Prevention & Screening: Maximizing Public Health Impact

1. Prevention and Screening MMS Year 4 Public Health Workshop in O&G

2. Why is prevention better than cure? Hygieia – goddess of health

3. McKeowan (1979) and Illich (1976) • Personal medical care has contributed relatively little to the improvements in mortality since 1800s

4. Medical care • Bunker 1995 • Only 2 years of the 23 years increase in longevity is due to medical intervention between 1900-1950 • But since 1950, 3 out of the 7 years increased life expectancy • Why?

5. Prevention • Primary prevention • Stopping a disease from occurring (incidence) • Secondary prevention • Detecting a disease early to allow early intervention to reduce mortality and morbidity • Tertiary Prevention • Stopping the recurrence or complications of a disease

6. Why do we do them? • Intuitive • Cost • Benefit • Quality of life • Morbidity • Mortality • So why don’t we do them for everything?

7. Why don’t we do them • Cost • Benefit • Evidence based • Early detection • Treatment • Ethical (Abdominal Aortic Aneurysm) • Intuitive • Persuasion • Side effects (pertussis) • Harm (MMR, psychological) • Risk no longer exists

8. TASK: Why do you wear a seatbelt?

9. Prevention Paradox • Rose 1981 • “A preventative measure that brings large benefits to the community offers little to each participating individual.”

10. Secondary/Tertiary Prevention

11. Screening: doing more good than harm Mission of the National Screening Committee

12. What is screening? • ‘Healthy’ population • Asked a question or offered a test • Identifies individuals who are more likely to be helped than harmed by further tests or treatment to reduce the risk of a disease or its complications

13. Limitations of Screening • Ethical differences from clinical practice for people who present with symptoms • Should help individuals to make informed choices about their health • Risks as well as benefits

14. TASK: What screening programmes are NHS funded? • Rapid list on the whiteboard

16. Animation http://cpd.screening.nhs.uk/timeline

17. Criteria for Screening Programmes Ideally all criteria should be met!

18. The Condition • Important health problem • Natural history understood • development from latent to declared disease • detectable risk factor • disease marker • Cost-effective primary prevention interventions implemented • Carriers identified as a result of screening • natural history of this status understood • psychological implications

19. The Test • Validated screening test • Simple • Safe • Precise • Test values in the target population known • suitable cut-off level defined and agreed • Acceptable to the population • Agreed investigation if positive test • With choices • For mutations • the criteria for subset of mutations to be covered

20. The Treatment • Effective treatment or intervention • evidence of early treatment leading to better outcomes • Evidence based policies covering • individuals should be offered treatment • appropriate treatment to be offered • Clinical management and patient outcomes should be optimised

21. The Screening Programme • Randomised Controlled Trials • effective in reducing mortality or morbidity • evidence from trials for informed decision making that the test accurately measures risk • Acceptable to professionals and the public • Clinically • Socially • Ethically • Benefit outweigh the physical and psychological harm • Opportunity cost • value for money

22. The Screening Programme • Plan for managing and monitoring programme • quality assurance standards • Adequate staffing and facilities • All other options considered • more cost effective intervention? • Evidence-based info allowing informed choice • consequences of testing • investigation and treatment • Anticipate public pressure • widening the eligibility criteria • reducing the screening interval

23. Sensitivity • The proportion of truly diseased patients in the screened population who are identified as diseased by the screening test. • The probability of correctly diagnosing a case • True positive rate

24. Specificity • The proportion of truly non-diseased persons who are so identified by the screening test. • The probability of correctly identifying a non-diseased person with a screening test • True negative rate • This is what a clinician or the patient wants to know

27. Example 1

28. Sensitivity 20/100 0.2 20% Specificity 890/900 0.99 99% Answer

29. Example 2

30. Answer • Sensitivity • 80/100 • 0.2 • 80% • Specificity • 895/900 • 0.99 • 99%

31. Positive Predictive Value • The probability that a person with a positive test is a true positive • Does have the disease • This is what a clinician or the patient wants to know

32. Negative Predictive Value • The probability that a person with a negative test is a true negative • Does not have the disease

33. Positive Predictive Value = a/ (a+b) Negative Predictive Value = d/(c+d)

35. Example 3

36. PPV 20/30 0.67 67% NPV 890/970 0.92 92% Answer

37. Example 4

38. Answer • PPV • 80/85 • 0.94 • 94% • NPV • 895/915 • 0.97 • 97%