This presentation has been prepared to highlight the most important points about screening. It builds on previous -even little-knowledge about screening in biomedical sciences.

1. Screening in Biomedical
sciences
Dr. Abbas A Assayed, MBBS, MPH, MSc
24.1.2017

2. Presentation outlines
• Definition of Screening
• Wilson and Junger Screening criteria
• New/ additional criteria
• Principles of screening
• Types of screening
• Clinical framework of screening
• Differences between screening and diagnostic tests
• Bias in screening
• Criteria of a good screening test
• Examples, sensitivity, specificity and predictive values (+/-)
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3. Questions?
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4. Definition
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5. Wilson and Jungner screening criteria
1968))
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6. Should we screen for Gout?
1. The condition sought should be an important health problem.
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7. 2. There should be an accepted treatment for patients with recognized
disease.
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8. Should we screen for Tetralogy of Fallot?
3. Facilities for diagnosis and treatment should be available.
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9. Should we screen for CORONA?
4. There should be a recognizable latent or early symptomatic stage.
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10. Should we screen for Irritable Bowel Syndrome (IBS(?
5. There should be a suitable test or examination.
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11. Should we screen for Cervical cancer?
6. The test should be acceptable to the population.
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12. 7. The natural history of the condition, including development from
latent to declared disease, should be adequately understood.
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13. Should we screen for DM or HTN?
8. There should be an agreed policy on whom to treat as patients.
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14. Should we screen for fetal defects during pregnancy?
9. The cost of case-finding (including diagnosis and treatment of
patients diagnosed) should be economically balanced in relation to
possible expenditure on medical care as a whole.
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15. Is it OK to set screening program for vision and hearing problems
among pupils at the beginning of every school year?
10. Case-finding should be a continuing process and not a “once and for
all” project.
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16. Wilson and Jungner classic screening criteria
1. The condition sought should be an important health problem.
2. There should be an accepted treatment for patients with recognized disease.
3. Facilities for diagnosis and treatment should be available.
4. There should be a recognizable latent or early symptomatic stage.
5. There should be a suitable test or examination.
6. The test should be acceptable to the population.
7. The natural history of the condition, including development from latent to declared disease,
should be adequately understood.
8. There should be an agreed policy on whom to treat as patients.
9. The cost of case-finding (including diagnosis and treatment of patients diagnosed) should be
economically balanced in relation to possible expenditure on medical care as a whole.
10. Case-finding should be a continuing process and not a “once and for all”
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17. WHO’s Synthesis of emerging screening criteria proposed over the past
40 years
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18. The Principles of Screening
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19. Types of screening
 Mass screening
 High risk or selective or targeted screening.
 Multiphasic screening.
 Multipurpose screening.
 Opportunistic and case finding screening
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20. Clinical framework of screening
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21. Difference between
Screening and Diagnostic tests
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22. Screening and diagnostic tests are never perfect
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Screening Diagnostic
1. Purpose To detect potential disease
indicator or risk factor
To confirm/exclude disease

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2. Target population Large numbers of asymptomatic,
but potentially at risk individuals
Symptomatic individuals or
asymptomatic with positive
screening test

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3. Test method Simple and acceptable to clients
and staff
Maybe invasive and expensive,
but justifiable

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4. Positive result threshold generally chosen towards high
sensitivity not to miss potential
disease
Chosen towards high specificity (true
negatives). More weight given to accuracy
and precision than to patient acceptability

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5. Positive result Essentially indicates suspicion of
disease that warrants confirmation
provides a definite diagnosis

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6. cost Cheap Higher costs associated with diagnostic test maybe justified to
establish diagnosis

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Screening Diagnostic
Purpose To detect potential disease indicator or risk
factor
To confirm/exclude disease
Target
population
Large numbers of asymptomatic, but
potentially at risk individuals
Symptomatic individuals or asymptomatic with positive
screening test
Test method Simple and acceptable to clients and staff Maybe invasive and expensive, but justifiable
Positive result
threshold
generally chosen towards high sensitivity not
to miss potential disease
Chosen towards high specificity (true negatives). More
weight given to accuracy and precision than to patient
acceptability
Positive result Essentially indicates suspicion of disease that
warrants confirmation
provides a definite diagnosis
cost Cheap Higher costs associated with diagnostic test maybe
justified to establish diagnosis

30. Bias in screening
• Lead time bias:
– is the apparent improvement in the length of survival due to screening and
results from artificially adding extra months or years provided by the earlier
date of diagnosis compared to the later date of diagnosis.
• Length time bias:
– also gives an apparent increase in the length of survival in screened versus
the unscreened because screening is more likely to pick up slow growing
tumors with a better prognosis.
• Diagnostic bias:
– can also lead to overestimation of the benefit of screening because of the
inclusion of detected cases with pre-invasive conditions which would not
have led to invasive or fatal disease.
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31. Characteristics of a good screening test
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32. Validity
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33. Reliability
 The degree to which an assessment tool produces stable and consistent
results.
 Intra-observer or intra measurement reliability:
• the questionnaire or measurement would be used by the same
interviewer on the same subject on two or more occasions
separated b a time interval.
 Inter-observer reliability:
• the questionnaire or measurement is carried out on the same
subject by two or more observers.
 Internal consistency
- similar or opposite questions relate
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36. Positive predictive value
The proportion of all test positives that are truly positive.
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37. Negative predictive value
The proportion of all test negatives that are truly negative.
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Gold Standard/True Disease Status
Screening
test results
Present Absent
Positive "True positives"
a
"False positives"
b
Negative "False negatives"
c
"True negatives"
d
Sensitivity of screening test = a
a + c
Specificity of screening test = d
b + d
Predictive value of positive test = a
a + b
Predictive value of negative test = d
c + d

39. Fever and malaria
Blood Smear
Positive Negative
Fever?
Yes 45 40 85
No 5 10 15
50 50 100
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40. 40
Blood Smear
Positive Negative
Fever?
Yes 45 40 85
No 5 10 15
50 50 100
Sensitivity = a/(a+c) = 90%
Specificity = d/(d+b) = 20%
Positive predictive value = 53%
Negative predictive value = 67%

41. Some common screening tests
➔Fasting blood cholesterol for heart disease
➔Fasting blood sugar for diabetes
➔Blood pressure for hypertension
➔Mammography for breast cancer
➔Pap smear for cervical dysplasia or cervical cancer
➔PSA test for prostate cancer
➔Fecal occult blood for colon cancer
➔Ocular pressure for glaucoma
➔PKU test for phenolketonuria in newborns
➔TSH for hypothyroid and hyperthyroid
➔…...etc
Sukon Kanchanaraksa, PhD Johns Hopkins University
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42. Thank you very much
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