Evaluating Screening tests

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Evaluating Screening tests

• Thomas B. Newman, MD, MPH
• Andi Marmor, MD, MSEd
• October 23, 2008

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Outline

• Overview and definitions
• Can screening be bad?
• Evaluating studies of screening tests
• Observational studies of screening
• Randomized trials of screening
• Conclusion

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Outline

• Overview and definitions
• Can screening be bad?
• Evaluating studies of screening tests
• Observational studies of screening
• Randomized trials of screening
• Conclusion

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What is screening?

• Common definition
• Testing to detect asymptomatic disease
• Better definition
• Application of a test to detect a potential
  disease or condition in people with no known
  signs or symptoms of that disease or condition

Common screening tests. David M. Eddy, editor.
Philadelphia, PA American College of Physicians,
1991
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What is screening?

• Common definition
• Testing to detect asymptomatic disease
• Better definition
• Application of a test to detect a potential
  disease or condition in people with no known
  signs or symptoms of that disease or condition

Common screening tests. David M. Eddy, editor.
Philadelphia, PA American College of Physicians,
1991
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What is screening?

• Common definition
• Testing to detect asymptomatic disease
• Better definition
• Application of a test to detect a potential
  disease or condition in people with no known
  signs or symptoms of that disease or condition
• Condition includes a risk factor for a disease

Common screening tests. David M. Eddy, editor.
Philadelphia, PA American College of Physicians,
1991
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Screening Spectrum
Recognized symptomatic disease
Unrecognized symptomatic disease
Presymptomatic disease
Risk factor

• Fewer people
• Easier to demonstrate benefit
• Less potential for harm

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Screening Spectrum
Recognized symptomatic disease
Unrecognized symptomatic disease
Presymptomatic disease
Risk factor

• Fewer people
• Easier to demonstrate benefit
• Less potential for harm

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Issues in Screening for Risk Factors

• Risk factor treatment disease
• Does risk factor predict disease?
• Does treatment of risk factor reduce disease?
• Does treatment reduce risk factor?
• Test accuracy must measure incidence of disease
  over time
• Most measures of test accuracy apply to disease
  that is prevalent at the time the test is done
• Potential for harm greatest when screening for
  risk factors!

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Screening Spectrum
Recognized symptomatic disease
Unrecognized symptomatic disease
Presymptomatic disease
Risk factor

• Fewer people recognized and treated
• Easier to demonstrate benefit
• Less potential for harm

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Goals of Screening for Presymptomatic Disease

• Detect disease in earlier stage than would be
  detected by symptoms
• Only possible if an early detectable phase is
  present (latent phase)
• Only beneficial if earlier treatment is more
  effective than later treatment
• Do this without incurring harm to the patient
• Net benefit must exceed net harm
• Long follow up and RCT may be needed to prove

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Special Case Screening for Cancer

• Natural history heterogeneous
• Screening test may pick up slower growing or less
  aggressive cancers
• Not all patients diagnosed with cancer will
  become symptomatic
• Diagnosis is subjective
• There is no gold standard

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Outline

• Overview and definitions
• Can screening be bad?
• Evaluating studies of screening tests
• Observational studies of screening
• Randomized trials of screening
• Conclusion

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Possible harms from screening

• To all
• To those with negative results
• To those with positive results

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Possible harms from screening

• To all
• To those with negative results
• To those with positive results

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Possible harms from screening

• To all
• To those with negative results
• To those with positive results

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Source Funny Times. (1-888-Funnytimes x 476)
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Possible harms from screening

• To all tested
• To those with negative results
• To those with positive results

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Forces Behind Excessive Screening

• Economic
• Political
• Health care providers
• Public/cultural

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Forces Behind Excessive Screening

• Economic
• Political
• Health care providers
• Public/cultural

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Continue to Part 2
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