prevention and screening mms year 4 public health workshop in o&g
TRANSCRIPT
Prevention and ScreeningPrevention and Screening
MMS Year 4 Public HealthMMS Year 4 Public HealthWorkshop in O&GWorkshop in O&G
Why is prevention better Why is prevention better than cure?than cure?
Hygieia – goddess of Hygieia – goddess of healthhealth
McKeowan (1979) and McKeowan (1979) and Illich (1976)Illich (1976)
• Personal medical care has Personal medical care has contributed relatively little to contributed relatively little to the improvements in mortality the improvements in mortality since 1800ssince 1800s
Medical careMedical care
• Bunker 1995Bunker 1995– Only 2 years of the 23 years Only 2 years of the 23 years
increase in longevity is due to increase in longevity is due to medical intervention between 1900-medical intervention between 1900-19501950
– But since 1950, 3 out of the 7 years But since 1950, 3 out of the 7 years increased life expectancyincreased life expectancy
– Why? Why?
PreventionPrevention
• 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
Why do we do them?Why do we do them?
• IntuitiveIntuitive• CostCost• BenefitBenefit
– Quality of lifeQuality of life– MorbidityMorbidity– MortalityMortality
• So why don’t we do them for So why don’t we do them for everything?everything?
Why don’t we do themWhy don’t we do them
• CostCost• BenefitBenefit
– Evidence basedEvidence based• Early detectionEarly detection• TreatmentTreatment• Ethical (Abdominal Aortic Aneurysm)Ethical (Abdominal Aortic Aneurysm)
• IntuitiveIntuitive– PersuasionPersuasion– Side effects (pertussis)Side effects (pertussis)– Harm (MMR, psychological)Harm (MMR, psychological)
• Risk no longer existsRisk no longer exists
TASK: WHY DO YOU WEAR A SEATBELT?
Prevention ParadoxPrevention Paradox
• Rose 1981– “A preventative measure that brings
large benefits to the community offers little to each participating individual.”
Secondary/Tertiary Prevention
Screening: doing more good Screening: doing more good than harmthan harm
Mission of the National Mission of the National Screening CommitteeScreening Committee
What is screening?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
Limitations of ScreeningLimitations 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
TASK: What screening programmes are NHS funded?
• Rapid list on the whiteboard
Criteria for Screening Criteria for Screening ProgrammesProgrammes
Ideally all criteria should be Ideally all criteria should be met!met!
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
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
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
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
The Screening ProgrammeThe 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
SensitivitySensitivity
• The proportion of truly diseased The proportion of truly diseased patients in the screened patients in the screened population who are identified as population who are identified as diseased by the screening test.diseased by the screening test.– The probability of correctly diagnosing a
case• True positive rate
SpecificitySpecificity
• The proportion of truly non-The proportion of truly non-diseased persons who are so diseased persons who are so identified by the screening test.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
Example 1Example 1
Has Has DiseaseDisease
No No DiseaseDisease
Test Test PositivePositive
2020 1010
Test Test NegativeNegative
8080 890890
AnswerAnswer
• SensitivitySensitivity
• 20/10020/100• 0.20.2
• 20%20%
• SpecificitySpecificity
• 890/900890/900• 0.990.99
• 99%99%
Example 2Example 2
Has Has DiseaseDisease
No No DiseaseDisease
Test Test PositivePositive
8080 55
Test Test NegativeNegative
2020 895895
AnswerAnswer
• SensitivitySensitivity
• 80/10080/100• 0.20.2
• 80%80%
• SpecificitySpecificity
• 895/900895/900• 0.990.99
• 99%99%
Positive Predictive ValuePositive Predictive Value
• The probability that a person The probability that a person with a positive test is a true with a positive test is a true positive positive – Does have the disease
– This is what a clinician or the patient wants to know
Negative Predictive ValueNegative Predictive Value
• The probability that a person The probability that a person with a negative test is a true with a negative test is a true negative negative – Does not have the disease
Positive Predictive Value = a/ (a+b)
Negative Predictive Value = d/(c+d)
Example 3Example 3
Has Has DiseaseDisease
No No DiseaseDisease
Test Test PositivePositive
2020 1010
Test Test NegativeNegative
8080 890890
AnswerAnswer
• PPVPPV
• 20/3020/30• 0.670.67
• 67%67%
• NPVNPV
• 890/970890/970• 0.920.92
• 92%92%
Example 4Example 4
Has Has DiseaseDisease
No No DiseaseDisease
Test Test PositivePositive
8080 55
Test Test NegativeNegative
2020 895895
AnswerAnswer
• PPVPPV
• 80/8580/85• 0.940.94
• 94%94%
• NPVNPV
• 895/915895/915• 0.970.97
• 97%97%