Screening Screening By: Michael OReilly Technical Advisor FETP - - PowerPoint PPT Presentation

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Screening Screening By: Michael OReilly Technical Advisor FETP - - PowerPoint PPT Presentation

Nov 11, 2023 105 likes •523 views

Screening Screening By: Michael OReilly Technical Advisor FETP Thailand Session Objectives Session Objectives Distinguish screening from diagnosis Distinguish screening from diagnosis Understand the basic principles of

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Screening Screening

By: Michael O’Reilly Technical Advisor FETP Thailand

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Session Objectives Session Objectives

  • Distinguish screening from diagnosis

Distinguish screening from diagnosis

  • Understand the basic principles of screening

Understand the basic principles of screening

  • Distinguish between reliability (repeatability) and

Distinguish between reliability (repeatability) and accuracy (validity) accuracy (validity)

  • Distinguish between efficacy and effectiveness

Distinguish between efficacy and effectiveness

  • Calculate sensitivity, specificity, predictive value

Calculate sensitivity, specificity, predictive value positive, and predictive value negative positive, and predictive value negative

  • Describe the effect of prevalence on each of the

Describe the effect of prevalence on each of the above measures above measures

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Screening Programs Screening Tests

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To screen

  • r

not to screen?

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Screening Definition Screening Definition

Screening is the presumptive identification Screening is the presumptive identification preclinically preclinically of asymptomatic disease by using

  • f asymptomatic disease by using

tests, examinations, or other rapidly conducted tests, examinations, or other rapidly conducted

  • procedures. Persons with a suspicious finding
  • procedures. Persons with a suspicious finding

must be referred for follow must be referred for follow-

  • up diagnostic tests

up diagnostic tests and necessary treatment. Screening implies a and necessary treatment. Screening implies a public health intervention applied to populations, public health intervention applied to populations, whereas diagnosis implies a clinical intervention whereas diagnosis implies a clinical intervention applied to individuals. applied to individuals.

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Screening Screening -

  • Summary

Summary

Population: healthy Disease: asymptomatic Test: not diagnostic

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The decision to screen The decision to screen

  • Disease

Disease

  • Screening test

Screening test

  • Follow

Follow-

  • up

up

  • Social and economic factors

Social and economic factors

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Principles of Screening Principles of Screening

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The disease The disease

  • Important

Important

  • Latent stage

Latent stage

  • Natural history understood

Natural history understood

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The screening test The screening test

  • Suitable

Suitable

  • Acceptable

Acceptable

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Follow Follow-

  • up

up

  • Policy

Policy

  • Available Diagnostic Test

Available Diagnostic Test

  • Available Treatment

Available Treatment

  • Acceptable treatment

Acceptable treatment

  • Available facilities

Available facilities

  • Impact on prognosis

Impact on prognosis

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Social factors Social factors

  • Costs

Costs

  • Process

Process

  • Ethics

Ethics

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Factors influencing screening Factors influencing screening test performance test performance

I ntrinsic I ntrinsic

  • Properties of the test

Properties of the test

Extrinsic Extrinsic

  • Characteristics of the population

Characteristics of the population

  • Skill and judgment of the

Skill and judgment of the screener screener

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Efficacy of a Screening Test Efficacy of a Screening Test

Efficacy refers to the ability of the Efficacy refers to the ability of the screening test to produce a precise result screening test to produce a precise result under ideal conditions. Efficacy is related under ideal conditions. Efficacy is related to the intrinsic properties of the test: to the intrinsic properties of the test: namely, its reproducibility (reliability) and namely, its reproducibility (reliability) and its accuracy (validity). its accuracy (validity).

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Intrinsic test properties Intrinsic test properties

  • Validity

Validity -

  • Accuracy

Accuracy

  • Reproducibility

Reproducibility -

  • Reliability

Reliability

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Validity vs Reproducibility

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Accuracy

(validity)

Measure by:

  • Sensitivity
  • Specificity
  • Predictive value positive
  • Predictive value negative

Reproducibility

(Reliability, Consistency, Precision)

Variation:

  • Biologic
  • Test Method
  • Intraobserver
  • Interobserver

Efficacy—Intrinsic Property

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Reproducibility (reliability) is the ability of the Reproducibility (reliability) is the ability of the test to achieve the same result when repeated. test to achieve the same result when repeated. Concerning reliability, there are four sources of Concerning reliability, there are four sources of possible variation: possible variation:

1. 1. Biological variation Biological variation 2. 2. Test method variation Test method variation 3. 3. Intraobserver Intraobserver variation variation 4. 4. Interobserver Interobserver variation variation

Reproducibility

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Accuracy Accuracy-

  • validity

validity

Accuracy is the test Accuracy is the test’ ’s ability to do what s ability to do what it is supposed to do and is measured in it is supposed to do and is measured in terms of four indices: sensitivity, terms of four indices: sensitivity, specificity, predictive value positive, and specificity, predictive value positive, and predictive value negative (see table predictive value negative (see table below). below).

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Gold Standard

Positive Negative Present Absent

Disease Test

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Sensitivity

  • Proportion of persons with the

condition who test positive Specificity

  • Proportion of persons without the

condition who test negative

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Sensitivity = Specificity = A A + C D B + D

+

  • Disease

Total A + C B + D

+

  • Test

X 100 X 100

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Screening test results Screening test results

Dichotomous: culture Dichotomous: culture (positive/negative) (positive/negative) Continuous: blood pressure (mm Hg) Continuous: blood pressure (mm Hg) serum cholesterol (mg/dl) serum cholesterol (mg/dl)

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x

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x x

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x x x

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Effects of varying the diagnostic threshold

  • n sensitivity and specificity

Test Result

100 95 70 50 Sensitivity (%) 50 70 95 100 Specificity (%)

Number of Persons

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Costs of false positives Costs of false positives

  • Unnecessary tests

Unnecessary tests

  • Labeling

Labeling

Costs of false negatives

  • Care for advanced disease
  • Premature death or disability
  • Expose others to infections
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Predictive Value Positive Predictive Value Positive

  • Proportion of persons with a positive

Proportion of persons with a positive test who have the condition test who have the condition

Predictive Value Negative

  • Proportion of persons with a

Proportion of persons with a negative test who do not have the negative test who do not have the condition condition

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Predictive value positive = Predictive value negative = A A + B D C + D

+

  • Disease

+

  • Test

Total A + B C + D

X 100 X 100

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Predictive value positive Predictive value positive depends on depends on what two what two factors?* factors?*

* Answer at end of session * Answer at end of session

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Predictive Value Positive and Prevalence Predictive Value Positive and Prevalence

Testing Conditions Size of Population = 100,000 Sensitivity of Test = 90% Specificity of Test = 90%

Disease Prevalence = 1%

Cancer Present Cancer Absent Positive Test Negative Test

PVP = A A+B

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Predictive Value Positive (PVP) and Predictive Value Positive (PVP) and Prevalence Prevalence

Testing Conditions Size of Population = 100,000 Sensitivity of Test = 90% Specificity of Test = 90%

Disease Prevalence = 0.1%

Cancer Present Cancer Absent Positive Test Negative Test

PVP = A A+B

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How do you increase PVP in a How do you increase PVP in a low Prevalence Diseases? low Prevalence Diseases?

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Perform a second screening test Perform a second screening test

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Screening program should be Screening program should be followed by surveillance to followed by surveillance to evaluate the effectiveness of the evaluate the effectiveness of the screening program. screening program.

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Session Objectives Session Objectives

  • Distinguish screening from diagnosis

Distinguish screening from diagnosis

  • Understand the basic principles of screening

Understand the basic principles of screening

  • Distinguish between reliability (repeatability) and

Distinguish between reliability (repeatability) and accuracy (validity) accuracy (validity)

  • Distinguish between efficacy and effectiveness

Distinguish between efficacy and effectiveness

  • Calculate sensitivity, specificity, predictive value

Calculate sensitivity, specificity, predictive value positive, and predictive value negative positive, and predictive value negative

  • Describe the effect of prevalence on each of the

Describe the effect of prevalence on each of the above measures above measures

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Are these Are these “ “screening screening” ” programs? programs?

  • Testing vision for driver

Testing vision for driver’ ’s license s license

  • “

“Screen Screen” ” an industry for risky work an industry for risky work habit habit – – are workers wearing safety are workers wearing safety helmets? helmets?

  • Testing pregnant women for Down

Testing pregnant women for Down’ ’s s Syndrome Syndrome

  • Screening for diabetes

Screening for diabetes

  • Testing applicants for a job for drug use

Testing applicants for a job for drug use