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Screening For Disease
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
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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Screening


Purpose:

 Delay onset of symptomatic or
 clinical disease. Improve survival.



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Screening

For screening to be successful you need a:
   Suitable disease
   Suitable test
   Suitable screening program




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Screening for disease (ravi)
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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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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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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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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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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Suitable Test


 Reliable Test: Gives you same results on
  repetition



 Validity is more important than reliability



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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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                                                      ng
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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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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?)
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
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)
Screening for disease (ravi)
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
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
Reliability (reproducibility)


             Inter-Observer Agreement in Grading Severity of Cataract
                     Examiner 1: Grade
Examiner<1      1-<2     2-<3      3-<4      4
   2
<1             10          2          1              0          0
1-<2           1           20         2              0          0
2-<3           0           1         20              1          0
3-<4           0           0          1              10         2
4              0           0          0              2          5
               % Agreement = 81.3%
               Kappa = 0.76
Yield from a Screening Test for Disease X
                  Predictive Value


                       Screening Test
                                                     X
X                                                X


                                          X


                                                     X
              X


                  Negatives          Positives
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
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%
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
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
Lead Time Bias

       screen-detectable    clinically evident


                                                   death


                           5 years

                              clinically evident


                                                   death

                                         2 years
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
Diagnostic and Screening
Tests
Tests

Diagnostic     Screening
PSA Performance (ROC) Curve
Sensitivity (TP/[TP+FN])   100

                           80

                           60                            Urologic
                                                         practice
                           40
                                                         Community
                           20                            screening

                            0
                                 0    20     40     60   80    100
                                  1-Specificity (FP/[TN+FP])
Lead-time bias.




               Jaar B G et al. CJASN 2008;3:601-609



©2008 by American Society of Nephrology
Screening Generally
        Is to seek about certain
        problem in certain high
        risk gp.
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
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
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
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
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
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
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
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?
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.
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.
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
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
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
Comparison between :
VIA and Cytology

            Sensitivity(%)    Specificity (%)
 Cytology   47--62           60-95

 VIA             76-84             79-83
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.
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.

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Screening for disease (ravi)

  • 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. Scr eeni ng 3
  • 4. Screening Purpose: Delay onset of symptomatic or clinical disease. Improve survival. Scr eeni ng 4
  • 5. Screening For screening to be successful you need a:  Suitable disease  Suitable test  Suitable screening program Scr eeni ng 5
  • 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 Scr eeni ng 7
  • 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 Scr eeni ng 8
  • 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 Scr eeni ng 9
  • 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 Scr eeni ng 10
  • 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 Scr eeni ng 11
  • 12. Suitable Test  Reliable Test: Gives you same results on repetition  Validity is more important than reliability Scr eeni ng 12
  • 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 Scr eeni ng 13
  • 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 Scr eeni ng 14
  • 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
  • 21. Reliability (reproducibility) Inter-Observer Agreement in Grading Severity of Cataract Examiner 1: Grade Examiner<1 1-<2 2-<3 3-<4 4 2 <1 10 2 1 0 0 1-<2 1 20 2 0 0 2-<3 0 1 20 1 0 3-<4 0 0 1 10 2 4 0 0 0 2 5 % Agreement = 81.3% Kappa = 0.76
  • 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
  • 30. Tests Diagnostic Screening
  • 31. PSA Performance (ROC) Curve Sensitivity (TP/[TP+FN]) 100 80 60 Urologic practice 40 Community 20 screening 0 0 20 40 60 80 100 1-Specificity (FP/[TN+FP])
  • 32. Lead-time bias. Jaar B G et al. CJASN 2008;3:601-609 ©2008 by American Society of Nephrology
  • 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.