Wrap up SCREENING TESTS

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Screening test

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The basic tool of a screening program

•

easy to use, rapid and inexpensive.

Screening Test

Definition:

PRESUMPTIVE identification of unrecognized disease or defect by the application of tests, examinations, 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.

A screening test is not intended to be diagnostic. Persons with positive or suspicious findings must be referred to their physicians for diagnosis and necessary treatment.

[Commission on Chronic Illness, 1951]

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PURPOSES OF SCREENING

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To designate people with preclinical disease “ Positive” and people without preclinical disease “ negative”

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Types of Screening Programs

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Selective screening



specific people at risk for disease

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Mass screening



of people test large number

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Lead time and screening test

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Lead time is the time interval from detection by screening test to the time at which diagnosis would have been made without that screening.

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Length of lead time interval may vary from person to person (short and long lead time)

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Importance of lead time is for disease control and by early detection and early treatment to prevent spread and disability of affected persons. Screening test is valuable in reducing severe morbidity and mortality

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Measurements used in screening tests

• • •

Validity – test is able to differentiate presence or absence of disease Yield – brought unrecognized disease to diagnosis Reliability – consistent results when tested more than once

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VALIDITY

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Validity is the rate at which a test is capable of differentiating the presence or absence of a disease concerned SENSITIVITY = ability of test to detect people who actually have the disease (True Positives/All Positives) SPECIFICITY = ability of test to identify correctly people who actually do not have the disease (True Negatives/All Negatives)

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Creation of 2 x 2 table: initial step for calculation TEST “ Screening ” + DISEASE “ Gold standard ” Yes No True pos

a c

False neg

b

False pos

d

True neg

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Validity of screening test

1.

2.

3.

Sensitivity = proportion of subjects with disease who have the positive test from screening = a / a+c or = TP / TP + FN Specificity = proportion of subjects without disease who have the negative test from screening = d / b+d or = TN / FP + TN Accuracy of the test = a + d / a + b + c + d = TP + TN / Total screened 10

Trade off point Cut off point

“Criterion of Positivity”

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Real situation of screening test

4 .

5 4 3 .

5 3 2 .

5 2 1 .

5 1 0 .

5 0 Persons without disease

d

Persons with Disease

c b Persons with disease = a + c

Trade off point

Persons without disease = b + d a

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4 .

5 4 3 .

5 3 2 .

5 2 1 .

5 1 0 .

5 0

Shifting of trade off point

A

d Healthy Trade off point A b c a Sick 13

4 .

5 4 3 .

5 3 2 .

5 2 1 .

5 1 0 .

5 0

Shifting of trade off point B

d Healthy c a Sick Trade off point B 14

Setting of trade off point A on sensitivity and specificity of HIV EIA assay Negative test Positive test

4 .

5 4 3 .

5 3 HIV-free population FALSE POS 2 .

5 2 1 .

5 1 0 .

5 0

High SEN / Low SPEC

HIV-positive population c

A B

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Setting of trade off point B on sensitivity and specificity of HIV EIA assay

4 .

5

Negative test Positive test

4 3 .

5 3 HIV-free population 2 .

5 2 1 .

5 1 0 .

5 0

FALSE NEG

HIV-positive population

Low SEN / High SPEC A B

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4 .

5 4 3 .

5 3 2 .

5 2 1 .

5 1 0 .

5 0

Hypothetical best screening test

d Healthy Trade off point a Sick 17

YIELD

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Yield is the amount of previously unrecognized disease which is diagnosed and brought to treatment as a result of the screening PREDICTIVE VALUE POSITIVE (PVP) is the likelihood that an individual with a positive test has the disease PREDICTIVE VALUE NEGATIVE (PVN) is the likelihood that an individual with a negative test does not have the disease

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Yield of screening test

• Predictive Value Positive (PVP) PVP = a / a + b or = TP / TP + FP • Predictive Value Negative (PVN) PVN = d / c + d or = TN / TN + FN

RELIABILITY (Precision)

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Reliability is consistency of results when the test is performed more than once on the same individual under the same conditions. It is also called ‘Repeatability’

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Reliability**Precision**Repeatability Number of agreed positive = ------------------------------------ Number of positive either time a = --------------

(a + b + c)

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Thank you for your attention

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