the definitions used in the epidemiology of infectious diseases

19.04.2319.04.23 Dr.Ozlem TanrioverDr.Ozlem Tanriover 11

The definitions used in the epidemiology of infectious diseases


Learning ObjectivesLearning Objectives

At the end of this lecture you (will) be able to:At the end of this lecture you (will) be able to:

ExplainExplain common definitions and basic concepts used in common definitions and basic concepts used in

epidemiology.epidemiology.

ApplyApply these definitions in different situations. these definitions in different situations.

Identify the epidemiological approach to study health Identify the epidemiological approach to study health

problemsproblems

Describe natural history and spectrum of infectious Describe natural history and spectrum of infectious

diseases and their diseases and their implicationsimplications for public health. for public health.


EpidemiologyEpidemiology is the study of is the study of

distributiondistribution and and

determinantsdeterminants of of

health-related events in specified populations and health-related events in specified populations and

the application of this study to the application of this study to preventprevent and and controlcontrol

of health problems.of health problems.

DefinitionsDefinitions


PersonPerson………. ……….

PlacePlace………….. …………..

TimeTime…………… ……………

Describe disease Describe disease frequencyfrequency– Incidence Rate……Incidence Rate……– Prevalence Rate……Prevalence Rate……

DistributionDistribution


The Epidemiologic TriadThe Epidemiologic Triad

HOST

AGENT ENVIRONMENT

Disease DeterminantsDisease Determinants


The epidemiological approach:The epidemiological approach:

An Epidemiologist’ approach to study health-An Epidemiologist’ approach to study health-

related events involves answering five related events involves answering five

questions:questions:

What?What?

Who?Who?

Where?Where?

When?When?

Why? Why?


Infection:Infection:

It is the entry, development and multiplication of an It is the entry, development and multiplication of an

infectious agent in the body of man or animal.infectious agent in the body of man or animal.

Outcome of infection varies.Outcome of infection varies.

Infectious disease:Infectious disease:

A clinically manifest disease of man or animal A clinically manifest disease of man or animal

resulting from infection.resulting from infection.

In-apparent infection:In-apparent infection:

The infection does not become manifest at any The infection does not become manifest at any

stage.stage.


An illness due to a specific infectious agent or its An illness due to a specific infectious agent or its

toxic products that arises through toxic products that arises through transmissiontransmission

of that agent from reservoir to susceptible host.of that agent from reservoir to susceptible host.

Communicable Disease (typhoid, influenza,..Communicable Disease (typhoid, influenza,..

Non- Communicable Disease (DM, cancer,..Non- Communicable Disease (DM, cancer,..

Communicable DiseaseCommunicable Disease


Contamination:Contamination:

The presence of living infectious agents on the

exterior surface of the body or on the clothes or

articles of the person or on any inanimate object in

the environment including water and food.

Contagious diseasesContagious diseases

A disease that is capable of being transmitted from

one person to another by contact or close

proximity. e.g. scabies, trachoma and leprosy.


EndemicEndemic

The The constantconstant presence of a disease or infectious presence of a disease or infectious agent within a given geographic area or population agent within a given geographic area or population group; group;

It may also refer to the usual prevalence of a given It may also refer to the usual prevalence of a given disease within such area or group.disease within such area or group.

EpidemicEpidemicThe occurrence of The occurrence of moremore cases of a disease than cases of a disease than

expected in a given area or among a specific expected in a given area or among a specific group of people over a particular period of time.group of people over a particular period of time.


PandemicPandemic

An epidemic occurring over a very wide area (several An epidemic occurring over a very wide area (several

countries or continents) at the same time and usually countries or continents) at the same time and usually

affecting a large proportion of the population.affecting a large proportion of the population.

e.g. e.g. Influenza,Influenza, choleracholera

OutbreakOutbreak

A more or less localized epidemic affecting large number of A more or less localized epidemic affecting large number of

a group, in the communitya group, in the community

e.g. outbreak of e.g. outbreak of food poisoningfood poisoning


SporadicSporadic

Cases occur irregularly, haphazardly from time to Cases occur irregularly, haphazardly from time to

time and generally infrequently. time and generally infrequently.

Cases are few and separated widely in space and Cases are few and separated widely in space and

time showing no connection to each other.time showing no connection to each other.


Nosocomial infection Nosocomial infection (Hospital acquired infection):(Hospital acquired infection):

It is an infection occurring in a patient while in a It is an infection occurring in a patient while in a

hospital or other health care facilityhospital or other health care facility

It should not be present or incubating at the time of It should not be present or incubating at the time of

admission. admission.


Elimination of diseaseElimination of disease

Disease incidence is reduced to a minimal level at Disease incidence is reduced to a minimal level at

which the disease is no longer considered a public which the disease is no longer considered a public

health problem, while infection may still occurhealth problem, while infection may still occur

Example:Example:

The aim of elimination of Neonatal tetanus is The aim of elimination of Neonatal tetanus is

reduction of its incidence to less thanreduction of its incidence to less than

one case/1000 live births.one case/1000 live births.


Eradication:Eradication:

It means worldwide disappearance of a disease It means worldwide disappearance of a disease

(permanent reduction to zero level) with complete (permanent reduction to zero level) with complete

destruction of the agent. destruction of the agent.

The organism can be present only in laboratories The organism can be present only in laboratories

and no need for interventions. and no need for interventions.

e.g. smallpox eradication from the world since 1979. e.g. smallpox eradication from the world since 1979.


Natural historyNatural history

NaturalNatural historyhistory of disease refers to the progress of a of disease refers to the progress of a

disease process in an disease process in an individualindividual over time, in the over time, in the

absenceabsence of intervention. of intervention.

The natural historyThe natural history of a disease of a disease describes the course describes the course

of the disease in an individual starting from the moment of the disease in an individual starting from the moment

of of exposureexposure to the causal agents till one of the possible to the causal agents till one of the possible

outcomesoutcomes occurs. occurs.


Spectrum of diseaseSpectrum of disease

The idea that an exposure can lead to varying signs, The idea that an exposure can lead to varying signs,

symptoms and severity of the same disease in the symptoms and severity of the same disease in the

populationpopulation is the spectrum of disease. is the spectrum of disease.

Why do we have varying degrees of severity? Prognosis?Why do we have varying degrees of severity? Prognosis?

The outcome will depend on the interactions of host, agent The outcome will depend on the interactions of host, agent

and environmental factors.and environmental factors.


The Epidemiologic TriadThe Epidemiologic Triad

HOST

AGENT ENVIRONMENT

Disease DeterminantsDisease Determinants


•Agent factorsAgent factors refer to an infectious microorganism— refer to an infectious microorganism—

virus, bacterium, parasite, or other microbes. virus, bacterium, parasite, or other microbes.

•They are They are necessarynecessary but but notnot always always sufficientsufficient

alone to cause disease.alone to cause disease.

•Host factorsHost factors are are intrinsicintrinsic factors that influence an factors that influence an

individual’s exposure, susceptibility, or response to a individual’s exposure, susceptibility, or response to a

causative agent.( age, nutrition, race,…causative agent.( age, nutrition, race,…

•Environmental factorsEnvironmental factors are are extrinsicextrinsic factors which factors which

affect the agent and the opportunity for exposure.affect the agent and the opportunity for exposure.


The pyramid and iceberg of diseaseThe pyramid and iceberg of disease

11 Diseased, diagnosed & controlledDiseased, diagnosed & controlled

22 Diagnosed, uncontrolledDiagnosed, uncontrolled

33 Undiagnosed or wronglyUndiagnosed or wronglydiagnosed diseasediagnosed disease

44 Risk factors for diseaseRisk factors for disease

55 Free of risk factors Free of risk factors

Diagnosed Diagnosed diseasedisease

Undiagnosed orUndiagnosed orwrongly diagnosed diseasewrongly diagnosed disease


Cases of illness correctly diagnosed by clinicians in the Cases of illness correctly diagnosed by clinicians in the

community often represent only the “community often represent only the “tip of the icebergtip of the iceberg.”.”

Many additional cases may be too early to diagnose or may Many additional cases may be too early to diagnose or may

remain asymptomatic.remain asymptomatic.

Examples: Tuberculosis, meningitis, polio, hepatitis A, AIDS.Examples: Tuberculosis, meningitis, polio, hepatitis A, AIDS.

The riskThe risk is that persons with in-apparent or undiagnosed is that persons with in-apparent or undiagnosed

infections may be infections may be able to transmitable to transmit infection to others. infection to others.

Iceberg phenomenon Iceberg phenomenon


The pyramid and iceberg of diseaseThe pyramid and iceberg of disease

11 Diseased, diagnosed & controlledDiseased, diagnosed & controlled

22 Diagnosed, uncontrolledDiagnosed, uncontrolled

33 Undiagnosed or wronglyUndiagnosed or wronglydiagnosed diseasediagnosed disease

44 Risk factors for diseaseRisk factors for disease

55 Free of risk factors Free of risk factors

Diagnosed Diagnosed diseasedisease

Undiagnosed orUndiagnosed orwrongly diagnosed diseasewrongly diagnosed disease


Applications of the concepts of natural history Applications of the concepts of natural history and spectrum of diseaseand spectrum of disease

Persons with in-apparent or undiagnosed Persons with in-apparent or undiagnosed

infections can transmit infections to others.infections can transmit infections to others.

Control measures must be directed toward all Control measures must be directed toward all

infections capable of being transmitted to others;infections capable of being transmitted to others;

– both clinically apparent cases and both clinically apparent cases and

– those with in-apparent or undiagnosed infections.those with in-apparent or undiagnosed infections.

INFECTION PREVENTIONINFECTION PREVENTION

AT HEALTH CARE AT HEALTH CARE FACILITIESFACILITIES

PURPOSE OF INFECTION PREVENTION:PURPOSE OF INFECTION PREVENTION:

* * to minimize infections due to to minimize infections due to microorganisms causing serious wound microorganisms causing serious wound infections,abscesses, PID, gangrene and infections,abscesses, PID, gangrene and tetanus etc.tetanus etc.to prevent transmission of serious life-to prevent transmission of serious life-threatening diseases such as Hep.B and threatening diseases such as Hep.B and AIDSAIDSto prevent the spread of infections at health to prevent the spread of infections at health care facilities can be possible only if we can care facilities can be possible only if we can break the inf. chain somewherebreak the inf. chain somewhere

Infection Transmission at Health Infection Transmission at Health Care Facilities Care Facilities

1.Transmission from patient to health worker1.Transmission from patient to health worker:: preventable accidents (puncture wounds), preventable accidents (puncture wounds), splash of body fluids into the eye or on to broken splash of body fluids into the eye or on to broken skin (scratch, ache, dermatitis)skin (scratch, ache, dermatitis)

Infection PreventionInfection Prevention

2.2. Transmission from health worker to Transmission from health worker to patientpatient::touching the patient with dirty hands, sneezing, touching the patient with dirty hands, sneezing, coughing during cleaning the wound.coughing during cleaning the wound.3.3. Transmission from patient to patient : Transmission from patient to patient :occurs through use of contaminated equipment, occurs through use of contaminated equipment, instruments, environment, surfaces, air in the instruments, environment, surfaces, air in the operation room.operation room.

Health care workers should know Health care workers should know these and teach these!these and teach these!

3.3. Transmission from patient to patient : Transmission from patient to patient :

occurs through use of contaminated equipment, occurs through use of contaminated equipment, instruments, environment, surfaces, air in the instruments, environment, surfaces, air in the operation room.operation room.

PROTECTIVE BARRIERSPROTECTIVE BARRIERSat health care facilitiesat health care facilities

Placing a physical, mechanical or chemical barrier Placing a physical, mechanical or chemical barrier between inf.agent and an individual will break the between inf.agent and an individual will break the inf. chain:inf. chain:-handwashing-handwashing-wearing gloves, either for surgery or to protect -wearing gloves, either for surgery or to protect clinic staff when handling contaminated waste clinic staff when handling contaminated waste materials and instrumentsmaterials and instruments- using antiseptic solutions for cleaning wounds or - using antiseptic solutions for cleaning wounds or preparing the skin prior to surgery, decontaminating, preparing the skin prior to surgery, decontaminating, cleaning, sterilizing or high –level disinfecting cleaning, sterilizing or high –level disinfecting solutions.solutions.-wearing face-masks, eye glasses, gowns, caps etc.-wearing face-masks, eye glasses, gowns, caps etc.

ASEPSISASEPSIS

Is a general term used to describe the Is a general term used to describe the combination of efforts made to prevent entry of combination of efforts made to prevent entry of microorganisms into any area of the body where microorganisms into any area of the body where they are likely to cause infection (Living and they are likely to cause infection (Living and nonliving objects)nonliving objects)

ANTISEPSISANTISEPSIS

Prevention of infection by killing or inhibiting the Prevention of infection by killing or inhibiting the growth of microorganisms using a chemical growth of microorganisms using a chemical agent (antiseptic) on living surfaces agent (antiseptic) on living surfaces

Iodophors, 70% alcoholIodophors, 70% alcohol

DECONTAMINATIONDECONTAMINATION

The process that makes nonliving objects safer The process that makes nonliving objects safer to be handled by staff before cleaningto be handled by staff before cleaning

Soak in 0.5 % chlorine solution for ten minutesSoak in 0.5 % chlorine solution for ten minutes

1 (household bleach) + 9 (water) = prevents 1 (household bleach) + 9 (water) = prevents from Hep B and AIDS from Hep B and AIDS

HIGH LEVEL DISINFECTIONHIGH LEVEL DISINFECTION

The process that eliminates all microorganisms The process that eliminates all microorganisms except for some bacterial endosporesexcept for some bacterial endospores

Boiling for 10 minutesBoiling for 10 minutes

oror

Soak in 0.5 % chlorine solutionSoak in 0.5 % chlorine solution

for 20 minutesfor 20 minutes

STERILIZATIONSTERILIZATION

The process that eliminates all microorganismsThe process that eliminates all microorganisms

Heat sterilization (autoclaving or dry heating)Heat sterilization (autoclaving or dry heating)

Chemical sterilization (glutaraldehide or Chemical sterilization (glutaraldehide or formaldehide)formaldehide)

Processes for reuse of contaminated Processes for reuse of contaminated instrumentsinstruments

DecontaminationWash and rinse

Sterilizationpreferred

HLDacceptable

Autoclaving Or dry heat

Boiling or chemical

CORRECTCORRECT

WRONG WRONG

MEDICAL waste disposal MEDICAL waste disposal

* * allow persons to accidentally step and allow persons to accidentally step and injureinjure

* * are a fire hazard are a fire hazard

* * produce bad odors produce bad odors

attract insectsattract insects

NONCONTAMINATED WASTE:NONCONTAMINATED WASTE:* * poses no infectious risk to persons who poses no infectious risk to persons who handle themhandle themCONTAMINATED WASTE:CONTAMINATED WASTE:* * may carry microorganisms which are may carry microorganisms which are potentially infectious to any body who potentially infectious to any body who contact or handle the waste and to the contact or handle the waste and to the community if not disposed properlycommunity if not disposed properly* * blood,pus,urine,stool and other body fluids blood,pus,urine,stool and other body fluids as well as items which contact them such as as well as items which contact them such as used dressingsused dressings

* * wastes from operating rooms and wastes from operating rooms and laboratories should be considered laboratories should be considered contaminatedcontaminated* * items which are capable of inflicting injury items which are capable of inflicting injury (e.g. used needles, scalpel blades) and are (e.g. used needles, scalpel blades) and are capable of spreading blood-borne diseases capable of spreading blood-borne diseases such as hepatitis-B and AIDSsuch as hepatitis-B and AIDS* * Proper handling of waste items minimizes Proper handling of waste items minimizes the spread of infection to clinic personnel the spread of infection to clinic personnel and to the local communityand to the local community

* * Where available noncontaminated wastes Where available noncontaminated wastes should be transported to disposal sites in should be transported to disposal sites in covered containers (covered containers (blueblue bags or bags or green green bagsbags))* * Contaminated wastes in Contaminated wastes in redred bags bags with the with the amblem of clinical waste in noncorrosive amblem of clinical waste in noncorrosive washable covered containerswashable covered containers* * Persons handling wastes should wear Persons handling wastes should wear heavy gloves,wastes should be buried or heavy gloves,wastes should be buried or incinerated(burnt)incinerated(burnt)

•

Reusable glass wastes should be Reusable glass wastes should be first decontaminated in chlorine first decontaminated in chlorine

solution then transported in solution then transported in black black bagsbags

* * Inside the health facility, the waste bags Inside the health facility, the waste bags should be transported in rust proof small should be transported in rust proof small cars, to transitory storage cars, to transitory storage containers ,rooms,small buildings etc.containers ,rooms,small buildings etc.

* * Waste must not wait in these transitory Waste must not wait in these transitory storage containers longer than 48 hoursstorage containers longer than 48 hours

* * Transitory storage containers have two Transitory storage containers have two departments (one for red badepartments (one for red baggs, the other for s, the other for the blue,green and black bags)the blue,green and black bags)

INCINERATION (BURNING):INCINERATION (BURNING):

* * Provides high temperatures which destroy Provides high temperatures which destroy the item as well as the item as well as microorganisms ;therefore it is the best microorganisms ;therefore it is the best method for disposal of contaminated wastesmethod for disposal of contaminated wastes

* Incineration also reduces the bulk size of * Incineration also reduces the bulk size of wastes to be buried and ensures that the wastes to be buried and ensures that the items are not reused dangerouslyitems are not reused dangerously

IF INCINERATION ISIF INCINERATION IS NOT POSSIBLE ,NOT POSSIBLE , ALL CONTAMINATED ALL CONTAMINATED WASTES MUST BE WASTES MUST BE BURIED TO PREVENTBURIED TO PREVENT SCATTERING OF THESCATTERING OF THE WASTESWASTES

Disease Frequency Disease Frequency 4949

Mortality and morbidity indices used in the epidemiology of

infectious diseases


Population Population

Group of people with a common characteristic like age, race, sex


Two types of populations, based on whether Two types of populations, based on whether membership is permanent or transientmembership is permanent or transient::

Fixed population: membership is permanent and defined by Fixed population: membership is permanent and defined by an eventan event

Ex. Atomic bomb survivorsEx. Atomic bomb survivors

Dynamic population: membership is transient and defined by Dynamic population: membership is transient and defined by being in or out of a "state.”being in or out of a "state.”

Ex. Residents of the City of BostonEx. Residents of the City of Boston


Want to quantify disease occurrence in a Want to quantify disease occurrence in a populationpopulation

Measures of disease frequency should take into Measures of disease frequency should take into account:account:

Number of individuals affected with the diseaseNumber of individuals affected with the disease

Size of source populationSize of source population

Length of time the population was followed Length of time the population was followed

Disease FrequencyDisease Frequency


Ex. Hypothetical Frequency of AIDS in Two CitiesEx. Hypothetical Frequency of AIDS in Two Cities

# New Cases# New Cases TTime Periodime Period PPopulationopulationCity ACity A 58 58 1995 1995 25,000 25,000City BCity B 35 35 1995-96 1995-96 7,000 7,000 Annual "rate" of AIDSAnnual "rate" of AIDSCity A = 58/25,000/1yr = 232/100,000/1 yrCity A = 58/25,000/1yr = 232/100,000/1 yrCity B= 35/7,000/2yrs = 17.5/7000/1 yr = 250/100,000/1 yrCity B= 35/7,000/2yrs = 17.5/7000/1 yr = 250/100,000/1 yr

Make it easy to compare rates by using same population unit (say, per Make it easy to compare rates by using same population unit (say, per 100,000 people) and time period (say, 1 year)100,000 people) and time period (say, 1 year)


P = Number of existing cases of disease / P = Number of existing cases of disease / Number in total population (at a point or during a Number in total population (at a point or during a period of time)period of time)

Ex. City A has 7000 people with Ex. City A has 7000 people with Hepatitis BHepatitis B on on Jan 1Jan 1stst, 1999 , 1999

Population of City A = 70,000 Population of City A = 70,000 Prevalence of arthritis on Jan 1Prevalence of arthritis on Jan 1stst = .10 or 10% = .10 or 10%

PrevalencePrevalence(P) Quantifies number of existing cases of disease in a population at a point or during a period of time


IncidenceIncidence - Quantifies number of - Quantifies number of newnew cases of cases of disease that develop in a population at risk disease that develop in a population at risk during a specified time periodduring a specified time period

Three key conceptsThree key concepts::New disease events, or for diseases that can New disease events, or for diseases that can

occur more than once, usually first occurrence occur more than once, usually first occurrence of diseaseof disease

Population at risk (candidate population) Population at risk (candidate population) Time must pass for a person to move from Time must pass for a person to move from

health to diseasehealth to disease


Cumulative IncidenceCumulative Incidence (Abbreviated CI) (Abbreviated CI)

Incidence RateIncidence Rate (Abbreviated IR) (Abbreviated IR)

Two Types of Incidence MeasuresTwo Types of Incidence Measures


Cumulative incidence = Cumulative incidence =

Number of new cases of disease Number of new cases of disease

Number in candidate population over a specified Number in candidate population over a specified period of timeperiod of time

Two Types of Incidence Measures Two Types of Incidence Measures

(Continued)(Continued)


Cumulative incidenceCumulative incidence estimates the probability estimates the probability or risk that a person will develop disease or risk that a person will develop disease DURING A SPECIFIED TIMEDURING A SPECIFIED TIME. .

Note that the candidate population is comprised of Note that the candidate population is comprised of people who are “at risk” of getting the diseasepeople who are “at risk” of getting the disease

Used mainly for fixed populations because its Used mainly for fixed populations because its assumes that everyone is followed for the entire assumes that everyone is followed for the entire time periodtime period

Two Types of Incidence Measures Two Types of Incidence Measures

(Continued)(Continued)


Example: Cumulative incidence of Example: Cumulative incidence of an an infection infection during first year of lifeduring first year of life

PopulationPopulation 1,000 livebirths1,000 livebirths

Cases of Cases of infection infection 10 10

Cumulative IncidenceCumulative Incidence 10/1,000 10/1,000 or or 1% over 1% over one yearone year

Note that all livebirths are ‘at risk’Note that all livebirths are ‘at risk’


Cumulative incidence calculation assumes that you have Cumulative incidence calculation assumes that you have followed the entire population for the entire follow-up followed the entire population for the entire follow-up period. For example, it assumes you have followed all of period. For example, it assumes you have followed all of the livebirths for one year or until the livebirths for one year or until infection infection occurred.occurred.

Often you can't follow everyone for entire time periodOften you can't follow everyone for entire time period

In a dynamic population, individuals enter population over In a dynamic population, individuals enter population over time, become lost, etc. time, become lost, etc.

So length of follow-up is not uniform for all So length of follow-up is not uniform for all

Incidence rates do not make assumption of complete Incidence rates do not make assumption of complete follow-upfollow-up


Incidence rateIncidence rate (IR) = (IR) = # new cases of disease# new cases of disease

person-time of person-time of observation observation in candidate in candidate populationpopulation

This measure is a true rate because it directly This measure is a true rate because it directly integrates time into the denominator.integrates time into the denominator.


Accrual of Person-TimeAccrual of Person-Time

Jan Jan Jan1980 1989 1999

------------------x

------------------x

------------------------------------

10 Person-Years (PY)

10 PY

20 PY40 PY

Subject 1

Subject 2

Subject 3

X = outcome of interest, thus the incident rate is 2/40 PY


Some Ways to Accrue 100PYSome Ways to Accrue 100PY

100 people followed 1 year each = 100 py100 people followed 1 year each = 100 py

10 people followed 10 years each= 100 py10 people followed 10 years each= 100 py

50 people followed 1 year plus 25 people 50 people followed 1 year plus 25 people

followed 2 years = 100 pyfollowed 2 years = 100 py

Time unit for person-time = year, month or dayTime unit for person-time = year, month or dayPerson-time = person-year, person-month, person-Person-time = person-year, person-month, person-

dayday


Prevalence depends on incidence rate and Prevalence depends on incidence rate and duration of disease (duration lasts from onset of duration of disease (duration lasts from onset of disease to its termination)disease to its termination)

If incidence is If incidence is lowlow but duration is but duration is longlong - prevalence - prevalence

is relatively is relatively highhigh

If incidence is If incidence is highhigh but duration is but duration is shortshort - -

prevalence is relatively prevalence is relatively lowlow

Relationship between prevalence and Relationship between prevalence and incidenceincidence


Prevalence: administration, planning, some Prevalence: administration, planning, some research research

Incidence: etiologic research (problems with Incidence: etiologic research (problems with prevalence since it combines IR and D), prevalence since it combines IR and D), planningplanning

Uses of Prevalence and Incidence Uses of Prevalence and Incidence MeasuresMeasures


Common measures of disease Common measures of disease frequency for public healthfrequency for public health

– Crude death (mortality) rate:Crude death (mortality) rate:

Total number of deaths from all causesTotal number of deaths from all causes

1,000 people For one year1,000 people For one year

(also cause-specific, age-specific, race-specific (also cause-specific, age-specific, race-specific death rate)death rate)

Common measures of disease frequency for Common measures of disease frequency for public healthpublic health

Live-birth rateLive-birth rate::

total number of livebirthstotal number of livebirths For one year For one year1,000 people (sometimes women of childbearing age)1,000 people (sometimes women of childbearing age)

Infant mortality rateInfant mortality rate::

# deaths of infants under 1 year of age# deaths of infants under 1 year of age For one year For one year1,000 live-births1,000 live-births


Attack rate:Attack rate:

# cases of disease that develop during ________defined period_______

# in pop. at risk at start of period

(usually used for infectious disease outbreaks)


Case fatality rate:Case fatality rate:

# of deaths for a defined period of time# cases of disease

Survival rate:

# living cases for a defined period of time# cases of disease

Screening tests used in

the epidemiology of infectious

diseases

Which screening test?Which screening test?

•incidence

• prevalence

• morbidity

• mortality

•Sensitivity (Duyarlılık)

• Specificity (Özgüllük)

•Positive Predictive Value

•Negative predictive Value

Analytical Tests include both:

• Screening Tests • Diagnostic Tests

•Two Important Objectives

To distinguish between people in the population who have the diseases and those who do not

To determine how good the test is in separating populations of people with and without the disease in question?

ScreeningScreening

DefinitionDefinition - use of quick and simple testing - use of quick and simple testing procedures to identify and separate persons: procedures to identify and separate persons: – who have a disease from those that do notwho have a disease from those that do not

OROR– who are apparently (appear to be) well, but who may who are apparently (appear to be) well, but who may

be at risk of a disease, from those who probably don’t be at risk of a disease, from those who probably don’t have the disease.have the disease.

Terms Related to Terms Related to Screening TestsScreening Tests

ValidityValidity - relates to accuracy (correctness) - relates to accuracy (correctness)

ReliabilityReliability - repeatability - repeatability

Terms Related to Screening Tests Terms Related to Screening Tests (cont’d)(cont’d)

SensitivitySensitivity - ability of a test to identify those - ability of a test to identify those who have diseasewho have disease

SpecificitySpecificity - ability of a test to exclude those - ability of a test to exclude those who don’t have diseasewho don’t have disease

An important public health consideration, particularly in screening free-living populations, is:

How good is the test at identifying people with the disease and without the disease?

In other words:If we screen a population, what proportion of people who have the disease will be correctly identified?

POPULATION

Test Results With Disease Without Disease

PositiveTrue Positive

(TP)False Positive

(FP)

NegativeFalse Negative

(FN)True Negative

(TN)

In the clinical setting, a more important question is:

If the test results are positive (or negative) in a given patient, what is the probability that this patient has (or does not have) the disease?

In other words:What proportion of patients who test positive (or negative) actually have (or do not have) the disease in question?

Predictive ValuePredictive Value

Pos. PV = X 100 = % True Positives TP + FP

Neg. PV = X 100 = % True Negatives TN + FN

PPredictive value of redictive value of a ca complaint or omplaint or a a symptomsymptom for for a disease varies a disease varies dependdependinging on the prevalence of on the prevalence of the disease.the disease.

for example: a patients with complaintfor example: a patients with complaint of of fatigue, fatigue, tiredness, tiredness, weaknessweakness

If come to primary health care center If come to primary health care center

If come to If come to Hematology clinic Hematology clinic

Is your preliminary diagnoses is same in both cases?Is your preliminary diagnoses is same in both cases?


Further reading:Further reading:

Principles of EPIDEMIOLOGYPrinciples of EPIDEMIOLOGYin Public Health Practicein Public Health PracticeThird Edition

An Introduction toAn Introduction toApplied Epidemiology and BiostatisticsApplied Epidemiology and Biostatistics

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICESU.S. DEPARTMENT OF HEALTH AND HUMAN SERVICESCenters for Disease Control and Prevention (CDC)Centers for Disease Control and Prevention (CDC)Office of Workforce and Career DevelopmentOffice of Workforce and Career Development

Atlanta, GA 30333 (Atlanta, GA 30333 (free Online)free Online)


EPIDEMIOLOGY IN MEDICINEEPIDEMIOLOGY IN MEDICINE

Charles H. Hennekens, M.D., Dr.P.H.Charles H. Hennekens, M.D., Dr.P.H.

Julie E. Buring, Sc.D.Julie E. Buring, Sc.D.

Park’s textbook of PREVENTIVE AND SOCIAL Park’s textbook of PREVENTIVE AND SOCIAL MEDICINE MEDICINE

John E. ParkJohn E. Park

the definitions used in the epidemiology of infectious diseases

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