1

Wrap upSCREENING TESTS

2

Screening test

•The basic tool of a screening program

•easy to use, rapid and inexpensive.

1.2

3

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]

4

PURPOSES OF SCREENING

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

5

Types of Screening Programs

• Selective screening specific people at risk for disease

• Mass screening test large number of people

6

Lead time and screening test

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

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

• 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

7

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

8

VALIDITY

• 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)

9

Creation of 2 x 2 table:initial step for calculation

TEST “Screening”

+

-

Yes No

DISEASE “Gold standard”

a bc d

True pos False pos

False neg True neg

10

1. Sensitivity = proportion of subjects with disease who have the positive test from screening

= a / a+c

or = TP / TP + FN2. Specificity = proportion of subjects without

disease who have the negative test from screening

= d / b+d

or = TN / FP + TN3. Accuracy of the test = a + d / a + b + c + d

= TP + TN / Total screened

Validity of screening test

11

Trade off pointCut off point

“Criterion of Positivity”

12

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5Persons without disease

Persons with Disease

Trade off point

abc

d

Num

ber

o f p

ers o

n sReal situation of screening test

Persons with disease = a + c

Persons without disease = b + d

13

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Trade off point A

Healthy

Sick

Num

ber

of p

ers o

n sShifting of trade off point A

abc

d

14

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Trade off point B

Healthy

Sick

Num

ber

of p

ers o

n sShifting of trade off point B

ab

c

d

X

15

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

HIV-free population

HIV-positive population

Num

ber

o f p

ers o

n sSetting of trade off point A on sensitivity and specificity of HIV EIA assay

c

A B

Negative test Positive test

FALSE POS

High SEN / Low SPEC

16

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

HIV-free population

HIV-positive population

Num

ber

o f p

ers o

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

A B

Negative test Positive test

FALSE NEG

Low SEN / High SPEC

17

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

HealthySick

Trade off point

a

d

Num

ber

o f p

ers o

n sHypothetical best screening test

18

YIELD

• 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

19

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

9

20

RELIABILITY (Precision)

• 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’

21

Reliability**Precision**Repeatability

Number of agreed positive= -------------------------------------

Number of positive either time

a= --------------

(a + b + c)

22

Thank you for your attention