Measurement

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Measurement

• Measuring disease and death frequency
• FETP India

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Competency to be gained from this lecture

• Calculate incidence, prevalence, mortality and
  case fatality

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Key areas

• Prevalence
• Incidence
• Relation between prevalence and incidence
• Mortality

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Population at risk

• Portion of a population that is susceptible to a
  disease
• Can be defined on the basis of demographic or
  environmental factors

Prevalence
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Population at risk Examples

• Population at risk of developing carcinoma of the
  cervix
• Female population
• Age gt 30 and lt 70 years
• Population at risk of hepatitis B
• Those individuals anti-HBc negative

Prevalence
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Prevalence (P)

• Number of existing cases (old and new) in a
  defined population at a specified point of time
• people with disease at a specified time
  
• P ------------------------------------------
  -------- x 10n
• Population at risk at the specified time
• In some studies the total population is used as
  an approximation if data on population at risk is
  not available

Prevalence
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Point prevalence

• Measures the frequency of disease at a given
  point in time
• Applies when the data has been collected at one
  point in time
• P C / N
• C of observed cases at time t
• N Population size at time t

Prevalence
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Example of point prevalence

• 150 children in a school
• Screening for refractory errors at time t
• 15 children require glasses
• Prevalence of refractory errors
• 15 / 150 10

Prevalence
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Period prevalence - (PP)

• Measures the frequency of disease over some time
• Applies when the data has been collected over a
  period of time
• PP C I / N
• C of prevalent cases at the beginning of the
  time period
• I of incident cases that develop during the
  period
• N size of the population for this same time
  period

Prevalence
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Exercise

• Scenario
• Population of 150 persons
• Follow-up for one year
• 25 had a disease of interest at the beginning
• Another 15 new cases developed during the year
• Calculate
• Point prevalence at the start of the period
• Period prevalence for the year

P C/N 25 /150 0.17 (17 ) PP (CI)/N
(2515)/150 0.27 (27 )
Prevalence
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Factors influencing prevalence

• Number of new cases
• Duration of the illness
• If the disease is short, the prevalence is
  reduced
• The prevalence of sudden infant death 0
• If the disease is long, the prevalence is
  increased
• Rare-lifelong disease can accumulate to build up
  a large prevalence

Prevalence
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Causes of increase and decrease of prevalence

• Increase
• Long duration
• Low cure rate
• Low case fatality
• Increase in new cases
• Immigration of patients
• Improved detection
• Emigration of healthy people

• Decrease
• Shorter duration
• High cure rate
• High case fatality
• Decrease in new cases
• Emigration of patients
• Improved cure rate
• Immigration of healthy people

Conclusion Changes in prevalence may have many
causes and are difficult to interpret
Prevalence
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Uses of prevalence data

• Assessing health care needs
• Planning health services
• Measure occurrence of conditions with gradual
  onset
• Study chronic diseases

Prevalence
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Incidence (I)

• Number of new cases in a given period in a
  specified population
• Time, (i.e., day, month, year) must be specified
• Measures the rapidity with which new cases are
  occurring in a population
• Can be expressed
• In absolute numbers
• In terms of cumulated incidence
• In terms of incidence density

Incidence
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Cumulated incidence - (CI)

• of new cases
• CI -----------------------------------------
  --------- x 10n
• Population at risk at the beginning
• Also known as
• Attack rate
• Assumes that the entire population at risk at the
  beginning was followed-up for the time period of
  observation

Incidence
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Cumulated incidence calculation over 7 years of
observation
7 years
?
?
Person included8 Lost to follow
up 4 Illness 1 Cumulated incidence 25
?
?
Incidence
Development of illness
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Risk

• Probability that an individual will experience a
  health status change over a specified followup
  period
• This assumes that the individual does not
• Have disease at the beginning
• Die from other causes during follow-up
• Corresponds to cumulated incidence

Incidence
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Incidence density - (ID)

• of new cases
• ID -----------------------------------------
  --------- x 10n
• Total person-time of observation
• Also known as
• Incidence rate
• Reflects more exactly the person-time observed

Incidence
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Incidence density calculation with annual
observations during 7 years
Person-year at risk41 Illness 2 Incidence
density 4.9 / person -year
Incidence
One year
Development of illness
Censored
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Uses of incidence data

• Describe trends in diseases
• Evaluate impact of primary prevention programmes

Incidence
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The dynamics of incidence and prevalence
New cases
Incidence
Prevalence
Death Cure
Incidence and prevalence
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The relation between prevalence and incidence

• Prevalence depends on
• Incidence (I)
• Duration of the disease (D)
• P I x D
• Change in prevalence from one time period to
  another may be the result of changes in incidence
  rates, changes in the duration of disease, or both

Incidence and prevalence
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Patterns of incidence and prevalence

• High prevalence and low incidence
• e.g., Diabetes Mellitus
• Low prevalence and high incidence
• e.g., Common cold

Incidence and prevalence
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Evolution of HIV prevalence in a country scaling
up public health efforts

• Increase in prevention
• Reduction in incidence (Difficult to measure)
• Increase in care and support (treatment)
• Increase in disease duration (reduced mortality)
• Increase in prevalence (easier to measure)
• Incidence measures the impact of prevention
  efforts
• Prevalence may be used to plan care and support
• The immediate consequence of the plan may be an
  increased prevalence

Incidence and prevalence
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Crude mortality rate - (CMR)

• deaths in a specified period
• CMR --------------------------------------------
  ---- x 10n Average total population
• Does not take into account factors such as age,
  sex, race, socio-economic status, etc.
• Provides information on trends in a populations
  health status

Deaths
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Disease-specific mortality rate - (SMR)

• deaths from a disease in a specified period
• SMR --------------------------------------------
  ---- x 10n Average total population
• Reflects the impact of a disease on a population
  in terms of death
• Should not be confused with case fatality

Deaths
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Case fatality

• Places in relation the number of deaths from a
  disease to the number of cases
• Reflects severity
• Can be expressed as
• Proportion
• Ratio
• Not as rate (although often referred to as case
  fatality rate)

Deaths
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Case fatality proportion

• deaths among the cases identified
• CF ---------------------------------------------
  --- x 10n of cases
• Applies to a situation where follow-up
  information is available individually for all
  case-patients
• The numerator is a part of the denominator

Deaths
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Case fatality ratio

• reported deaths from a disease
• CFR --------------------------------------------
  ---- x 10n of reported cases from
  the disease
• Used in surveillance when no specific information
  is available on follow-up of individual patients
• The deaths in the numerator may not correspond to
  the cases in the denominator

Deaths
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Take-home messages

• Prevalence is a static measure taken at a point
  in time
• Incidence is a dynamic measure taken over a
  certain time
• Mortality is calculated using population
  denominators to reflect burden while case
  fatality is calculated using cases as
  denominators to reflect severity