screening for diseases, public health, public health dentistry

1. SCREENING FOR
DISEASE

2. CONTENTS
o INTRODUCTION
o DEFINITION
o CONCEPT OF SCREENING
o HISTORY OF SCREENING
o SCREENING AND PERIODIC HEALTH EXAMINATIONS
o SCREENING TEST VS DIAGNOSTIC TEST
o CONCEPT OF LEAD TIME
o USES OF SCREENING
o TYPES OF SCREENING
o CRITERIA FOR SCREENING-DISEASE AND SCREENING TEST
o EVALUATION OF SCREENING TEST
o ROC CURVE
o EVALUATION OF SCREENING PROGRAMES
o PUBLIC HEALTH SIGNIFICANCE
o CONCLUSION
o REFERENCES
2

3. INTRODUCTION
• Only one-ninth of the volume of an iceberg is above water.
• The shape of the underwater portion can be difficult to
judge by looking at the portion above the surface.
• Metaphor-health problems.
3

4. ICEBERG PHENOMENON OF DISEASE..
• The tip of the iceberg represents- what the clinicians see
• submerged is the part explored and made visible by the epidemiologists.
4

5. • Screening is defined as the search for unrecognized disease or defect by means of rapidly
applied tests, examinations or other procedures in apparently healthy individuals.
-The United States Commission of Chronic Illness, 1951
5

6. CONCEPT OF SCREENING
The active search for disease among apparently healthy people.
6

7. HISTORY OF SCREENING
• During the World war II(1917)
• To exclude the persons with psychological disorders from joining the United States army.
7

8. SCREENING AND PERIODIC HEALTH
EXAMINATIONS
Screening differs from the periodic health examination in the following aspects:
8

9. SCREENING AND DIAGNOSTIC TESTS
SCREENING TEST DIAGNOSTIC TEST
Done on apparently healthy Done on those with indications or sick
Applied to groups Applied to single patients
Test results are arbitrary and final Diagnosis is not final, it is the sum of all evidence
Based on one criterion Based on evaluation of number of symptoms,
signs and laboratory findings.
9

10. SCREENING AND DIAGNOSTIC TESTS
SCREENING TEST DIAGNOSTIC TEST
Less accurate More accurate
Less expensive More expensive
Not a basis for treatment Used as a basis for treatment
Initiative comes from the investigator or
agency
Initiative comes from a patient with a complaint.
10

11. 11
SOME SCREENING TESTS
 Anaemia
 Hypertension
 Toxemia
 Rh status
 Syphilis (VDRL Test)
 Diabetes
 Cardiovascular disease
 Neural tube defects
 Down's syndrome
 HIV
PREGNANCY
 Congenital dislocation of hip
 Congenital heart disease
 Spina bifida
 Cerebral palsy
 Hearing defects
 Visual defects
 Hypothyroidism
 Developmental screening
tests
 Haemoglobinopathies
INFANCY

12. 12
 Hypertension
 Cancer
 Diabetes mellitus
 Serum cholesterol
 Obesity
MIDDLE-AGED MEN
AND WOMEN
 Nutritional disorders
 Cancer
 Tuberculosis
 Chronic bronchitis
 Glaucoma
 Cataract
ELDERLY

13. CONCEPT OF LEAD TIME
• “Lead time” is the length of time between the detection of disease and its usual
clinical presentation and diagnosis .
OR
• It is the time between early diagnosis with screening ,and diagnosis without
screening.
13

14. MODEL FOR EARLY DETECTION PROGRAMMES
• “Lead time” is the advantage gained by screening
• A –usual outcome of the disease
• B –outcome to be expected when the disease is detected at the earliest possible moment.
• Therefore the benefits of the programme are B-A
• considerable time lag between disease onset and the usual time of diagnosis should be
present.
14

15. POSSIBLE OUTCOMES OF SCREENING
APPARENTLY HEALTHY
APPARENTLY NORMAL
(PERIODIC RE-
SCREENING)
APPARENTLY ABNORMAL
AFTER PERFORMING DIAGNOSTIC TEST
•A)NORMAL-PERIODIC RE-SCREENING
•B)INTERMEDIATE-SURVEILLANCE
•C)ABNORMAL-TREATMENT
15
ON SCREENING

16. USES OF SCREENING
 Case detection
 Control of disease
 Research purpose
 Educational opportunities
16

17. CASE DETECTION
USES OF
SCREENING
• "prescriptive screening".
• It is defined as the presumptive identification of unrecognized disease, which does not
arise from a patient's request, e.g., neonatal screening, etc.
• Since disease detection is initiated by medical and public health personnel, they are
under special obligation to make sure that appropriate treatment is started early.
17

18. CONTROL OF DISEASE
USES OF
SCREENING
• "prospective screening".
• People are examined for the benefit of others, e.g., screening of immigrants from
infectious diseases such as tuberculosis and syphilis to protect the home population;
• The screening programme may, by leading to early diagnosis permit more effective
treatment and reduce the spread of infectious disease and/or mortality from the disease.
18

19. RESEARCH PURPOSES
USES OF
SCREENING
• Screening may aid in obtaining more basic knowledge about the natural history of such
diseases, as for example, initial screening provides a prevalence estimate and
subsequent screening, an incidence figure.
• no follow-up therapy will be available
19

20. EDUCATIONAL OPPORTUNITIES
USES OF
SCREENING
• Screening programmes (as for example, screening for diabetes) provide opportunities
for creating public awareness and for educating health professionals.
20

21. TYPES OF SCREENING
21

22. MASS SCREENING TYPES OF
SCREENING
• Mass screening simply means the screening of a whole population or a sub-group, as
for example, all adults.
• However, when a number of mass screening procedures were subjected to critical
review, there appeared to be little justification.
• Example- Screening for Tuberculosis
22

23. HIGH-RISK OR SELECTIVE SCREENING
TYPES OF
SCREENING
• Screening will be most productive if applied selectively to high-risk groups.
• For example,
• since cancer cervix tends to occur relatively less often in the upper social groups,
screening for cancer cervix in the lower social groups could increase the
yield of new cases.
• elevated serum cholesterol is associated with a high risk of developing coronary heart
disease.
23

24. MULTIPHASIC SCREENING
TYPES OF
SCREENING
• Application of two or more screening tests in combination to a large number of people
at one time.
• The procedure may also include a health questionnaire, clinical examination and a range
of measurements and investigations (e.g., chemical and haematological tests on blood
and urine specimens, lung function assessment, audiometry and measurement of visual
acuity) — all of which can be performed rapidly with the appropriate staffing
organization and equipment.
• On the other hand, it has increased the cost of health services without any observable
benefit.
24

25. CRITERIA FOR SCREENING
The criteria for screening are based on two considerations:
• the DISEASE to be screened,
• the TEST to be applied.
25

26. DISEASE
CRITERIA FOR SCREENING
• should be an important health problem (in general, prevalence should be high);
• should have a recognizable latent or early asymptomatic stage;
• The natural history of the condition, including development from latent to
declared disease, should be adequately understood (so that we can know at what
stage the process ceases to be reversible);
26

27. DISEASE
CRITERIA FOR SCREENING
• Facilities should be available for confirmation of the diagnosis;
• Effective treatment available for the disease;
• There is good evidence that early detection and treatment reduces morbidity and
mortality;
• The expected benefits (e.g., the number of lives saved) of early detection exceed the
risks and costs.
27

28. CRITERIA FOR SCREENING
28

29. ACCEPTABILITY
CRITERIA FOR SCREENING TEST
• Since a high rate of cooperation is necessary, it is important that the test should be
acceptable.
• In general, tests that are painful, discomforting or embarrassing (e.g., rectal or
vaginal examinations) are not likely to be acceptable.
29

30. REPEATABILITY
CRITERIA FOR SCREENING TEST
• The test must give consistent results when repeated more than once on the same
individual or material, under the same conditions.
• The repeatability of the test depends upon three major factors,
• observer variation
• biological (or subject) variation and
• errors relating to technical methods.
30

31. OBSERVER VARIATION
CRITERIA FOR SCREENING TEST-REPEATABILITY
All observations are subjected to variation (or error). These may be of two types:
• Intra-observer variation
• Inter-observer variation
31

32. OBSERVER VARIATION
Intra-observer variation
If a single observer takes two measurements (e.g., blood pressure, chest expansion)
in the same subject, and each time, he obtained a different result, this is termed as
intra-observer or within-observer variation.
Intra-observer variation may often be minimized by taking the average of several
replicate measurements at the same time.
CRITERIA FOR SCREENING TEST-REPEATABILITY
32

33. OBSERVER VARIATION
Inter-observer variation
• This is variation between different observers on the same subject or material, also
known as between-observer variation.
• Inter-observer variation has occurred if one observer examines a blood-smear and
finds malaria parasite, while a second observer examines the same slide and finds it
normal.
CRITERIA FOR SCREENING TEST-REPEATABILITY
33

34. Observational errors are common in the interpretation of X-rays, ECG tracings, readings
of blood pressure and studies of histopathologica1 specimens.
Observer errors can be minimized by
(a) standardization of procedures for obtaining measurements, and classifications
(b) intensive training of all the observers
(c) making use of two or more observers for independent assessment, etc.
OBSERVER VARIATION
CRITERIA FOR SCREENING TEST-REPEATABILITY
34

35. BIOLOGICAL (SUBJECT) VARIATION
CRITERIA FOR SCREENING TEST-REPEATABILITY
There is a biological variability associated with many physiological variables such as
blood pressure, blood sugar, serum cholesterol, etc.
The fluctuation in the variate measured in the same individual may be due to:
(a)Changes in the parameters observed
(b)Variations in the way patients perceive their symptoms and answer
(c)Regression to the mean
35

36. ERRORS RELATING TO TECHNICAL METHODS
CRITERIA FOR SCREENING TEST-REPEATABILITY
Lastly, repeatability may be affected by variations inherent in the method.
e.g., defective instruments, incorrect calibration, faulty reagents; or the test itself might
be inappropriate or unreliable. Where these errors are large, repeatability will be reduced,
and a single test result may be unreliable.
36

37. VALIDITY(ACCURACY)
CRITERIA FOR SCREENING TEST-VALIDITY
• The term validity refers to what extent the test accurately measures which it
purports to measure. In other words, validity expresses the ability of a test to
separate or distinguish those who have the disease from those who do not.
• For example, glycosuria is a useful screening test for diabetes, but a more valid
or accurate test is the glucose tolerance test.
37

38. VALIDITY(ACCURACY)…
CRITERIA FOR SCREENING TEST-VALIDITY
• Validity has two components — sensitivity and specificity.
• When assessing the accuracy of a diagnostic test, one must consider both these
components.
• Both measurements are expressed as percentages.
38

39. SENSITIVITY
• The term sensitivity was introduced by Yerushalmy in 1940
• It has been defined as the ability of a test to identify correctly all those who have
the disease, that is "true-positive".
• 90% sensitivity means that 90% of the diseased people screened by the test will
give a "true-positive" result.
39

40. SPECIFICITY
• It is defined as the ability of a test to identify correctly those who do not have
the disease, that is, "true-negatives".
• 90% specificity means that 90% of the non-diseased persons will give "true-
negative" result, 10 per cent of non-diseased people screened by the test will be
wrongly classified as "diseased" when they are not.
40

41. SCREENING TEST RESULT BY DIAGNOSIS
CRITERIA FOR SCREENING TEST-VALIDITY
SCREENING TEST
RESULTS
DIAGNOSIS TOTAL
DISEASED NOT
DISEASED
POSITIVE a (True-positive) b(False-positive) a+b
NEGATIVE c (False-
negative)
d(True-
negative)
c+d
TOTAL a+c b+d a+b+c+d
41
(a) Sensitivity = a/ (a + c) x 100
(b) Specificity = d/(b + d) x 100
(c) Predictive value of a positive test = a/(a + b)x 100
(d) Predictive value of a negative test = d/(c + d) x 100
(e) Percentage of false-negatives = c/(a + c) x 100
(f) Percentage of false-positive = b/(b + d)x 100

42. NAKED EYE RESULTS ORAL CANCER
PRESEN
T
ABSENT
POSITIVE 36 54,000
NEGATIVE 4 306,000
40 360,000
42
Sensitivity=36/40 * 100=90%
Specificity=306,000/360,000 *
100=85%
SMEAR
RESULTS
ORAL CANCER
PRESENT ABSENT
POSITIVE 39 18,000
NEGATIVE 1 342,000
40 360,000
Sensitivity=39/40 * 100=97.5%
Specificity=342,000/360,000 *
100=95%
EVALUATION OF A SCREENING TES

43. 43
BLOOD
GLUCOSE
LEVEL(mg/100
ml)
SENSITIVITY SPECIFICITY
80 100 1.2
90 98.6 7.3
100 97.1 25.3
110 92.9 48.4
120 88.6 68.2
130 81.4 82.4
140 74.3 91.2
150 64.3 96.1
160 55.7 98.6
170 52.9 99.6
180 50 99.8
EVALUATION OF A SCREENING TES
SENSITIVITY AND SPECIFICITY OF A 2-HOUR POSTPRANDIAL BLOOD TEST FOR GLUCOSE FOR 70
TRUE DIABETICS AND 510 TRUE NON-DIABETICS AT DIFFERENT LELVES OF BLOOD GLUCOSE

44. 44
• Reflects the diagnostic power of the test.
• The predictive accuracy depends upon sensitivity, specificity and disease
prevalence.
• The "predictive value of a positive test" indicates the probability that a patient with
a positive test result has, in fact, the disease in question.
• The more prevalent a disease is in a given population, the more accurate will be
the predictive value of a positive screening test.
PREDICTIVE ACCURACY
EVALUATION OF A SCREENING TES

45. 45
CULTURE
POSITIVE NEGATIV
E
SMEAR POSITIVE 25 95
NEGATIV
E
25 855
TOTAL 50 950
POSTIVE PREDICTIVE
VALUE=25/120 *
100=21%
PREVALENCE 5%
CULTURE
POSITIVE NEGATIV
E
SMEAR POSITIVE 75 85
NEGATIV
E
75 765
TOTAL 150 850
POSTIVE PREDICTIVE
VALUE=75/160 *
100=47%
PREVALENCE 15%
CULTURE
POSITIVE NEGATIV
E
SMEAR POSITIVE 125 75
NEGATIV
E
125 675
POSTIVE PREDICTIVE
VALUE=125/200 *
100=62.5%
PREVALENCE 25%
EVALUATION OF A SCREENING TES
PREVALENCE POSITIVE PREDICTIVE
VALUE

46. FALSE-NEGATIVES
46
• The term "false-negative" means that patients who actually have the disease are
told that they do not have the disease.
• might ignore the development of signs and symptoms and may postpone the
treatment.
• A screening test which is very sensitive has few "false negatives". The lower
the sensitivity, the larger will be the number of false negatives.
EVALUATION OF A SCREENING TES

47. FALSE-POSITIVES
47
• The term "false-positive" means that patients who do not have the disease are told
that they have the disease.
• In this case, normal healthy people may be subjected to further diagnostic tests, at
some inconvenience, discomfort, anxiety and expense — until their freedom from
disease is established.
• A screening test with a high specificity will have few false positives. False-
positives not only burden the diagnostic facilities, but they also bring discredit to
screening programmes.
EVALUATION OF A SCREENING TES

48. 48
SCREENING TEST
RESULTS
DIAGNOSIS TOTAL
DISEASE
D
NOT
DISEASE
D
POSITIVE 40
(a)
20
(b)
60
(a+b)
NEGATIVE 100
(c)
9,840
(d)
9,940
(c+d)
140
(a+c)
9,860
(b+d)
10,000
(a+b+c+d)
1)Sensitivity=(40/140)*100=28.57%
2)Specificity=(9840/9860)*100=99.79%
3)False-negative=(100/140)*100=71.4%
4)False-positive=(20/9860)*100=0.20%
5)Predictive value of a positive test=(40/60)*100=66.66% of a positive test
6)Predictive value of a negative test=(9840/9940)*100=98.9% of a negative test

49. YIELD
49
• "Yield" is the amount of previously unrecognized disease that is diagnosed as a
result of the screening effort.
• It depends upon many factors, viz. sensitivity and specificity of the test, prevalence
of the disease, the participation of the individuals in the detection programme.
• For example, by limiting a diabetes screening programme to persons over 40 years,
we can increase the yield of the screening test. High-risk populations are usually
selected for screening, thus increasing yield.
EVALUATION OF A SCREENING TES

50. THE PROBLEM OF THE BORDERLINE
50
The question arises which of the two qualities (sensitivity or specificity) is more
important in screening? No categorical answer can be given.

51. THE PROBLEM OF THE BORDERLINE…
51
• If the disease is bimodal, as may be expected in certain genetically transmitted
characteristics such as phenylketonuria, the shaded area or the "borderline" group will
comprise a mixture of persons with the disease and persons without the disease (i.e., a
mixture of false negatives and false positives).
• The point at which the distributions intersect (i.e., at level E) is frequently used as the
cut-off point.

52. UNIMODAL DISTRIBUTION
52
Examples-blood pressure, blood sugar and serum cholesterol
• The question arises whether the cut-off point between "disease" and "normality"
should be set at C or D. If the cut-off point is set at the level of C, it will render
the test highly sensitive, missing few cases but yielding many false positives.
• If the cut-off point is set at D, it will increase specificity of the test. Furthermore,
in the unimodal distribution, once a cut-off point level has been adopted, all persons
above that level (i.e., above level C or D) would be regarded as "diseased".

53. 53
BLOOD
GLUCOSE
LEVEL(mg/100
ml)
SENSITIVITY SPECIFICITY
80 100 1.2
90 98.6 7.3
100 97.1 25.3
110 92.9 48.4
120 88.6 68.2
130 81.4 82.4
140 74.3 91.2
150 64.3 96.1
160 55.7 98.6
170 52.9 99.6
180 50 99.8
190 44.3 99.8
SENSITIVITY AND SPECIFICITY OF A 2-HOUR POSTPRANDIAL BLOOD TEST FOR GLUCOSE FOR 70
TRUE DIABETICS AND 510 TRUE NON-DIABETICS AT DIFFERENT LELVES OF BLOOD GLUCOSE
• If the cut off point is lowered (say less than 120 mg),
the sensitivity of the test is increased at the cost of
specificity.
• If the cut-off point is raised (say to 180 mg per cent),
the sensitivity is decreased

54. 54
(a) Disease prevalence: When the prevalence is high in the community, the
screening level is set at a lower level, which will increase sensitivity.
(b)The disease: If the disease is very lethal (e.g., cervical cancer, breast cancer,
oral cancer) and early detection markedly improves prognosis, a greater
degree of sensitivity is desired.
In these cases, subsequent diagnostic work-up can be relied on to rule out the
disease in the false-positives. That is, a proportion of false-positives is
tolerable but not false-negatives.

55. 55
• On the other hand, in a prevalent disease like diabetes for which treatment
does not markedly alter outcome, specificity must be high and early cases
may be missed, but false-positives should be limited;
• Otherwise the health system will be overburdened with diagnostic
demands on the positives, both true and false

56. RECEIVER OPERATING CHARECTERISTIC CURVE
(ROC CURVE)
56
This is an effective method for assessing the performance of
a diagnostic test.
This curve is useful in
(i) finding optimal cut-off point to least misclassify diseased
or non-diseased subjects
(ii) comparing the efficacy of two or more tests for assessing
the same disease
(iii) comparing two or more observers measuring the same
test (inter-observer variability).
The area under the curve (AUC) is an effective and
combined measure of sensitivity and specificity for
assessing validity of a diagnostic test. Maximum
AUC = 1 and it means the test is perfect

57. EVALUATION OF SCREENING PROGRAMMES
57
Many screening tests were introduced in the past without subjecting them to
proper evaluation. They were introduced because it was thought a good thing to
detect and treat cases before they should reach an advanced stage.
The modern view is that new screening programs should be introduced only after
proper evaluation.

58. 58
(1) Randomized controlled trials
Ideally RCT should be performed in the setting where the screening programme
will be implemented, and should employ the same type of personnel, equipment
and procedures that will be used in that programme.
If the disease has a low frequency in the population, and a long incubation period
(e.g., cancer) RCT may require following tens of thousands of people for 10-20
years with virtually perfect record keeping. The cost and logistics are often
prohibitive.

59. 59
Uncontrolled trials:
Sometimes, uncontrolled trials are used to see if people with disease
detected through screening appear to live longer after diagnosis and treatment
than patients who were not screened. One such example is uncontrolled studies
of cervical cancer screening which indicated that deaths from that disease could
be very much reduced if every woman was examined periodically.

60. 60
Other methods: There are also other methods of evaluation such as case control
studies and comparison in trends between areas with different degrees of
screening coverage. Thus it can be determined whether intervention by screening
is any better than the conventional method of managing the disease.

61. National programme for prevention and control of cancer,
diabetes, cardiovascular diseases and stroke(NPCDCS)
61
Health promotion, awareness
generation and promotion of healthy
lifestyle
Screening and early detection
Timely, affordable and accurate
diagnosis
Access to affordable treatment
Rehabilitation
STRATEGIES
PUBLIC HEALTH SIGNIFICANC

62. ALGORITHM FOR
MANAGEMENT OF
DIABETES
62

63. 63
PUBLIC HEALTH SIGNIFICANC

64. 64
PUBLIC HEALTH SIGNIFICANCE

65. Register for screening common NCDs
65

66. Checklist for
screening of
common NCDs
66

67. 67
PUBLIC HEALTH SIGNIFICANC

68. VITAL
STAINING
68
VIZILITE VELSCOPE BRUSH
BIOPSY

69. RASHTRIYA BAL SWASTHYA KARYAKRAM
• This programme has been launched (2013) under National rural health mission
initiated by the Ministry of health and family welfare, therefore, aims at early
detection and management of the 4Ds prevalent in children.
• Defects at birth, Diseases in children, Deficiency conditions, Developmental
delays including disabilities.
• Rashtriya bal swasthya karyakram aims at screening over 27 crore children from
0 to 18 years
69
PUBLIC HEALTH SIGNIFICANC

70. RASHTRIYA BAL SWASTHYA KARYAKRAM…
70
Defects at Birth Deficiencies Diseases of childhood Developmental delays
and Disabilities
1.Neural tube defect
2. Down's Syndrome
3. Cleft Lip & Palate /
Cleft palate alone
4. Talipes (club foot)
5. Developmental
dysplasia of the hip
6. Congenital
cataract
7. Congenital
deafness
8. Congenital heart
diseases
9. Retinopathy of
Prematurity
10. Anemia
especially Severe
anemia
11. Vitamin A
deficiency (Bitot
spot)
12. Vitamin D
Deficiency, (Rickets)
13. Severe Acute
Malnutrition
14. Goiter
15. Skin conditions
(Scabies, fungal
infection and
Eczema)
16. Otitis Media
17. Rheumatic heart
disease
18. Reactive airway
disease
19.Dental conditions
20. Convulsive
disorders
21. Vision
Impairment
22. Hearing
Impairment
23. Neuro-motor
Impairment
24. Motor delay
25. Cognitive delay
26. Language delay
27. Behavior disorder
(Autism)
28. Learning
disorder
29. Attention deficit
hyperactivity
disorder
30. Congenital Hypothyroidism, Sickle cell anemia, Beta thalassemia (Optional)
PUBLIC HEALTH SIGNIFICANC

71. RASHTRIYA BAL SWASTHYA KARYAKRAM…
• Babies born at public health facilities and home - birth to 6 weeks
• Preschool children in rural areas and urban slum - 6weeks to 6 years
• School children enrolled in class 1st and 12th in
Government and government aided schools - 6yrs to 18 yrs
71
TARGET GROUP
PUBLIC HEALTH SIGNIFICANC

72. 72
Screening helps to detect the disease at its early stage and prevent disabilities arising
out of the disease.
To conclude, the screening concept, filled with potential has been overburdened
with problems, many of which remain unsolved. The construction of accurate tests
that are both sensitive and specific is a key obstacle to the wide application of
screening.
CONCLUSION
AN OUNCE OF PREVENTION IS WORTH A POUND OF CURE!!!!!

73. REFERENCES
73
1) Park K. Textbook of preventive and social medicine; 24th ed. Bhanot Publishers 2017.
2) Jong AW. Community Dental Health. 3rd ed. Mosby Publsihers 1993
3) Operational Framework – Management of Common Cancers. Ministry of Health and Family
Welfare: Government of India; 2016
4) Operational Guidelines on Prevention Screening and Control of Common Non Communicable
Diseases. Ministry of Health and Family Welfare, Government of India; 2016.
5) Training Module for Medical Officers for Prevention, Control and Population Level Screening of
Hypertension, Diabetes and Common Cancer (Oral, Breast & Cervical). Ministry of Health and
Family Welfare, Government of India; 2017.
6) Lingen MW et al., Critical evaluation of diagnostic aids for the oral cancer. Oral Oncol
2007;6(11):1-13.

74. 74