2. SCREENING: DEFINITION
“The PRESUMPTIVE identification of UNRECOGNIZED
disease or defect by the application of tests, exams or
other procedures which can be applied RAPIDLY to sort out
apparently well persons who PROBABLY have a disease
from those who PROBABLY do not”*
Key Elements: disease/disorder/defect
screening test
population
*Commission on Chronic Illness, 1957
3. Screening
Definition: Presumptive identification
of an unrecognized disease or
defect by the application of tests,
examinations, or other procedures.
Classifies asymptomatic people as
likely or unlikely to have a disease or
defect. Usually not diagnostic.
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5. Screening
For screening to be successful you need a:
Suitable disease
Suitable test
Suitable screening program
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7. Suitable Disease
Has serious consequences
Is progressive
Disease treatment must be effective at an
earlier stage
Prevalence of the detectable pre-clinical
phase must be high
Examples of suitable diseases: breast
cancer, cervical cancer, hypertension
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8. Natural History of Disease
20 30 40 50 60 70 Years
A B C D
Biological Disease Symptoms Death
Onset Detectable Develop
By Screening
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9. Natural History of Disease
Total pre-clinical phase = A to C (Age 30
to Age 60) = 30 years
Detectable pre-clinical phase (DPCP) = B
to C (Age 45 to Age 60) = 15 years
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10. Natural History of Disease
DPCP varies with the test, the disease,
and the individual
Lead Time: Duration of time by which the
diagnosis is advanced as a result of
screening. B to C (Age 45 to Age 60) = 15
years
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11. Suitable Test
Ideally, it's inexpensive, easy to
administer, has minimal discomfort has
high level of validity and reliability
Valid Test: Does what it's supposed to do,
that is, correctly classify people with pre-
clinical disease as positive and people
without pre-clinical disease as negative
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12. Suitable Test
Reliable Test: Gives you same results on
repetition
Validity is more important than reliability
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13. Suitable Test
Disease Status (Truth)
Yes No Total
Positive a b a+b
Screening Negative c d c+d
Test Result
Total a+c b+d a + b + c+ d
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14. Suitable Test
Measures of test validity
Sensitivity - enables you to pick up the cases of
disease
Sensitivity = a / a + c = those that test
positive / all with disease
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15. Issues in Screening
Disease
-Disease/disorder should be an important public health
problem
High prevalence
Serious outcome
-Early Detection in asymptomatic (pre-clinical)
individuals is possible
-Early detection and treatment can affect the course of
disease (or affect the public health problem?)
16. Criteria for Evaluating a Screening Test
•Validity: provide a good indication of who does and
does not have disease
-Sensitivity of the test
-Specificity of the test
•Reliability: (precision): gives consistent results when
given to same person under the same conditions
•Yield: Amount of disease detected in the population,
relative to the effort
-Prevalence of disease/predictive value
17. Validity of Screening Test (Accuracy)
- Sensitivity: Is the test detecting true cases of
disease? (Ideal is 100%: 100% of cases are
detected)
-Specificity: Is the test excluding those without
disease? (Ideal is 100%: 100% of non-cases are
negative)
19. Screening for Glaucoma using IOP
True Cases of Glaucoma
Yes No
IOP > 22: Yes 50 100
No 50 1900
(total) 100 2000
Sensitivity = 50% (50/100) False
Negative=50%
Specificity = 95% (1900/2000) False
20. Where do we set the cut-off for a screening test?
Consider:
-The impact of high number
of false positives:
anxiety, cost of further
testing
-Importance of not missing a
case:
seriousness of disease,
likelihood of re-screening
22. Yield from a Screening Test for Disease X
Predictive Value
Screening Test
X
X X
X
X
X
Negatives Positives
23. Yield from the Screening Test: Predictive
Value
•Relationship between Sensitivity, Specificity, and
Prevalence of Disease
Prevalence is low, even a highly specific test will
give large numbers of False Positives
•Predictive Value of a Positive Test (PPV): Likelihood
that a person with a positive test has the disease
•Predictive Value of a Negative Test (NPV): Likelihood
that a person with a negative test does not have the
disease
24. Screening for Glaucoma using IOP
True Cases of Glaucoma
Yes No
IOP > 22: Yes 50 100
No 50 1900
(total) 100 2000
Specificity = 95% (1900/2000) False
Positive=5%
Positive Predictive Value =33%
25. How Good does a Screening Test have
to be?
-Seriousness of disease, consequences of high false
positivity rate:
-Rapid HIV test should have >90% sensitivity, 99.9%
specificity
-Screen for nearsighted children proposes 80%
sensitivity, >95% specificity
-Pre-natal genetic questionnaire could be 99%
sensitive, 80% specific
26. Principles for Screening Programs
1. Condition should be an important health problem
2. There should be a recognizable early or latent stage
3. There should be an accepted treatment for persons with
condition
4. The screening test is valid, reliable, with acceptable yield
5. The test should be acceptable to the population to be
screened
6. The cost of screening and case finding should be
economically balanced in relation to medical care as a whole
27. Lead Time Bias
screen-detectable clinically evident
death
5 years
clinically evident
death
2 years
28. Lead Time Bias
Because of lead-time bias, it is necessary
to look at disease-specific and age-
specific death rates in screened and
unscreened groups when assessing a
screening intervention.
Time from diagnosis to death does not
tell you if a screening test is effective
33. Screening Generally
Is to seek about certain
problem in certain high
risk gp.
34. Validity of Screening Test
Validity of test determined by ability to correctly
categorise subjects to test-positive or test-
negative
Disease status
Test Positive Negative Total
Positive
result a b a+b
Negative c d c+d
Total a+c b+d
35. Validity of Screening Test cont...
Sensitivity = ability of test to give a positive
result when disease is present
= a / a+c
Specificity= ability of test to give a negative
result when disease is absent
= d / b+d
36. Validity of Screening Test cont...
Predictive value is determined by sensitivity &
specificity and also by the prevalence of preclinical
diseas
Positive predictive value
= probability that a person with a positive test
actually has the disease = a / a+b
Negative predictive value
= probability that a person with a negative test is
truly disease-free = d / c+d
37. What is a Pap Smear?
“Papanicolaou test” - 1941
Dr. Babes & Dr. Papanikolaou
> 50% decrease in rates of cervical cancer in developed
countries over last 30 yrs due to widespread screening
A sample cervix cells from transformation zone.
junction of endocervix and ectocervix
Use of spatula +/- cytobrush, broom stick
2 types – conventional, liquid-based
Send for cytologic interpretation
38. Screening Guidelines
Start at age 21 (regardless of age of first intercourse)
Age 21-29 screen every 2 years
Age 30 + screen every 3 years if…
Negative cytology x3 previous Paps
NIELM and negative HR HPV test in 1 year
No history of high grade lesions
Annual screening if…
Immunocompromised (ie. HIV, transplant pts)
History of CIN II, III or cancer
Exposure to DES in utero
Stop screening at 65 or 70 yrs if …
3 prior consecutive normal Paps
No history of abnormal screening in last 10 yrs
Stop screening if hysterectomy for benign disease with no history of
abnormal Pap smears
39. CRITERIA FOR SCREENING
Disease:
Must be serious enough
Must be widespread enough
Must be fairly reliably diagnosable
Must be treatable
Must be affordable
Hopefully legally defensible
40. Criteria for Screening Test
1. Simple & quick
2. Capable of being performed by paramedics
3. Inexpensive
4. Acceptable to population
5. Accurate
6. Repeatable
7. Sensitive
8. Specific
41. Secondary Prevention of
Ca.Cx.
Key Point is to detect precancerous
lesions –BY
- A good screening method
- PAP smear test is considered to be the gold
standard – Has limitations ?
Alternatives to Pap Smear – What are they?
42. Why screening for cervical
cancer?
1. Is relatively common in unscreened
women.
2. Has a relatively good prognosis if
found early stage in its natural course of
disease.
3.Has a characteristic natural course
that is a slow progression through a
premalignant stage.
43. Why screening for cervical
cancer? Cont…
4. A premalignant stage can be detected
by noninvasive means (the Pap smear ,
cervicography&VIA).
5. There are effective treatment modalities
to eradicate premalignant lesions and early
invasive cervical cancer.
44. Screening by Pap. Cx. Smear
a. Importance: unscreened female have ten fold
risk > screened female
b. To whom :
- Every sexually active female (18-35 y)
- Specially, high risk group.
c. When:
- Annually up to the age of 35y
- No need to extend screening > 35y if smear is N.
- At each pregnancy
- If new risk factors appear after 35y.
d- If + ve smear colposcopy
45. Alternatives to Cytology
Visual Inspection of the cervix:
Unaided: Downstaging.
Aided with acetic acid: VIA:
Naked eye
Aided with acetic a and magnification( VIAM)
Cervicography
Colposcopy
Speculoscopy
Automated pap smear
HPV DNA test
Infrared Spectroscopy & Laser Fluorescence
46. Limitations of Pap Smear
• Complex laboratory test
• Requires trained cytotechnician for reading and
pathologist for review
• Continuous monitoring needed to maintain high-
quality results
• Reports often take minimum 1-2 weeks to obtain
• Follow-up of women is difficult
• Usually available only in large cities in many
countries
47. Comparison between :
VIA and Cytology
Sensitivity(%) Specificity (%)
Cytology 47--62 60-95
VIA 76-84 79-83
48. Breast
• Population - women, age 20 +
Breast self-examination Monthly, starting at age 20
Clinical breast examination Every three years,
age 20-39
Annual, starting at age 40
*
Mammography Annually, starting at age
40 *
Beginning at age 40, annual clinical breast examination should be
performed prior to mammography. Most other affluent countries
recommend mammography every other year between ages 50 and 70.
49. Levels of Prevention
Disease Onset Clinical
Diagnosis
No Disease Asymptomatic Clinical Course
Disease
Primary Secondary Tertiary
Remove Risk Early Detection Reduce
Factors and Treatment Complications
Fletcher RF, Fletcher SW, Wagner EH. Clinical Epidemiology: The Essentials, 3rd ed. Williams and
Wilkins, Baltimore, 1996.