Session 7 Lecture 1&2 - Screening

Question 1

what is screening?

Answer 1

the process of identifying healthy people who may have an increased chance of a disease or condition

Question 2

Determinants of Screening

Answer 2

• the condition
• the test
• the intervention
• screening programme
• implementation criteria

Question 3

“The condition”- criteria

Answer 3

-an important health problem with understood epidemiology, incidence, prevalence and natural history

Question 4

“The test”-criteria

Answer 4

• simple. safe, precise and validated screening test
• accepted to target pop.
• distribution of test values in the pop. must be known and an agreed cut-off level must be agreed upon

Question 5

4 ways in which the effectiveness of a screen test is assessed

Answer 5

• sensitivity
• specificity
• positive predictive value
• negative predictive value

Question 6

Definition of Sensitivity

Answer 6

the proportion of cases which the test correctly detects

Question 7

Definition of Specificity

Answer 7

The proportion of non-cases which the test correctly detects

Question 8

Definition of Positive Predictive Value

Answer 8

the proportion of positive tests who are cases

Question 9

Definition of Negative predictive value

Answer 9

the proportion of negative tests who are non-cases

Question 10

How to calculate sensitivity

Answer 10

a/a+c

Question 11

How to calculate specificity

Answer 11

d/b+d

Question 12

How to calculate Postive predictive value

Answer 12

a/a+b

Question 13

How to calculate Negative predictive value

Answer 13

d/c+d

Question 14

“The intervention” - criteria

Answer 14

-evidence that intervention at a presymptomatic phase leads to better outcomes compared with usual care

Question 15

“The screening programme”- criteria

Answer 15

• proven effectiveness in reducing mortality or morbidity through high-quality research
• evidence that the complete screening programme is clinically, socially and ethically acceptable to health professionals and public
• the opportunity cost of the screening programme should be economically balanced in relation to expenditure on medical care as a whole

Question 16

“Implementation”- criteria

Answer 16

• adequate staff and facilities available
• managemnet and monitoring programmes
• evidence based info available to potential participants to make informed choices

Question 17

state the difficulties for evaluation of screening programmes

Answer 17

• lead time bias
• length time bias
• selection bias

Question 18

What is Lead time bias?

Answer 18

• early diagnosis falsely appears to prolong survival
• screening patients appear to survive longer but only because they were diagnosed earlier
• patients actually live the same length of time, but longer knowing they have the disease

Question 19

What is length time bias?

Answer 19

• screening programmes better at picking up slowing growing unthreatening cases than aggressive fast-growing ones
• could lead to a false conclusion that screening is beneficial in lengthening the lives of those found positive

Question 20

What is selection bias?

Answer 20

• studies of screening are often skewed by “healthy volunteer” effect
• those who have regular screening likely to also do other things that protect them from disease