Measuring the Occurrence of Disease

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Measuring the Occurrence of Disease

• PH2610
• Introduction to Epidemiology
• Fall 2002

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Counting cases

• Ones knowledge of science begins when he can
  measure what he is speaking about and express it
  in numbers Lord Kelvin (1824-1907)
• To examine the transmission of disease in human
  populations, we need to be able to measure the
  frequency of disease occurrence and of deaths
  from the disease

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Measures

• How do we express the extent of morbidity and
  mortality resulting from disease?
• Counts
• Proportions
• what fraction of the population is affected
• Rates
• how fast things are occurring

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Measures

• Measures of morbidity
• Prevalence - a proportion
• Cumulative incidence - a proportion
• Incidence density - a rate
• Measures of mortality
• Mortality rate - a proportion
• Standardized mortality (SMR)

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Defining case

• Natural course of disease
• Exposure ? onset ? symptoms ? dx ? outcome

Incubation period
Recovery Death Chronic disease
Clinical Stage
Subclinical Stage
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The disease iceberg
Died
Hospitalized
Diagnosed by PMD
Self-reported
Population screening
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Counts

• Prerequisite for epidemiologic investigation
• Simplest measure of disease frequency
• Frequency of affected individuals
• Useful for planning adequacy of health care
  allocation at a particular level
• For example
• Number of West Nile virus cases

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Counts West Nile Virus
Rate/100,000
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Ratio

• A fraction with no specified relationship between
  numerator and denominator
• Range 0 to ?
• A/B
• Examples
• sex ratio (MF)

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Ratio

• Number of men with syphilis, 1991
• 2,412
• Number of women with syphilis. 1991
• 2,314
• Ratio of male to females
• 2,412/2,314 1.04
• (The numerator is not included in the denominator)

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Example of Ratio Measures

• Male to Female
• Urban to Rural
• Black to White
• Young to Old
• South to Midwest (West Nile Virus)
• LA, MS, TX 32/100,000
• Il, MI, OH 11/100,000
• Ratio 2.87

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Proportion

• Type of ratio
• Numerator included in denominator
• May be expressed as percentage
• Range 0 to 1
• A/(AB)
• Example
• Prevalence
• Cumulative Incidence

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Rate

• A special type of proportion
• Unit of time in denominator
• A/(AB) per time interval
• Common to use the population as the denominator
• Always two components
• New cases and time

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Basic Population Measures

• Numerator
• Denominator

Deaths Cases Events
Population Population at risk Time at risk
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Prevalence

• All individuals with a disease at a given point
  in time
• Dimensionless - should not be described as a rate
  - may be described as a percent
• number of cases (A) today
• P
• total population (AB) today

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Prevalence

• Proportion of individuals in a population who
  have the disease or condition of interest at a
  specific time period
• Used to measure disease burden
• Depends on incidence and duration

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Prevalence Incidence X Duration
Prevalence (existing cases)
Incident Cases
Population at risk
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Counts U.S. Population
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Counts 9/11
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Comparison
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END - 9/11
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Counts / Proportion
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Prevalence

• Types
• Point prevalence proportion of all cases at a
  specific point in time
• Period prevalence proportion of all cases
  during a period of time
• Frequency of disease at autopsy all cases among
  those autopsied
• Birth defect rate number of new births with
  abnormality among all live births

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Point prevalence

• Do you currently have asthma?
• Since asthma is a diagnosis that stays constant,
  the point prevalence and period prevalence may be
  the same.
• Are you having asthma symptoms right now?
• Do you have a sunburn?

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Period prevalence

• Have you had an asthma attack in the last 6 weeks
• Have you had a sun burn in the last 10 years

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Disease at autopsy

• What is the prevalence of prostate cancer at
  autopsy
• More men are likely to die with prostate cancer
  than from prostate cancer

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Disease at birth

• What is the congenital syphilis rate
• How many cases of congenital syphilis were
  identified at the time of birth
• For congenital syphilis
• the numerator are cases among live and still
  births
• The denominator is total live births during that
  period

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Prevalence

• Utility
• Describe health burden of a population
• Status of disease in a population
• Estimate the frequency of exposure
• Project health care needs of affected individuals

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Prevalence

• Limitations
• All cases with a disease at a given point in time
  are a product of
• Factors influencing development of disease
• Factors influencing survival
• Identifying who has disease (definition)
• How to find diseased (case ascertainment)
• Denominator (population at risk)

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Prevalence

• Prevalence of Hepatitis B Virus
• 4.4 of the population has evidence of current or
  previous Hepatitis B Virus infection
• Prevalence of Hepatitis C Virus
• 1.0 of the population has evidence of current or
  previous Hepatitis C Virus infection
• Data from NHANES III

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Incidence

• Rate of development of disease during a given
  period of time
• Three key elements
• Only new cases included in numerator
• Total population at risk in the denominator
• Time element period over which new cases
  developed

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Incidence

• Types
• Cumulative Incidence
• estimate of probability (risk) that an individual
  will develop disease during a specified period of
  time
• new cases in a given period of time
• CI
• total population at risk during that time

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Incidence

• Measures the frequency of new cases of disease or
  condition in a population at risk during a
  specified period of time 
• Number of new cases of a disease
• during a specified period of time
• CI x multiplier
• Total population at risk during
  (e.g., 100,000)
• during that time
• Assumption the entire population at the
  beginning of the study has been followed for the
  entire interval for the development of the
  outcome of interest

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Incidence

• Types
• Incidence Density (ID) measure of the true rate
  of disease development
• True force of morbidity or mortality
• new cases in a given period of time
• ID
• total person-time of observation

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Incidence, Prevalence
What was prevalence of disease in 1992? What was
the incidence of disease between 1992 and 1994?
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Calculating person-time
x
x
ID 2/17 11.8/100 person-years of observation
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Calculating person-time
Number of subjects length of Observation Person
Years 20 10 200 10 9 90 8
8 64 3 7 21 1 1
1 42 376 There were 5 incident
cases during the 10 year period. Incidence
density 5/376 Five-year incidence 13.29 per
1000 person year of observation Or can be
expressed 1329.78 per 100,000 person years
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Factors that affect Prevalence

• Increases
• Immigration of ill cases
• Migration of healthy people
• Prolongation of life with cure/treatment
• Increase incidence

• Decreases
• Immigration of healthy persons
• Migration of ill cases
• Improved cure rate
• (decreased duration)
• Increased death rate
• Decreased incidence
• Death

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Factors that affect Incidence

• Increases
• Increased risk
• (more people exposed)
• Failure of prevention programs

• Decreases
• Changes in the natural history of disease
• (changes in the virulence of infection)
• Successes in prevention programs
• Fewer people exposed