1. 1. Dr. Mohan Lal Associate Professor Department of Community Medicine Govt. Medical College ,Amritsar
2. 2. Iceberg Phenomenon of Disease Major submerged portion consists of :- Subclinical cases Carriers Undiagnosed cases (apparently healthy individuals) Constitute a mass of unrecognized disease in the community. Responsible for the constant prevalence of disease . Detection and a control is challenge .
3. 3. 1 Diseased, diagnosed & controlled 2 Diagnosed, uncontrolled 3 Undiagnosed or wrongly diagnosed disease 4 Risk factors for disease 5 Free of risk factors Diagnosed disease Undiagnosed or wrongly diagnosed disease
4. 4. Defined as the search for unrecognized disease or defect by means of rapidly applied test, examination or other procedures in apparently healthy individuals
5. 5. Detection programmes should be restricted to those conditions in which there is considerable time lag between disease onset and usual time of diagnosis Lead time is advantage gained by screening i.e. period between diagnosis by early detection and diagnosis by other means
6. 6. Aims To sort out those having the disease and those not having the disease from a group of apparently healthy individuals.
7. 7. Objectives To provide treatment to those detected persons, so that the disease is controlled in community
8. 8. Considerations of Screening Screening must be applied to those people most likely to benefit. Selection must be based on the persons age, sex, medical history, occupation, family history, etc. Test with greater accuracy may be more expensive and time consuming, so choice of test is based on compromise.
9. 9. Screening should be integrated in to existing health services The risks as well as expected benefits must be explained to people to be screened Regular patient follow up is also important
10. 10. Criteria for Screening Public health importance Recognizable early stage Disease can be diagnosed before the onset of signs and symptoms Facilities available to confirm diagnosis Must be effective treatment Disease with treatment , Good prognosis
11. 11. Screening Disease Important Public health problem- High Prevalence & Serious outcome Recognized in early/ latent phase Natural history well understood- Early detection and treatment can affect the course of disease Test Safe, Accurate, Cost effective, Suitable & Acceptable Treatment Diagnostic Test Agreed policy & Treatment Cost Cost of screening< Cost of medical care Continuing process and not a "once for all" project.
12. 12. Problem of borderline In screening for disease, a prior decision is made about the cut off point The following factors are take into consideration: Disease prevalence: when the prevalence is high in the community, its screening level is set at lower level, which will increase sensitivity The disease: if the disease is very lethal and early detection markedly improves prognosis, a greater degree of sensitivity, even at the expense of specificity, is desired
13. 13. Uses of Screening Case detection (Prescriptive screening) Control of the disease(Prospective screening) Research purpose Educational purpose
14. 14. Case detection It is also known as prescriptive screening. Defined as presumptive identification of unrecognized disease which does not arise from patients request. e.g. neonatal screening. Diseases sought include breast cancer, cervical cancer, diabetes, tuberculosis, HDN etc. Initiated by medical and public health personnel, they are under special obligation to make sure that appropriate treatment is started early.
15. 15. Control of the disease Also known as prospective screening. People are examined for benefit of others. e.g. screening of immigrants from infectious diseases such as tuberculosis, syphilis, streptococcal infection etc. 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.
16. 16. Educational opportunities Screening programs provide opportunities for creating public awareness and for educating health professionals. eg screening for diabetes.
17. 17. Research purposes Screening helps in obtaining basic knowledge about natural history of chronic diseases. Initial screening provides prevalence estimate and subsequent screening, an incidence figure. Where screening is done for research purposes, the investigator should inform the study participant that no follow up therapy will be available.
18. 18. DISEASE The disease to be screened should fulfill the following criteria before its considered suitable for screening 1) The condition sought should be an important health problem. 2) There should be a recognizable latent or early asymptomatic stage. 3) The natural history of the condition, including development from latent to declared disease, should be adequately understood.
19. 19. Flow Chart Apparently healthy Screening test Probably have disease Diagnostic test Probably do not have disease Have disease Do not have disease Given treatment Under surveillance Periodic screening (Screening)
20. 20. Types of Screening Mass screening High risk screening Multipurpose screening Multiphase screening Opportunistic screening
21. 21. Mass screening Means screening of the whole population or a sub group It is offered to all irrespective of particular risk individual may run of contracting disease in question. Indiscriminate mass screening is not a useful preventive measure unless backed up by suitable treatment that will reduce duration of illness or alter final outcome.
22. 22. High risk or selective screening Screening is most productive if applied selectively to high risk groups, groups defined on epidemiological research. Diseases tend to aggregate in family so screening other members of family can detect additional cases. Epidemiologists have extended concept of screening to risk factors.
23. 23. Multi phasic screening Defined as application of two or more screening test in combination to large number of people at one time than to carry out separate screening test for single diseases. Procedure includes health questionnaire, clinical examination and range of measurements and investigations. E.g. chemical and hematological tests on blood and urine, lung function assessment, audiometry etc.
24. 24. Recently multi phasic screening has not shown any benefit to population in terms of mortality and morbidity reduction. Rather it has increased the cost of health services without any observable benefit.
25. 25. Thus screening test divides the apparently healthy population in to two groups Those probably having the disease Those probably not having disease
26. 26. The first group is then further subjected to history taking ,clinical examination and diagnostic tests Which divides this group into two sub- groups Those who have the disease , requiring treatment Those not having the disease ,requiring Surveillance and periodic screening
27. 27. Population (1) Examination of urine sugar Persons with negative result Persons with positive result (2) Examination of random blood for sugar level Persons with blood glucose level 120 mgm % (3) Oral glucose tolerance test Blood sugar 180 mgm% 2 hrs after glucose Non- diabetic Impaired glucose tolerance diabetic
28. 28. Screening differs from periodic health examination o Capable of wide application o Relatively inexpensive and o Requires little physician- time
29. 29. Screening test Diagnostic test Done on apparently healthy people Done on sick people Done on groups Done on individual cases Done by the epidemiologist Done by the physician Test results are final Diagnosis is not final but based on other criteria such as history and clinical findings The purpose is to do community diagnosis ,to launch a control programme The purpose is to make a diagnosis in the patient to give treatment Less expensive More expensive Initiative is from the epidemiologist Initiative is from patient
30. 30. Criteria for screening test Should be Simple, Safe cheap and rapidly applied Acceptable by the people Reliable (repeatable or reproducible)
31. 31. Some Screening Tests Pregnancy Infancy Anemia Hypertension Toxemia Rh status Syphilis Cardiovascular disease Neural tube defects Down syndrome HIV Congenital heart disease Spina bifida visual defects Hypothyroidism Haemoglobinopathies Sickle cell anemia Middle-aged men and women Elderly hypertension cancer Diabetes mellitus Serum cholesterol Nutritional disorders Cancer Glaucoma Cataract
32. 32. SCREENING TEST The test must satisfy the criteria of: Acceptability Repeatability Validity besides other such as yield, simplicity, safety, rapidity, ease of administration and cost also.
33. 33. ACCEPTABILITY Since a high rate of cooperation is necessary, its important that the test should be acceptable to the people at whom it is aimed. REPEATABILITY The test must give consistent results when repeated more than once. It depends on 3 major factors: -- Observer variation -- Biological (Subject) variation -- Errors relating to technical methods
34. 34. Variations Observer variations A) Intra-observer B) Inter-observer Biological Variations Mechanical variations Valid (accurate)
35. 35. OBSERVER VARIATIONS These are of 2 types: a) Intra-observer variations If a single observer takes two measurements (e.g. BP & Chest expansion) in the same subject, at the same time and each time he obtains a different result, this is termed as intra-observer or within-observer variation. b) Inter-Observer variation this is variation b/w diff. observers on the same subject or material, also known as Between-observer variation.
36. 36. BIOLOGICAL (SUBJECT) VARIATIONS There is a biological availability associated with many physiological variables such as BP, 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. E.g. Cervical smears taken from the same woman may be normal one day, and abnormal on other day.
37. 37. b) Variations in the way patients perceive their symptoms and answers. c) Regression to mean : There is tendency for values at the extreme of distribution, to regress toward the mean or average on repeat measurements.
38. 38. Contd. E.g. elevated serum cholesterol is associated with high risk of developing coronary heart disease. In this way preventive measures can be applied before the disease occurs.
39. 39. ERRORS RELATING TO TECHNICAL METHODS: Repeatability may be affected by variations inherent in the method. e.g. -- Defective instruments -- Erroneous calibration -- Faulty reagents -- Test inappropriate or unreliable
40. 40. VALIDITY (ACCURACY) 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.
41. 41. Diagnosis (screening test results) Diseased Not diseased Total Positive (True positive ) a (False positive) b (Total positive) a+b Negative (False negative) c (True negative) d (Total negative) c+d Total (Total disease) a+c (Total non diseased) b+d (Grand total) a+b+c+d
42. 42. Validity has two components: Sensitivity Specificity When assessing the accuracy of a diagnostic test, one must consider both these components. Both measurements are expressed in percentages. Sensitivity & Specificity are usually determined by applying the test, to one group of persons having disease, and to a reference group not having the disease.
43. 43. Evaluation of screening test
44. 44. Indicators To evaluate screening test: Sensitivity Specificity Predictive value of positive test Predictive value of negative test Percentage of false negatives Percentage of false positives
45. 45. Sensitivity Introduced by Yerushalmy as statistical index of diagnostic accuracy. Defined as the ability of test to identify correctly all those who have the disease, that is true positive.
46. 46. a Sensitivity = x100 a+c Percentage of diseased persons ,showing the test result positive Ability of a test to correctly identify those having disease (true positive )
47. 47. d Specificity= x100 b+d Ability of the test to correctly identify those not having the disease Percentage of non-diseased persons showing the negative results
48. 48. Combination of tests Two or more tests can be used in combination to enhance specificity or sensitivity of screening. For example syphilis screening (RPR test) has high sensitivity, yet will yield false positives. However, all those positive to RPR are then submitted to FTA-ABS which is more specific test and the resultant positives now truly have syphilis.
49. 49. Predictive accuracy Performance of screening test is measured by its predictive value which reflects diagnostic power of test. Predictive accuracy depends upon sensitivity, specificity and disease prevalence. More prevalent a disease in a given population, more accurate will be the predictive value of a positive screening test.
50. 50. a Predictive value ( + test)= x100 a+b Percentage of positive probably having the disease
51. 51. d Predictive value (-test)= x100 c+d Percentage of negative probably not having the disease
52. 52. False positives Means patients who do not have the disease are told that they have. It causes inconvenience, discomfort, anxiety and expense to normal healthy people. Screening test with high specificity will have few false positives
53. 53. b False positive = x100 b+d Percentage of non-diseased persons wrongly identified as having a disease
54. 54. False negatives Means patients who actually have the disease are told that they do not have the disease. So patient might ignore the development of signs and symptoms and may postpone the treatment. A screening test which is very sensitive has few false negatives. Lower the sensitivity, larger the number of false negatives.
55. 55. False negative = x100 Percentage of diseased persons wrongly identified as not having the disease C a+c
56. 56. That is, a proportion of false positives is tolerable but not false negatives. On the other hand, in a prevalent disease like diabetes for which treatment does not markedly alter outcome, specificity must be high and false positives should be limited.
57. 57. Yield It is the amount of previously unrecognized disease that is diagnosed as a result of screening effort. It depend on sensitivity and specificity of the test, prevalence of the disease and participation of individuals in the detection program.
58. 58. Gratitude for all authors and sources whose material has been used for this Presentation