PH2610 Applied Epidemiology: Screening for Disease Prevention and Control

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PH2610 - Applied EpidemiologyScreening for
Disease Prevention and Control

• Lowell E. Sever, Ph.D.
• September 23, 2002

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Outline

• The basic concepts of screening in public health
• Screening tests Sensitivity, specificity and
  predictive value
• Selecting cut points for screening tests
• Screening and the natural history of disease
• Guidelines for screening programs

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Outline

• Genetic screening and public health
• Why is screening for breast and cervical cancer
  important?
• Selected issues in cancer screening policy

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Outcomes in the Screening Decision Tree

• False positive when a screening test indicates
  that the individual has a disease but the person
  in fact does not have the disease.
• False negative when a screening test indicates
  that the individual does not have a disease but
  the person in fact has the disease.

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Outcomes in the Screening Decision Tree

• True positive when the test says the person has
  a disease and the person indeed has the disease.
• True negative when the test says the person
  does not have the disease and the person in fact
  is disease free.

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Screening Tests

• Validity of test is shown by how well the test
  actually measures what it is supposed to measure.
  Validity is determined by the sensitivity and
  specificity of the test.
• Reliability is based on how well the test does in
  use over time - in its repeatability.

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Sensitivity and Specificity Tests of Validity

• Sensitivity is the ability of a screening
  procedure to correctly identify those who have
  the disease--the percentage of those who have the
  disease and are proven to have the disease as
  demonstrated by a diagnostic test.
• Specificity is the ability of a screening
  procedure to correctly identify the percentage of
  those who do not have the disease--those who do
  not have the disease and are proven to not have
  the disease as demonstrated by a diagnostic test.

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Screening

• Diagnosed Disease
• Status
• Positive Negative Total
• Screening test
• Positive a b a b
• Negative c d c d
• Total a c b d

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Sensitivity and Specificity of Breast Cancer
Screening Examination (HIP Program)

• Breast Cancer
• Cancer confirmed Cancer not Total
• confirmed
• Screening test
• Positive 132 983 1155
• Negative 45 63,650 63,695
• Total 177 64,633 64,820

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Screening

• Predictive Value Positive Probability that a
  person actually has the disease given a positive
  screening test
• Predictive Value Negative Probability that a
  person is actually disease-free given a negative
  screening test

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Screening

• Diagnosed Disease
• Status
• Positive Negative Total
• Screening test
• Positive a b a b
• Negative c d c d
• Total a c b d

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Predictive Value Positive and Predictive Value
Negative of Breast Cancer Screening Examination
(HIP Program)

• Breast Cancer
• Cancer confirmed Cancer not
  confirmed Total
• Screening test
• Positive 132 983 1155
• Negative 45 63,650 63,695
• Total 177 64,633 64,820

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Effect of Prevalence on Predictive Value Positive
with Constant Sensitivity and Specificity

• Prevalence PV () Sensitivity Specifici
  ty
• () () ()
• 0.1 1.8 90 95
• 1.0 15.4 90 95
• 5.0 48.6 90 95
• 50.0 94.7 90 95

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CONDITIONS FOR WHICH SCREENING IS
RECOMMENDED(U.S. PREVENTIVE SERVICES TASK FORCE
1996)
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Levels of Prevention(Mausner and Kramer 1985)

• Primary Prevention - Prevention of the occurrence
  of disease (reduce incidence of disease)
• Secondary Prevention - Early detection and prompt
  treatment of disease for cure, to slow
  progression, to prevent complications, or to
  limit disability (reduce prevalence of disease)
• Tertiary Prevention - Limitation of disability
  and rehabilitation where disease has already
  occurred and left residual damage

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Levels of Prevention

• Primary Prevention
• Prevention of the occurrence of disease through
• General health promotion
• Specific preventive measures
• (Mausner and Bahn)

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Levels of Prevention

• Secondary Prevention Involves
• Curing disease at the earliest stage possible
• Slowing disease progression
• Preventing complication
• Limiting disability
• (Mausner and Bahn)

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Levels of Prevention

• Tertiary Prevention
• Consists of limitation of disability and
  rehabilitation where disease has already occurred
  and left residual damage
• (Mausner and Bahn)

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Potential Types of Bias in Screening Programs

• Lead time bias the perception that the cases
  detected through screening have longer survival
  rates simply because the disease was identified
  earlier in its natural history.

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Potential Types of Bias in Screening Programs

• Length bias tumors detected by screening tend
  to be slower growing and hence have an inherently
  better prognosis than more rapidly growing tumors
  detected as a result of clinical manifestations.
• Selection bias individuals motivated to get
  screened may be different than those who are not.

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Screening Program Considerations (Guidelines)

• The disease or condition being screened for
  should be major medical problem.
• The natural history of the disease or condition
  should be adequately understood, including the
  regular phases and course of the disease, with an
  early period identifiable through screening.

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Screening Program Considerations (Guidelines)

• A suitable and effective screening test or
  examination for the disease(s) should be
  available.
• The screening process should be acceptable to the
  general population.
• The process should be simple enough to encourage
  large groups of persons to participate.

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Screening Program Considerations (Guidelines)

• Policies, procedures, and levels on screening
  tests should be determined in order to establish
  who should be referred for further testing,
  diagnosis, and possible treatment.
• Access to health care facilities and services for
  follow-up diagnosis and treatment should be
  available for potential cases and for diagnosed
  cases.

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Screening Program Considerations (Guidelines)

• Acceptable treatment should be available for
  individuals with diseases identified through the
  screening process.
• Screening should not be an occasional activity,
  but should be done on a regular and ongoing
  process.

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Conditions to be Considered in Establishing a
Screening Program

• The Disease
• causes significant morbidity and mortality
• can be identified at a pre-symptomatic stage
  before the individual would ordinarily seek
  medical care
• responds to acceptable, available and effective
  intervention and treatment
• is prevalent in the population undergoing
  screening

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Conditions to be Considered in Establishing a
Screening Program

• The Test
• is acceptable to those at risk for disease
• has adequate sensitivity, specificity and
  predictive value in the target population
• is available at a reasonable cost
• is sufficiently standardized to be performed with
  consistency, accuracy and reproducibility

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Genetic Screening and Public Health

• Some kinds of genetic screening programs raise
  concerns about the use of genetic screening to
  improve the gene pool - leading to concerns
  about eugenics and programs of racial
  purification.
• Many of these concerns have been exacerbated in
  recent years with the growth of the Human Genome
  Project.

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Genetic Screening and Public Health

• From a public health perspective, genetic
  screening should enable people to make informed
  decisions about their health and that of their
  children in a non-coercive atmosphere.

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Genetic Screening and Public Health

• Types of genetic screening that potentially have
  important public health applications and
  implications
• Heterozygote screening for carriers of recessive
  genetic diseases
• Prenatal screening to determine if a conceptus
  has specific types of genetic disorders
• Newborn screening to identify babies with genetic
  abnormalities that are amenable to treatment,
  thus preventing genetic disease by modifying
  the environment

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Genetic Screening and Public Health

• Types of genetic screening that potentially have
  important public health applications and
  implications
• Screening individuals to identify specific genes
  that lead to major diseases before they are
  manifested clinically (presymptomatic testing)

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Genetic Screening and Public Health

• Types of genetic screening that potentially have
  important public health applications and
  implications
• Screening individuals for genes that increase
  their risk for specific diseases
• Screening individuals for genes that increase
  their risk for developing particular diseases,
  given a specific exposure, that is genetic
  hypersusceptibility

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NIH CONSENSUS STATEMENTBreast Cancer Screening
for Women Ages 40-49

• Is there a reduction in mortality from breast
  cancer due to screening women ages 40-49 with
  mammography, with or without physical
  examination?
• How large is the benefit?
• How does this change with age?

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NIH CONSENSUS STATEMENT Breast Cancer Screening
for Women Ages 40-49

• What are the risks associated with screening
  women ages 40-49 with mammography, and with
  physical examination?
• How large are the risks?
• How do they change with age?

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Cervical Cancer Screening Policy/Guideline
Issues Under DiscussionBy CDC

• Screening intervals for Pap testing
• Upper and lower age limits for Pap testing
• New Pap testing technologies
• Recommendations on utilization of Human
  Papillomavirus (HPV) testing

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Cervical Cancer Screening Interval and Age Limits

• How often should women be screened?
• At which age should screening begin?
• At which age, if any, should screening end?
• Should the screening interval depend on previous
  screening history?
• What are other considerations (e.g., risk-based
  screening, practitioners concerns, etc.)?

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National Breast and Cervical Early Detection
Program (NBCCEDP)

• Policy Issue I Screening Intervals
• Reasons why some providers are hesitant to
  discontinue annual screenings
• The idea that women believe it is important to
  have an annual Pap test
• The idea that women may receive other preventive
  interventions while visiting the clinician for a
  Pap test

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National Breast and Cervical Early Detection
Program (NBCCEDP)

• Reasons why some providers are hesitant to
  discontinue annual screenings (cont)
• Questionable data indicating that annual Pap
  tests are unnecessary after 3 normal tests
• Concern that an interval longer than 1 year will
  allow aggressive cancers to escape early
  detection
• Concern that women will obtain Pap tests at a
  lower frequency than recommended

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Rescreening Women in the NBCCEDP Conclusions

• Clinically significant cervical cytologic
  abnormalities are uncommon in the three years
  following a recent normal smear, especially in
  women over age 50
• Screening more often than every three years in
  such women will likely increase morbidity from
  unnecessary diagnostic evaluations without
  decreasing cervical cancer morbidity or
  mortality.
• (Sawaya and Washington 1999)