Epidemiology

Screening

Screening
“...the identification of unrecognized disease or defect by the application of tests, examinations or other procedures...”
“...sort out apparently well persons who probably have disease from those who probably do not.”
“...not intended to be diagnostic...”

Types of screening

Mass screening, no selection of population (e.g., checking all infants for hearing problems)
Selective screening (e.g., by age and sex: mammograms for women aged over 40)
Multiphasic screening (a series of tests, as family doctors do at annual health exams)

When should we screen?

Screen when:
It is an important health problem (think about how to define ‘important’?)
There is an accepted and effective treatment
Disease has a recognizable latent or early symptomatic stage
There are adequate facilities for diagnosis and treatment
There is an accurate screening test
There is agreement as whom to consider as cases


Characteristics of a good screening test
Valid (e.g., sensitive and specific)
Reliable (gives consistent results; no random errors)
Cost – benefit (compare costs avoided due to early detection of the disease against cost of the screening. Does the test merely uncover more disease that is expensive to treat without appreciable advantage?)
Acceptable (discomfort, invasiveness, cost of obtaining test)
Follow-up services (plan needed to deal with positive results)

Validity – get the correct result

• Sensitivity
• Specificity
• Predictive values
Reliable – get same result each time

How good is the test?

What is used as a “gold standard”
1. Most definitive diagnostic procedure e.g. microscopic examination of a tissue specimen

2. Best available laboratory test

e.g. polymerase chain reaction (PCR) for HIV virus


3. Comprehensive clinical evaluation
e.g. clinical assessment of arthritis

8
Sensitivity and specificity
Assess correct classification of:
Sensitivity means probability of having a positive test results among those with disease
Specificity means probability of having a negative test results among those without the disease (specificity)

True

positive

True

negative
False
positive
False
negative
Sensitivity =
True positives


All cases
a + c
b + d
=
a

a + c

Specificity =
True negatives

All non-cases

=
d
b + d
a + b
c + d
True Disease Status
Cases
Non-cases
Positive
Negative
ScreeningTestResults
a
d
b
c


X 100
X 100

10
True Disease Status

Cases

Non-cases
Positive
Negative
ScreeningTestResults
a
d
1,000
b
c
60
Sensitivity =
True positives

All cases

200
20,000
=
140


200
Specificity =
True negatives

All non-cases

=
19,000

20,000

1,140
19,060

140

19,000

=
=
70%
95%

Uses of sensitive test:

1. In emergency department.2. In screening.3. In diseases with low frequency.4. In highly serious communicable disease.* Best use of sensitive test when test result is –v.
Uses of specific test:1. Chronic cases as in wards and clinic.2. To confirm the diagnosis.3. When the treatment is harmful as cytotoxic drugs.4. When cost of treatment is very high.* Best use of specific test when test result is +v.


12
Interpreting test results: predictive value
Probability (proportion) of those tested who are correctly classified
Having disease/ all positive tests
Not having disease/ all negative tests

13

True

positive

True

negative
False
positive
False
negative
PPV =
True positives

All positives

a + c
b + d
=
a


a + b
NPV =
True negatives

All negatives

=
d
c + d
a + b
c + d
True Disease Status
Cases
Non-cases
Positive
Negative
ScreeningTestResults
a
d
b
c


X 100
X 100

True Disease Status

Cases
Non-cases
Positive
Negative
ScreeningTestResults
a
d
1,000
b
c
60
PPV =
True positives

All positives

200
20,000
=
140


1,140
NPV =
True negatives

All negatives

=
19,000

19,060

1,140
19,060

140

19,000

=
=
12.3%
99.7%

15
Positive predictive value, Sensitivity, specificity, and prevalence
Se (%) Sp (%) Prevalence (%) PV+ (%)
0.1 1.4 70 95


1.0 12.3 70 95

5.0 42.4 70 95

50.0 93.3 70 95

Cut off point: the point at which a test results is considered to change from +v to –v. so by moving the cut off point will change every parameter in the test.

Lower cut-point:

increases sensitivity, reduces specificity

Higher cut-point:

reduces sensitivity, increases specificity

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Considerations in selection of cut-point
Implications of false positive results
burden on follow-up services
labelling effect
Implications of false negative results
Failure to intervene


Ethics in screening
Informed consent obtained?
Implications of positive result?
Number and implications of false positives?
Labeling and stigmatization