The document outlines the National Screening Committee criteria for evaluating population health screening programs. It describes the 19 criteria under 4 categories: the condition, the test, the treatment, and the screening program. It discusses factors to consider in policy making like evidence, resources, and values. It also covers epidemiological study designs, biases in screening like length time and selection bias, and how to interpret test performance measures like sensitivity, specificity, and number needed to treat.

1. Evidence based
population health
screening

2. Objectives
 Describe the National Screening Committee criteria.
 Describe epidemiological study designs.
 Describe length time bias and repeatability in relation to
cancer screening.
 Interpret sensitivity, specificity, positive predictive value,
negative predictive value and number needed to treat.

3. The NSC Screening Criteria (2003)
 The condition
 The test
 The treatment
 The screening programme

4. The condition
1. The condition should be an important health problem
2. The epidemiology and natural history of the condition,
including development from latent to declared disease, should
be adequately understood and there should be a detectable
risk factor, disease marker, latent period or early symptomatic
stage.
3. All the cost-effective primary prevention interventions should
have been implemented as far as practicable.
4. 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.

5. The test
5. There should be a simple, safe, precise and
validated screening test.
6. The distribution of test values in the target
population should be known and a suitable cut-off
level defined and agreed.
7. The test should be acceptable to the population.
8. Agreed policy on the further diagnostic investigation
of individuals with a +ve test result.
9. Select mutations.

6. The treatment
10. Availability of effective treatment or intervention
for patients identified through early detection, with
evidence of early treatment leading to better
outcomes than late treatment.
11. Evidence based policies covering which
individuals should be offered treatment and the
appropriate treatment to be offered.
12. Clinical management of the condition and
patient outcomes should be optimised in all health
care providers prior to participation in a screening
programme.

7. The Screening Programme
13. There should be evidence from high
quality RCTs that the screening programme is
effective in reducing mortality or morbidity.
14. Programme is acceptable.
15. Benefit should outweigh harm.
16. Should be cost-effective.

8. 17. All other options for managing the condition should have
been considered.
18. There should be a plan for managing and monitoring the
screening programme and an agreed set of quality assurance
standards.
19. Adequate staffing and facilities for testing, diagnosis,
treatment and programme management are available.
20. Evidence-based information
21. Public pressure for widening the eligibility criteria for
reducing the screening interval, and for increasing the
sensitivity of the testing process, should be anticipated.
22. If screening is for a mutation the programme should be
acceptable to people identified as carriers and to other family
members.

9. Factors in policy making
evidence resources
values beliefs

10. Factors in policy making
evidence resources
values beliefs

11. Assessing the evidence
 Evaluation/study methods
 Sources of Bias
 Performance of the test

12. Study methods
Randomised
controlled
trials (RCTs)
Cohort
studies
Case-control
studies

13. Randomised controlled trials
Reference population
Target (study) population
Excluded
Refused
Study participants
Random
allocation
Screening No Screening
Outcome
Loss to follow up Loss to follow up

14. Cohort studies
Cancer
Screened Not Screened
Cancer No CancerNo Cancer
Prospective

15. Case-control studies
Cancer No Cancer
Screened Not Screened Screened Not Screened
Retrospective

16. Bias
Length time
effect
Lead time
effect
Selection
bias
Over
diagnosis
bias

17. Selection bias
 The healthy screenee - Self-selection
process.
 Those who attend screening may differ in
their underlying risk of disease/mortality
compared to those who don’t.
 Characteristics of attendees? – Week 2

18. Over diagnosis bias
 Cancers detected that would not have had
any clinically morbidity associated with
them and would not be fatal.

19. A randomized controlled trial is designed
to examine bowel screening. The
intervention group gets FOBt tests on an
annual basis, the control group has no
intervention.
Screening Assessment

20. After five years
 There are more incident bowel cancers
identified in the screened group compared to
the control group.
 The cancers in the screened group are of an
earlier stage than those in the control group.
 Five year survival is higher for the people
with cancer in the screened group.
Can we conclude that this screening
program is effective?

21. Lead time bias
Mortality versus survival statistics
Gates TJ. America Family Physician, 2001

22. Length time bias
 Screening is more likely to detect cases of
slowly progressing disease
 Screening is less likely to pick up aggressive
disease due to its rapid progression
 Screen detected cases will automatically have
a better prognosis , even if screening makes
no difference, than those presenting with
symptoms.

23. Length time bias
Gates TJ. America Family Physician, 2001

24. Test performance
 Sensitivity
 Specificity
 Positive predictive values
 Negative predictive values

25. Condition being tested for
Present Absent Totals
Test
result
Positive A
true positives
B
false positives
A+B
Negative C
false negatives
D
true negatives
C+D
Totals A+C B+D A+B+C+D

26. Condition being tested for
Present Absent Totals
Test
result
Positive A b a+b
Negative c D c+d
Totals A+C B+D a+b+c+d
Sensitivity = A / (A + C)
The ability of the test to detect the condition
when it is actually present
Specificity = D / (B+D)
The ability of the test to detect that the
condition is not present, when in fact it is absent

27. Condition being tested for
Present Absent Totals
Test
result
Positive A b A+B
Negative c D C+D
Totals a+c b+d a+b+c+d
Positive predictive value = A / (A + B)
The probability that an individual has the condition,
given that the result is positive
Negative predictive value = D / (C+D)
The probability that an individual does not have
the condition, given than the result is negative

28. Number needed to Treat/Screen
 Number of people who would have to be
screened to prevent one death.
1/Absolute risk reduction.
 Number needed to be screened.
Accounts for participation and selection
effects.

29. Presenting the evidence
 Ease of understanding
 Framing effects
 Balanced presentation
 Asking the right questions
 Asking programme questions, not single issue
questions

31. Framing
Relative risk reduction 33%
Absolute risk reduction 0.2%
Percentage of event-free patients 99.4% versus 99.6%
Number needed to treat (NNT) 500
Six deaths in a thousand patients without treatment and
four deaths in a thousand with treatment

32. Nottingham randomised trial of bowel cancer screening
(Hardcastle et al. 1996)
Mortality reduction = 15%,
(95% confidence limits 2-26%)
‘bowel screening could cut deaths by as much as 30%’
Getting the balance right

33. Asking the right questions
Years between
tests
% reduction in
cancer incidence
% of women with
false positive
smear tests in
their lifetime
10 64.1 15
5 83.6 30
3 90.8 50
2 92.5 75
1 93.5 100

34.  Single issue question:
 Would bowel screening do more benefit than harm to
the population?
 Programme question:
 Would bowel screening be more beneficial to the
population than any current healthcare activity that
could be stopped to release the necessary funding?
 If we had an extra £3m to spend per year, would
bowel screening be the most beneficial thing we could
do with this investment?
Programme vs single issues