Evidence based

population health

screening

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.

The NSC Screening Criteria (2003)

The condition

The test

The treatment

The screening programme

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.

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.

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.

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.

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.

Factors in policy making

evidence resources

values beliefs

Factors in policy making

evidence resources

values beliefs

Assessing the evidence

Evaluation/study methods

Sources of Bias

Performance of the test

Study methods

Randomised

controlled

trials (RCTs)

Cohort

studies

Case-control

studies

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

Cohort studies

Cancer

Screened Not Screened

Cancer No Cancer No Cancer

Prospective

Case-control studies

Cancer No Cancer

Screened Not Screened Screened Not Screened

Retrospective

Bias

Length time

effect

Lead time

effect

Selection

bias

Over

diagnosis

bias

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

Over diagnosis bias

Cancers detected that would not have had

any clinically morbidity associated with

them and would not be fatal.

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

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?

Lead time bias

Mortality versus survival statistics Gates TJ. America Family Physician, 2001

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.

Length time bias

Gates TJ. America Family Physician, 2001

Test performance

Sensitivity

Specificity

Positive predictive values

Negative predictive values

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

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

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

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.

Presenting the evidence

Ease of understanding

Framing effects

Balanced presentation

Asking the right questions

Asking programme questions, not single issue questions

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

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

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

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