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 teste.g. polymerase chain reaction (PCR)

for HIV virus

3. Comprehensive clinical evaluatione.g. clinical assessment of arthritis

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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)

Truepositive

Truenegative

Falsepositive

Falsenegative

Sensitivity = True positives

All cases

a + c b + d

= aa + c

Specificity = True negatives All non-cases

= db + d

a + b

c + d

True Disease Status

Cases Non-cases

Positive

Negative

ScreeningTest

Results

a d b

c

X 100

X 100

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True Disease Status

Cases Non-cases

Positive

Negative

ScreeningTest

Results

a d

1,000 b

c60

Sensitivity = True positives

All cases

200 20,000

= 140200

Specificity = True negatives All non-cases

= 19,00020,000

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.

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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

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Truepositive

Truenegative

Falsepositive

Falsenegative

PPV = True positives

All positives

a + c b + d

= a

a + b

NPV = True negatives All negatives

= dc + d

a + b

c + d

True Disease Status

Cases Non-cases

Positive

Negative

ScreeningTest

Results

a d b

c

X 100

X 100

True Disease Status

Cases Non-cases

Positive

Negative

ScreeningTest

Results

a d

1,000 b

c60

PPV =True positives

All positives

200 20,000

= 1401,140

NPV = True negatives All negatives

= 19,00019,060

1,140

19,060

140

19,000

=

= 12.3%

99.7%

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Positive predictive value, Sensitivity, specificity, and prevalence

Se (%) Sp (%) Prevalence (%) PV+ (%) 0.1 1.47095

1.0 12.37095

5.0 42.47095

50.0 93.37095

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