Screening of disease

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Screening of disease:

Screening of disease Dr Manju Kerketta

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The pattern of disease in hospitals is different from disease in the community Large proportion of disease is hidden from the physicians or even the general population; iceberg phenomena = only the tip is seen – used to describe disease widely in the community

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Iceberg phenomena - Floating tip – what the physician sees in his practice Submerged portion of iceberg represents hidden mass of unrecognized disease (subclinical disease, carriers, undiagnosed cases) Detection and control of this hidden mass of disease is a challenge to modern techniques in preventive medicine


Screening Active search for disease in apparently healthy people is a fundamental aspect of prevention = screening Definition: ‘The search for unrecognized disease or defect by means of rapidly applied tests, examinations or other procedures in apparently healthy individuals’


Screening Historically- annual health examinations – meant for early detection of hidden disease Screening programs for individual diseases – Tb, syphilis or; selected groups such as antenatal mothers, school children, occupational groups Screening – a preventive care function – a logical extension of health care

Some screening tests:

Some screening tests Pregnancy Anemia Hypertension Toxemia Rh Status Syphilis (VDRL ) Diabetes CVD Neural tube defects Down’ Syndrome HIV Infancy LCB CDH CHD Spina Bifida Cerebral palsy Hearing defects Visual defects Hypothyroidism Developmental screening tests Hemoglobinopathies Sickle cell disease Undescended testis Middle aged men & women Hypertension Cancer Diabetes mellitus Serum cholesterol obesity Elderly Nutritional disorders Cancers Tuberculosis Chronic Bronchitis Glaucoma Cataract

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Screening differs from periodic health examination in the following respects: Capable of wide application Relatively inexpensive Requires little physician time – only to interpret the test result

Screening & diagnostic test:

Screening & diagnostic test Screening test used for initial examination Those with positive test result – referred to physician for further diagnostic work-up and treatment Some tests used for both screening and diagnosis. Example test for anemia and GTT

Screening & diagnostic test:

Screening & diagnostic test Screening Test Diagnostic Test Done on apparently healthy Applied to groups Test results are arbitrary and final Based on criterion or cut off point Less accurate Less expensive Not a basis for treatment The initiative come from the investigator or the agency providing care Done on those with indications or sick Applied to single patients Diagnosis is not final but modified in light of new evidence; diagnosis is the sum of all evidence Based on the evaluation of a number of signs /symptoms and lab findings More accurate More expensive Used a basis for treatment Initiative comes from patient with complaint


LEAD TIME Onset of Disease OUTCOME 1 st possible detection Final Critical Point Usual time of diagnosis LEAD TIME SCREENING TIME

Lead time:

Lead time Detection programs should be restricted to those conditions in which there is considerable time lag between disease onset and usual time of diagnosis Lead time is the advantage gained by screening

Possible outcomes of screening:

Possible outcomes of screening Apparently healthy (Screening tests) Apparently normal (periodic re-screening) Apparently abnormal Normal -periodic re-screening Intermediate - surveillance Abnormal -treatment

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Screening Is testing for infection or disease in population or individuals who are not seeking healthcare Serological testing of AIDS virus in blood donors, Neonatal screening and premarital screening for syphilis Case finding Usual clinical/lab test to detect disease in individuals seeking health care for other reasons VDRL to detect syphilis in pregnant women

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Diagnostic tests Clinical /lab procedures to confirm or refute the existence of disease or true abnormality in patients with signs and symptoms presumed to be caused by the disease VDRL – in patients with lesions suggestive of secondary syphilis

Uses of screening:

Case detection Control of disease Research purposes Educational opportunities Uses of screening

Case detection::

Case detection : Also known as prescriptive screening Definition: ‘the presumptive identification of unrecognized disease, which does not arise from a patient’s request’ Example : neonatal screening Appropriate treatment should be started in those found positive

Control of Disease::

Control of Disease: Also known as prospective screening - people screened for the benefit of others Example : screening of immigrants from infectious diseases such as tuberculosis and syphilis to protect the home population; screening for streptococcal infection to prevent rheumatic fever Screening – early diagnosis – effective treatment – reduce the spread of infectious disease and or mortality from the disease

Research purposes::

Research purposes: For chronic disease whose natural history is not fully known Educational opportunities For creating public awareness and educating health professionals

Types of screening:

Mass screening High risk screening Multi-phasic screening Types of screening

Mass screening:

Mass screening Screening of a whole population or a sub group Not a useful tool unless it is backed up a by suitable treatment that will reduce the duration of illness or alter its final outcome

High-risk or selective screening:

High-risk or selective screening Screening most productive if selectively applied to high risk groups Example Ca Cx less common in upper social groups, so screening for Ca Cx in lower social groups will increase the yield of new cases Certain disease tend to be aggregated in families (HTN, DM, Breast cancer) - screening other members of the family - can yield additional cases Screening for ‘risk factors’ – which antedate the development of actual disease Resource and cost effective

Multi-phasic screening:

Multi- phasic screening Definition : application of two or more screening tests in combination to a large number of people at one time than to carry out separate screening tests for single diseases

Criteria for screening:

Criteria for screening Disease sought to be screened should be a public health problem (high prevalence) Natural history of the condition is well known (to know the stage of reversibility) Disease should have an early pre-pathogenesis stage Facilities should be available for its diagnosis and treatment There should be a suitable test for examination of screening purpose There should be an agreed policy on whom to treat as patients There should be an accepted treatment for patients, if disease is subsequently confirmed Cost benefit ratio is high


Should be acceptable – anything painful, discomforting or embarrassing is unacceptable (rectal or vaginal examination) Repeatable – must give the same result, when repeated on the same individual/material under the same conditions Validity – the extent to which the test accurately measures what it is supposed to measure Sensitivity: Proportion of true positives correctly identified as positive Specificity: Proportion of true negatives correctly identified as negatives

Predictive Accuracy:

Predictive Accuracy Predictive value reflects the diagnostic power of the test Depends on sensitivity & specificity and disease prevalence Positive predictive value : Probability of disease in persons who have positive test result More prevalent a disease greater is the predictive value of a positive screening test Negative predictive value : Probability of not having the disease in persons who have negative test result.

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Sensitivity = a/( a+c ) x 100 Specificity = d/( b+d ) x 100 Positive Predictive Value = a/( a+b ) x 100 Negative Predictive value = d/( c+d ) x 100 Test Disease Present Absent Positive True Positive (a) False Positive (b) a + b Negative False Negative (c) True Negative (d) c + d a + c b + d

False negatives and false positives:

False negatives and false positives False negatives A very sensitive test will have few false negatives Lower sensitivity, large will be the number of false negatives False postives A test with high specificity will have few false postives Burden the diagnostic facilities


Yield The amount of previously unrecognized disease diagnosed as a result of screening effort Depends upon Sensitivity Specificity Prevalence of disease Participation of individuals in the detection program

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Percentage of false negatives = c/( a+c ) x 100 Percentage of false positives = b/( b+d ) x 100 Test Disease Present Absent Positive True Positive (a) False Positive (b) a + b Negative False Negative (c) True Negative (d) c + d a + c b + d



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