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
outcome
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
programs:
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
to
show decreased mortality among those screened
or a measurable improvement in an outcome.
Properties of a Screening test:
( Validity)
Sensitivity
Specificity
Predictive value
Properties of a Screening test:
(Reliability)
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
versa
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)
Sensitivity
The ability of the test to identify correctly those
who
Have the diease
Specificity
The ability of the test to identify correctly those
who
do not have the disease
Determining the Sensitivity, Specificity of
a New Test
Must know the correct disease status prior to
calculation
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)
(b)/(b+d)
Risks of screening
True Positives
Labeling effect
Difficulty obtaining insurance
Stress
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
disease
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
disease?)
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?
NO
is Senstivity a fixed characteristic of the test?
YES
Is specificity a fixed characteristic of the test?
YES
Is Positive Predictive value a fixed characteristics of the test
NO
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
values
specificity
The Relationship of Prevalence to Predictive
Value
(
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
0
Percent positive agreement
0