EPIDEMIOLOGY AND ITS CONTRIBUTION TO PUBLIC HEALTH (selected slides) Jan E. Zejda Department of Epidemiology, Medical University of Silesia

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







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


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 ?


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.







•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”




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



or


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



or


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 …)




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


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


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


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


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 !) ++


quick to perform +

easy to administer +

inexpensive +

safe +

acceptable sensitivity and specificity ++

Medical institutions ready to take care of positive cases +


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 !) - ←


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




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




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-

Documents

EPIDEMIOLOGY AND ITS CONTRIBUTION TO PUBLIC HEALTH (selected slides) Jan E. Zejda Department of Epidemiology, Medical University of Silesia