L13- Screening

Question 1

definition of screening

Answer 1

“The presumptive identification of unrecognised disease or defect by the application of tests, examinations or other procedures that can be applied rapidly to sort out apparently well persons who probably have a disease from those who probably do not”

Question 2

all screening tests require confirmation by a

Answer 2

diagnostic test after

Question 3

cost-effecitveness dictates that only

Answer 3

only high-risk screening test results are confirmed by a diagnostic test or procedure

Question 4

lower risk screening tests result are assumed to be correct

Answer 4

untill proven otherwise

Question 5

ethical and consent implications for screening

Answer 5

great responsibility for care required in screening than in clinical care:
‘Screening will inevitably turn some people who test ‘positive’ into patients – a transformation not to be undertaken lightly. If a patient asks a medical practitioner for help, the doctor does the best possible. The doctor is not responsible for defects in medical knowledge. If, however, the practitioner initiates screening procedures the doctor is in a very different situation. The doctor should, in our view, have conclusive evidence that screening can alter the natural history of the disease in a significant proportion of those screened.’

Question 6

five criteria need to be satisfied

Answer 6

1. The condition
2. The test
3. The intervention
4. The screening programme
5. Implementation

Question 7

the condition

Answer 7

• An important health problem with understood epidemiology, incidence, prevalence and natural history
• All cost effective primary prevention interventions should have been implemented as far as practicable
• If the carriers of a mutation are identified as a result of screening, the natural history of people with this status should be understood, including the psychological implications

Question 8

the test

Answer 8

• Simple, safe, precise and validated screening test
• Agreed cut off level must be defined and agreed
• Acceptable to target population
• Diagnostic test available for those who test positive

Question 9

the intervention

Answer 9

• Effective interventions for patients identified through screening, with evidence that intervention at a pre-symptomatic phase leads to better outcomes for the screened individual compared with usual care
• Treatment at early stage should be of more benefit than at a later stage

Question 10

the screening programme

Answer 10

• Proven effectiveness in reducing mortality or morbidity
• Evidence that the complete screening programme is clinically, socially and ethically acceptable to health professionals and the public
• Benefit gained by individual should outweigh any possible harms e.g. over treatment, false reassurance etc

Question 11

implementation

Answer 11

• Clinical management and patient outcomes should be optimised in all healthcare providers
• All other options for managing the condition should have been considered
• Management and monitoring programme (quality assurance)
• Adequate staff and facilities available
• Evidence based information available to potential participants (informed choice)
• Public pressure should be anticipated- decisions should be scientifically justifiable to the public

Question 12

any screen is going to make two types of errors

Answer 12

1) Its going to refer well people for further investigation- false positive
2) Its going to fail to refer people who actually do have an early form of the disease- false negatives

Question 13

sensitivity

Answer 13

is the proportion of cases which the test correctly detects

Question 14

specificity

Answer 14

is the proportion of non-cases which the test correctly detects

Question 15

positive predictive valie

Answer 15

is the proportion of positive tests who are cases

Question 16

NPV

Answer 16

is the proportion of negative tests who are not cases

Question 17

sensitivity =

Answer 17

true positives / TP + FN

Question 18

specificity =

Answer 18

TN/ TN + FP

Question 19

PPV=

Answer 19

TP/ TP+ FP

Question 20

NPV

Answer 20

TN/ TN + FN

Question 21

evaluating screening programmes

Answer 21

• Screening programmes need to be based on good quality evidence
• There can be a greater pressure to start screening programmes
  o E.g. Prostate cancer
   No robust evidence that earlier detection improves outcome
   Screening could cause harm- unnecessary treatment, can cause un wanted side effects

Question 22

difficult to evaluate due to

Answer 22

o Lead to time bias
o Length time bias
o Selection bias

Question 23

lead time bias

Answer 23

• Early diagnosis falsely appears to prolong survival
• Screened 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 24

length time bias

Answer 24

• Screening programmes better at picking up slowly growing, unthreatening cases than aggressive, fast growing ones
• Diseases that are detectable through screening are more likely to have a favourable prognosis, may indeed never have caused a problem
• Could lead to a false conclusion that screening is beneficial in lengthening lives of those found positive- curing people that didn’t need curing?

Question 25

selection bias

Answer 25

• 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
• Randomised control trials help with this bias

Question 26

need for informed choice

Answer 26

• Increasing emphasis on promoting informed choice about screening
• Important for individual patients even in the case of evidence- based and endorsed national screening programmes
• But communication benefits, harms and risks can be challenging

Question 27

positive of screening

Answer 27

can lead to better outcpomes

Question 28

negative sof screenign

Answer 28

risk over investigation and treatment and false reassurance