Question 7

How do you calculate test specificity?

Answer 7

True negatives / false positives + true negatives

Question 8

Name 4 features of test validity.

Answer 8

Sensitivity (detection rate)
Specificity
Positive predictive value
Negative predictive value

Question 9

What do we mean by positive predictive value (PPV)? What key factor can affect this?

Answer 9

If a person is tested, what is their risk of actually having the disease?

Population prevalence - high = high PPV

Question 10

What is the calculation for PPV?

Answer 10

True positives / true positives + false positives

Question 11

What is negative predictive value (NPV)? How do you calculate this?

Answer 11

What are you chances are not having the disease if I’m screened for it?

True negatives / false negatives + true negatives

Question 12

What are the implications of false positive results?

Answer 12

Unnecessary invasive testing
Slow uptake of later screening
Stressful

Question 13

What are the implications of false negative results?

Answer 13

false reassurance

Not ordered intervention

Question 14

Give some e.g.s of UK population screening programmes?

Answer 14

Bowel cancer 
Abdominal aneurysms 
Cervical cancer 
Breast screening - controversial - for every life saved 3 women were investigated and treatment unnecessarily 
Diabetic eye screening 
Newborn hearing
Sickle cell and thalassaemia

Question 15

What are some topics for debate regarding population screening?

Answer 15

How many abnormalities are spontaneous?
Are the right people being screened?
Has screening lead to an observed reduction in mortality?
Over - diagnosis and over - treatment?
Psychological impact?

Question 16

What are some difficulties in evaluating the effectiveness of screening programmes?

Answer 16

Lead time bias
Length time bias
Selection bias

Question 17

What is lead time bias?

Answer 17

Early diagnosis falsely appears to prolong survival, or patients may live the same length of time knowing they have the disease

Not difference in mortality outcomes.

Question 18

What is length time bias?

Answer 18

Screening better at picking up slow growing cases than aggressive, fast ones.
Could lead to conclusion that screening is beneficial in lengthening lives of those who are found positive.

Question 19

What is selection bias?

Answer 19

Skewed ‘heathy volunteer’ effect - those who have screening look after themselves.

Question 20

What are the sociological critiques of screening?

Answer 20

Victim blaming / individualising pathology - focus on prevention instead?
Surveillance society
Moral obligation - but difficulty in defining screening
Feminist critiques - aimed at women’s bodies e.g breast, cervical - perception at women’s bodies are unreliable