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)
How good is the test?
Validity – get the correct result
 Sensitivity
 Specificity
 Predictive values
Reliable – get same result each time
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
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)
8
True Disease Status
Cases Non-cases
Screening
Test
Results
Positive
Negative
True
positive
False
positive
ab
c d True
False
negative
negative
a+c
b+d
a
True positives
X 100
=
Sensitivity =
a+c
All cases
True negatives
d
X 100
Specificity =
=
All non-cases
b+d
a+b
c+d
True Disease Status
Cases Non-cases
Screening
Test
Results
Positive
140
1,000
1,140
19,000
19,060
ab
c d
Negative
60
200
20,000
True positives
140
Sensitivity =
=
= 70%
All cases
200
Specificity = True negatives = 19,000 = 95%
20,000
All non-cases
10
Uses of sensitive test:
Uses of specific 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.
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.
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
12
True Disease Status
Cases Non-cases
Screening
Test
Results
Positive
Negative
True
positive
False
positive
ab
c d True
False
negative
negative
a+c
b+d
True positives
a
X 100
PPV =
=
All positives
a+b
True negatives
d
X 100
NPV =
=
All negatives
c+d
a+b
c+d
13
True Disease Status
Cases Non-cases
Screening
Test
Results
Positive
140
1,000
1,140
19,000
19,060
ab
c d
Negative
60
200
20,000
True positives
140
PPV =
=
= 12.3%
All positives
1,140
19,000
NPV = True negatives =
= 99.7%
19,060
All negatives
Positive predictive value,
Sensitivity, specificity, and prevalence
Se (%) Sp (%) Prevalence (%)
95
70
1.4
PV+ (%)
0.1
95
70
12.3
1.0
95
70
42.4
5.0
95
70
93.3
50.0
15
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
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
17
Ethics in screening
•
•
•
•
Informed consent obtained?
Implications of positive result?
Number and implications of false positives?
Labeling and stigmatization