Screening
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Screening. “...the identification of unrecognized disease or defect by the application of tests, examinations or other procedures...” “...sort out apparently well persons who probably have disease from those who probably do not.” “...not intended to be diagnostic...”.

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Presentation Transcript
Screening l.jpg
Screening

  • “...the identification of unrecognized disease or defect by the application of tests, examinations or other procedures...”

  • “...sort out apparently well persons who probably have disease from those who probably do not.”

  • “...not intended to be diagnostic...”


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Different kinds of testing in medicine

  • “Diagnostic” - specifically looking for a suspected condition which is tested for and confirmed or excluded

  • “Case-finding” - usually in an investigation of exposed people, to sort the exposed and ill from the exposed and well. (E.g., test people who were in contact with a case of tuberculosis, or check b.p. of patient who is overweight)

  • “ Screening” - usually no specific exposure or indication that the individual has disease. (E.g., routine PSA testing in middle-aged males)


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Types of screening

  • Mass screening, no selection of population (e.g., checking all infants for hearing problems)

  • Selective screening (e.g., by age and sex: mammograms for women aged over 40)

  • Multiphased screening (a series of tests, as family doctors do at annual health exams)


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Logic of screening

Apparently well population

Screening test

Positive results: Diagnostic test

Negative results

Disease

No disease

Disease

No disease

(True positive)

(False positive)

(False negative)

(True negative)


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How good is the test?

Disease present?

No

Yes

Positive

False positive

True positive

Test result

False negative

True negative

Negative

Sensitivity = True positive

True positive + False negatives

Specificity = True negative

True negative + False positives


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Characteristics of a good screening test

  • Valid (e.g., sensitive and specific)

  • Reliable (gives consistent results; no random errors)

  • Yield (number of cases identified per thousand screened)

  • Cost – benefit (compare costs avoided due to early detection of the disease against cost of the screening. Does the test merely uncover more disease that is expensive to treat without appreciable advantage?)

  • Acceptable (discomfort, hassle, cost of obtaining test)

  • Follow-up services (plan needed to deal with positive results)


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When should we screen?

Screen when:

  • It is an important health problem (think about how to define ‘important’?)

  • There is an accepted and effective treatment

  • Disease has a recognizable latent or early symptomatic stage

  • There are adequate facilities for diagnosis and treatment

  • There is an accurate screening test

  • There is agreement as whom to consider as cases


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Ethics of medical care

Remember the basic ethical principles:

  • Autonomy

  • Non-maleficence

  • Beneficence

  • Justice


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Ethics in screening

  • Informed consent obtained?

  • Implications of positive result?

  • Number and implications of false positives?

  • Ditto for false negatives?

  • Labeling and stigmatization


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Periodic health examination

  • Canadian Task Force

  • Levels of evidence for interventions

    • I-1 Randomized controlled trials

    • II-1 Well-designed trial but not randomized

    • II-2 Cohort or case-control studies in more than one centre

    • II-3 “Natural experiments”

    • III Expert opinion


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Example of Periodic Health Exam: Women and men aged 16 - 44

Immunizations

  • Polio

  • Tetanus and diphtheria

  • Travel - related

  • Rubella

    Screening

  • Hypertension

  • CA cervix

  • Tuberculosis

  • STD