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Evidence based population health screening

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Evidence based population health screening

  1. 1. Evidence based population health screening
  2. 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. 3. The NSC Screening Criteria (2003)  The condition  The test  The treatment  The screening programme
  4. 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. 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. 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. 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. 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. 9. Factors in policy making evidence resources values beliefs
  10. 10. Factors in policy making evidence resources values beliefs
  11. 11. Assessing the evidence  Evaluation/study methods  Sources of Bias  Performance of the test
  12. 12. Study methods Randomised controlled trials (RCTs) Cohort studies Case-control studies
  13. 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. 14. Cohort studies Cancer Screened Not Screened Cancer No CancerNo Cancer Prospective
  15. 15. Case-control studies Cancer No Cancer Screened Not Screened Screened Not Screened Retrospective
  16. 16. Bias Length time effect Lead time effect Selection bias Over diagnosis bias
  17. 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. 18. Over diagnosis bias  Cancers detected that would not have had any clinically morbidity associated with them and would not be fatal.
  19. 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. 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. 21. Lead time bias Mortality versus survival statistics Gates TJ. America Family Physician, 2001
  22. 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. 23. Length time bias Gates TJ. America Family Physician, 2001
  24. 24. Test performance  Sensitivity  Specificity  Positive predictive values  Negative predictive values
  25. 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. 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. 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. 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. 29. Presenting the evidence  Ease of understanding  Framing effects  Balanced presentation  Asking the right questions  Asking programme questions, not single issue questions
  30. 30. 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
  31. 31. 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
  32. 32. 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
  33. 33.  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

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