EPIDEMIOLOGY AND ITS CONTRIBUTION TO PUBLIC HEALTH (selected slides) Jan E. Zejda Department of...
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Transcript of EPIDEMIOLOGY AND ITS CONTRIBUTION TO PUBLIC HEALTH (selected slides) Jan E. Zejda Department of...
EPIDEMIOLOGY AND ITS
CONTRIBUTION TO PUBLIC HEALTH
(selected slides)
Jan E. Zejda
Department of Epidemiology, Medical University of Silesia
EPIDEMIOLOGYThe study of the distribution and determinants of health related states or events in specified populations, and the application of this study to control of health problems
APPLICATION TO CONTROLto assess the public health importance of diseases, identify the population at risk, identify the causes of disease, describe the natural history of disease, and
evaluate the prevention and control of disease
THE CONCEPT OF PUBLIC HEALTH (definition*)
Public health is the art and science of protecting and improving the health of a community through an organized and systematic effort that includes education, assurance of the provision of health services and protection of the public from exposures that will cause harm
* - one out of many
ESSENTIAL FUNCTIONS OF MODERN PUBLIC HEALTH*
• Monitoring of health status and its determinants
• Prevention and control of diseases, injuries and disability
• Health promotion
• Environmental protection
* Bettcher D.W. et al..: Essential public health functions: results of the international Delphi Study. World Health Statistics Quarterly 1998;51:44-54
ESSENTIAL FUNCTIONS OF MODERN PUBLIC HEALTH*
• Monitoring of health status and its determinants
• Prevention and control of diseases, injuries and disability
• Health promotion
• Environmental protection
* Bettcher D.W. et al..: Essential public health functions: results of the international Delphi Study. World Health Statistics Quarterly 1998;51:44-54
EPIDEMIOLOGICAL STUDY
Measurement of:
1. Health events2. Exposures pertinent to health events3. Association of 1 with 2 to identify
- risk factors- protective factors- …
HEALTH EFFECT
„ANY CHANGE IN HEALTH STATUS OR BODY FUNCTION THAT CAN BE SHOWN TO BE DUE TO EXPOSURE DEFINED IN A BROAD SENSE (INCLUDES DISEASES)”
Local health effect (cough after inhalation of irritating agent)
Systemic health effect (asthma after prolonged inhalation of allergen)
Acute or chronic health effects (CO: dizzines or brain damage)
Baker D, Kjellstrom T., Calderon R., Pastides H.: Environmental Epidemiology, WHO 1999
MEASUREMENT OF HEALTH EFFECTS
CRUDE COUNTS OF DEATHS(deaths due to asthma)
↓CLINICALLY RECOGNIZED CASES OF DISEASE
(diagnosis of asthma)↓
OVERT SYMPTOMS(attacks of asthmatic dyspnea)
↓SPECIFIC PATHOLOGICAL CONDITIONS
(bronchial hypereactivity)↓
BIOCHEMICAL, PHYSIOLOGICAL CHANGES(increased IgE level)
DEFINITION OF HEALTH EFFECT
MORTALITY – standard procedurę (ICD)MORBIDITY – diagnostic criteria (agreed upon)
but:
my diagnostic decision concerning Alzheimer disease
is not necesserily
your diagnostic decision concerning Alzheimer disease
(experience of physician, diagnostic preferences, duration of differential diagnosis, access to medical technology, communication with a patient …)
CASE DEFINITION - II
1. Do you cough ? Yes/No
2. Do you usually bring up phlegm from your chest ? Yes/No
3. Do you usually bring up phlegm like this as much as twice a day, 4 or more days out of the week ? Yes/No
4. Do you bring up phlegm like this at most days for 3 consecutive months or more during the year ? Yes/No
5. For how many years have you had trouble with phlegm ? Years: 2
‘Yes’ to questions 1-3 and ‘Yes’ to question 4 and ‘2’ to question 5 =
diagnosis of chronic bronchitis
MEASUREMENT OF EXPOSURE AND HEALTH EFFECTS
BOTH COMPONENTS MUST BE MEASURED IN THE BEST POSSIBLE WAY
if not
WRONG CONCLUSION
Allergic Rhinitis
Recurrent Persitent(risk factors)
Pollen (tree, grass …) Dust, molds
ENVIRONMENTAL EPIDEMIOLOGY PARADIGM
occurrence of disease
vis-a-vis
environmental exposure
Occurrence of disease is expressed as
a) incidence
b) prevalence
INCIDENCE MEASURES
INCIDENCE RATE
number of cases
person-time
INCIDENCE PROPORTION
number of cases
number of persons
INCIDENCE ODDS
number of cases
number of noncases
OUR EXAMPLE
3/64 = 0.05 year -1
3/10 = 0.3 = 30%
3/(10-3) = 3:7
HEALTH EFFECT vs EXPOSURE
BASIC MEASURES OF ASSOCIATION AND IMPACT(RELEVANT TO EXPOSURES)
Relative Risk (association)
Attributable Risk (impact)
When you can measure what you are speaking about, and express it in numbers, you know something about it. But when you cannot
your knowledge is of a meager and unsatisfactory kind (Lord Kelvin)
THREE PRINCIPAL MEASURES OF EFFECT
(referred to using the generic term RELATIVE RISK)
RELATIVE RISK
Rate Ratio Risk Ratio Odds Ratio
based on
Incidence Rate Incidence Proportion Incidence Odds
RISK RATIO (RR)
- ASTHMA AND ENVIRONMENTAL TOBACCO SMOKING -
0
1
2
3
4
5
Physician Diagnosed Asthma
Cu
mu
lati
ve In
cid
ence
(%
)
No parent smokes (ETS-) Either parent smokes (ETS+)
Two different environments (two different exposures)&
Two different incidences (two different absolute risks: 4.2/100 and 3.6/100)
Risk Ratio: 4.2% / 3.6% = 1.16
(strength of association ‘asthma-ETS’ measured by RR)
PREVALENCE
Prevalence of a disease in a given population is based on the number of existing cases at a specific point in time, and estimated the prevalence proportion
number of existing cases
persons
PREVALENCE RATIO
The ratio of the prevalence proportion for the exposed to the prevalence proportion for the unexposed
(poor measure of risk)
ATTRIBUTABLE RISK
proportion of disease among the exposed population that can be attributed to the exposure
Attributable Fraction in the Population (AFP)
answers the question: „what fraction of the disease burden in a population would be averted if the effect of the exposure was to be removed ?”
Attributable Fraction in Exposed Cases (AFE)
answers the question: „what proportion of the disease incidence among the exposed population can be attributed to the exposure ?”
or „what is a relative magnitude of the environmental risk factor ?”
FROM EXPOSURE TO HEALTH EFFECT
Source emissions↓
Dispersion in environment↓
Human contact: exposure↓
Dose to the body↓
bioavailability↓
Absorbed (internal) dose↓
elimination, accumulation, transformation↓
Biologically effective dose↓
Early expression of disease↓
Health effect
Environmental Epidemiology, IPCS, EHC 1983
SOURCE OF EXPOSURE
Environmental Epidemiology, IPCS, EHC 1983
DOMESTIC („MICRO”) ENVIRONMENT OCCUPATIONAL
ENVIRONMENT
LOCAL („COMMUNITY”) ENVIRONMENT
REGIONAL ENVIRONMENT
?
RELIABLE ASSESSMENT OF EXPOSURE
Same agent, various sources
Various agents, same source
Various agents, various sources
PRINCIPAL QUESTIONS
- what agents need to be measured ?- how long and how often should samples be taken ?- where should samples be drawn from, or instruments located ?- what quality of data is needed ?- which instruments or analytical techniques should be used ?
Environmental Epidemiology, IPCS, EHC 1983
Proof of the association
THE PRESENCE AND MAGNITUDE OF A RISK FACTOR
Proof of the impact
ANALYTICAL EPIDEMIOLOGY - OUTCOME
RISK FACTOR
An aspect of personal behavior or life-style,
an environmental exposure,
or an inborn or inherited characteristic,
that, on the basis of epidemiologic evidence,
is known to be associated with health-related conditions(s)
considered important to prevent
Last J.M.: A Dictionary of Epidemiology.
ESSENTIAL FUNCTIONS OF MODERN PUBLIC HEALTH*
• Monitoring of health status and its determinants
• Prevention and control of diseases, injuries and disability
• Health promotion
• Environmental protection
* Bettcher D.W. et al..: Essential public health functions: results of the international Delphi Study. World Health Statistics Quarterly 1998;51:44-54
•Primary prevention
•Secondary prevention
•Teritary prevention
TOPICS
NATURAL HISTORY OF COLORECTAL CANCER
(simplification)
↑ ↑ ↑HEALTH FECAL BLOOD SYMPTOMS (WEIGHT
(INVISIBLE) LOSS, ABDOMINAL PAIN,OBSTRUCTION …)
NO
DISEASE
ASYPTOMATIC DISEASE SYMPTOMATIC DISEASE
„PRECLINICAL PHASE” „CLINICAL PHASE”
PREVENTION IN THE COURSE OF DISEASE
(natural history)
↑ ↑ ↑PRIMARY SECONDARY TERTIARY
PREVENTION
NO
DISEASE
ASYPTOMATIC DISEASE SYMPTOMATIC DISEASE
„PRECLINICAL PHASE” „CLINICAL PHASE”
•Primary prevention
•Secondary prevention
•Teritary prevention
TOPICS
PRIMARY PREVENTION
Primary prevention is the evidence-based action that seeks to avert disease before it develops. In practive and on individual level, its purpose is to limit the incidence of disease by:
(a) controlling exposure to risk factors (or known causes)
or
(b) increasing individual resistance against exposure effects.
PRIMARY PREVENTION
Primary prevention is the evidence-based action that seeks to avert disease before it develops. In practive and on individual level, its purpose is to limit the incidence of disease by:
(a) controlling exposure to risk factors (or known causes)
or
(b) increasing individual resistance against exposure effects.
Source: WHO: Preventing chronic disease: a vital investment
PREVENTION: FOCUS ON CHRONIC DISEASES
POTENTIAL FOR PRIMARY PREVENTION
NEXT 2 SLIDES
↓ ↓ ↓ ↓ ↓
POTENTIAL FOR PRIMARY PREVENTION (life-style
contribution*)
* - WHO Europe. Gaining health - The European Strategy for the Prevention and Control of Noncommunicable Diseases. WHO, Copenhagen, 2006. Available at: www.euro.who.int/InformationSources/Publications/Catalogue/20061003_1.
Almost 60% of the disease burden in Europe is accounted for by 7 risk factors:
High blood pressure 12.8%
Tobacco use 12.3%
Alcohol consumption 10.1%
High blood cholesterol 8.7%
Overweight 7.8%
Poor diet 4.4%
Physical inactivity 3.5%
PRIMARY PREVENTION
Primary prevention is the evidence-based action that seeks to avert disease before it develops. In practive and on individual level, its purpose is to limit the incidence of disease by:
(a) controlling exposure to risk factors (or known causes)
or
(b) increasing individual resistance against exposure effects.
BETTER RESISTANCE
a) non-specific tools: good diet, good sleep, control of stress, recreation, etc.
b) specific tools: immunization (e.i.vaccine against influenza)
Pioneer of smallpox vaccine, the world first vaccine (1796)
Hypotheses: Pus in the blisters that milkmaids received from cowpox protects them from smallpox. Infection with cawpox gives immunity to smallpox.
In 1979 the WHO declared smallpox an eradicated disease
Dr Edward Jenner
1749 - 1823
PROVIDERS OF PRIMARY PREVENTION
Health care institution (counseling, vaccination)
Public health institution (education re: lifestyle)
Non-health care institutions (regulations, environmental protection …)
•Primary prevention
•Secondary prevention
•Teritary prevention
TOPICS
SECONDARY PREVENTION
Secondary prevention refers to detection early disease when it is asymptomatic and when treatment can stop it from progressing. It can be performed
(a) in clinical setting or
(b) on a population level
Examples:
a) Blood pressure measurement in any patient
b) Mammography offered to all adult women in a town
(screen)
REALITY !!!
FALSE FALSE NEGATIVE POSITIVECASES CASES
SCREENING
MEMENTO
SCREENING TEST RESULT
≠
DIAGNOSTIC TEST RESULT
↓
FURTHER CLINICAL INVESTIGATION OF ABNORMAL RESULTS AND TREATMENT
THE PROCESS OF SCREENING* Perform negative result Record Inform the person
screening test the result screened
positive result
Perform negative result Record Inform diagnostic test the result the patient
positive result
Start negative response Revise treatment treatment
positive response
Continue treatment *- Jekel J.F. et al..: Epidemiology …W.B. Soundres Comp. 2001
TRUE POSITIVE & TRUE NEGATIVEFALSE POSITIVE & FALSE NEGATIVE
CASES
False True False True Negative Positive Positive Negative
Disease + Disease -
Test + a b
Test - c d
TRUE POSITIVE & TRUE NEGATIVEFALSE POSITIVE & FALSE NEGATIVE
CASES
Sensitivity a / (a + c)Specificity d / (b + d)Positive predictive value a / (a + b)Negative predictive value c / (c + d)
Disease + Disease -
Test + a b
Test - c d
Sensitivity is calculated as number of ‘true positives’ among all sick subjectsSpecificity is calcualted as number of ‘true negatives’ among healthy subjects
TRUE POSITIVE & TRUE NEGATIVEFALSE POSITIVE & FALSE NEGATIVE
CASES
Sensitivity = 60% Specificity = 80%Positive predictive value a / (a + b)Negative predictive value c / (c + d)
Disease + Disease -
Test + 60 20
Test - 40 80
TRUE POSITIVE & TRUE NEGATIVEFALSE POSITIVE & FALSE NEGATIVE
CASES
Predictive ValuePositive = 75% Negative = 50%
Disease + Disease -
Test + 60 20
Test - 40 80
SURVIVAL IN LUNG CANCER LEAD-TIME BIAS
Onset Death Survival since Dx
Person A Dx 4 years (unscreened)
Person B Dx 9 years (screened)
Age (years): 50 – 55 – 60 – 65 – 70 – 75
both A and B die at the same age – benefit of early Dx due to screening ?
SURVIVAL IN LUNG CANCER LEAD-TIME BIAS
Onset Death Survival since Dx
Person A Dx 4 years (unscreened)
5 years
Person B Dx 9 years (screened)
Age (years): 50 – 55 – 60 – 65 – 70 – 75
OVERDIAGNOSIS IN CANCER SCREENING* Size
Size at which cancer causes death →
Fast Slow
Size at which cancer causes symptoms →
Very slow
Nonprogressive
Abnormal cell →
Time Death from . other causes
* - Welsh HG, 2004
← SCREENING RESULTS
CRITERIA FOR INCLUSION OF A MEDICAL CONDITION IN
SCREENINGGOOD CANDIDATE, IF:
Important health problem (burden of suffering)
Known natural history
Presence of „preclinical phase”
Effective early treatment (lead-time bias !)
Availability of a screening test:
quick to perform
easy to administer acceptable to participants
safe
inexpensive
acceptable sensitivity and specificity
Medical institutions ready to take care of positive cases
CRITERIA FOR INCLUSION OF A MEDICAL CONDITION IN
SCREENING MAMMOGRAM
(BREAST CA)
Important health problem (burden of suffering) ++
Known natural history +
Presence of a long „preclinical phase” +
Effective early treatment (lead-time bias !) ++
Availability of a screening test:
quick to perform +
easy to administer +
inexpensive +
safe +
acceptable sensitivity and specificity ++
Medical institutions ready to take care of positive cases +
CRITERIA FOR INCLUSION OF A MEDICAL CONDITION IN
SCREENING CHEST X-RAY (LUNG
CA)
Important health problem (burden of suffering) ++
Known natural history +
Presence of a long „preclinical phase” - ←
Effective early treatment (lead-time bias !) - ←
Availability of a screening test:
quick to perform +
easy to administer +
inexpensive +
safe +
acceptable sensitivity and specificity - ←
Medical institutions ready to take care of positive cases +
World Health Organization, 2006
on behalf of the European Observatory
2006http://www.euro.who.int
•Primary prevention
•Secondary prevention
•Teritary prevention
TOPICS
TERTIARY PREVENTION
Tertiary prevention refers to clinical activities aiming at prevention of further deterioration or reduction of complications of the disease
Examples:
a) good control of glucose level (diabetes)
b) regular kidney function examination (diabetes)
c) regular ophtalmologic examination (diabetes)
HEALTH CARE SYSTEM (fees etc.)
THERAPEUTIC STANDARDS
PATIENTS COMPLIANCE
…
•Primordial prevention
•Primary prevention
•Secondary prevention
•Teritary prevention
TOPICS
PRIOMORDIAL PREVENTION
The aim of primordial prevention is to avoid the emergence and establishment of the social, economc, and cultural patterns of living that are known to contribute to an elevated risk of disease
Examples:
environmental protection (ambient air pollution …)
nutrition quality control (agriculture, food processing …)
control of unhealthy habits (antitobacco legislation …)
control of risky behaviors (speed-limit endorsement …)
MEDICAL
SCREENING
SOCIETY
-Est.:2002-