Epi week 14 (screening)

What is screening?

Using a method to identify an existing illness
or condition during its asymptomatic phase in
order to provide early treatment
- Secondary prevention
- Only useful if early treatment changes

Screening is only useful if?

early treatment changes the outcome

A diagnostic test

is used to determine the PRESENCE or ABSENCE of a disease when a subject shows signs or symptoms of the disease

A screening test

identifies asymptomatic individuals who may have the disease

Is a screening test or diagnostic test performed first?

The diagnostic test is performed after a positive screening test to establish a definitive diagnosis

Some Common Screening Tests:

Pap smear for cervical dysplasia or cervical cancer
Fasting blood cholesterol for heart disease
Fasting blood sugar for diabetes
Blood pressure for hypertension
Mammography for breast cancer
PSA test for prostate cancer
Fecal occult blood for colon cancer

Principles of successful mass screening

Condition is important health problem
Acceptable form of treatment
Available treatment
Natural history understood
Recognizable latent or early symptomatic stage
Suitable, acceptable screening test
Policy on whom to treat
Treatment in pre-symptomatic stage

Ethical Considerations for screenings

Purpose of screening ethically acceptable
Means and use of information appropriate for accomplishing the purpose
Use of high quality laboratory services
Right of informed consent
Right to be informed of results (+) or (-)
Counseling services before and af

Biases with Screening
(Lead time)

- Amount of survival time gained due to earlier detection
- Leads to false impression that survival is lengthened

Biases with Screening
Length time:

Illnesses which are slower in progression (e.g. slow
growing tumors) have more opportunity to be detected
-Screening may appear to lengthen survival when in fact the
slowly progressive illnesses are preferentially identified

Only way to determine effectiveness of screening is

show decreased mortality among those screened
or a measurable improvement in an outcome.

Properties of a Screening test:
( Validity)

Predictive value

Properties of a Screening test:

Percent agreement
Percent positive agreement

Validity is the

ability of a test to indicate which individuals have the disease and which do not

How well is a subject classified into disease
or non-disease group?

deally, all subjects who have the disease should
be classified as "having the disease" and vice
Practically, the ability to classify individuals into
the correct disease status depends on the
accuracy of the tests, among other things

Variation in Biologic Values
Many test results have a

continuous scale (are continuous variables)

The ability of the test to identify correctly those

Have the diease

The ability of the test to identify correctly those

do not have the disease

Determining the Sensitivity, Specificity of
a New Test

Must know the correct disease status prior to
Gold standard test is the best test available
It is often invasive or expensive
A new test is, for example, a new screening test
or a less expensive diagnostic test
Use a 2 x 2 table to compare the

Sensitivity calculation:

Sensitivity = TP / (TP + FN)
The number of true positives divided by the number of patients with the disease.
The proportion of individuals with the disease
(a+c) who test positive (a)
(a) / (a+c)

Specificity calculation

Specificity = TN / (TN + FP)
The number of true negatives divided by the number of patients without diease
The proportion of individuals without the disease
(b+d) who test negative (d)

Risks of screening
True Positives

Labeling effect
Difficulty obtaining insurance

Risks of screening
False Positives

Unnecessary stress
Cost from confirmatory tests
Adverse events from confirmatory tests

Risks of screening
True Negatives

Cost of test
Risk of test

Risks of screening
False Negatives

Delayed treatment due to ignoring symptoms of
Transmission if communicable

What the test shows

(a+b) = all the people with positive results
(c+d) = all the people with negative results

Positive predictive value (PPV)

The proportion of patients who test positive who
actually have the disease

Another interpretations of PPV

If a person tests positive, what is the probability
that he or she has the disease?
(And if that person tests negative, what is the
probability that he or she does not have the

Positive Predictive value

a / (a + b)
The proportion of patients who test positive (a+b)
who actually have the disease (a)

Negative predictive value (NPV)

The proportion of patients who test negative (c+d)
who are actually free of the disease (d)
d / (c+d)

Is NPV a fixed charactersitic of the test?


is Senstivity a fixed characteristic of the test?


Is specificity a fixed characteristic of the test?


Is Positive Predictive value a fixed characteristics of the test


PPV primarily depends on

The prevalence of the disease in the population
tested, and the test itself (sensitivity and specificity)
In general, it depends more on the specificity (and less on the sensitivity) of the test (if the disease prevalence is low)

In general, ______ has more impact on predictive


The Relationship of Prevalence to Predictive


Inter-observer variation

is a variation in the result of
a test due to MULTIPLE observers examining the result (inter = between)

Intra-observer variation

is a variation in the result of
a test due to the SAME observer examining the
result at different times (intra = within)

overall percent agreement


Percent positive agreement