Screening
Dr. Fahmida Akter
Associate Professor
Department of Epidemiology
NIPSOM
Definition
The presumptive identification of
unrecognized disease or defect by the
application of tests, exams or other
procedures which can be applied rapidly to
sort out apparently well persons who
probably have a disease from those who
probably do not
Screening is a process
Apparently healthy people
Screening may be-
»Questions
»Examinations
»Laboratory test
»Radiology exams
Difference between Screening and
Diagnostic Tests
Screening Diagnostic test
Purpose To detect potential disease To establish presence/absence of
indicators disease
Target population Large number of asymptomatic Symptomatic individuals to establish
but potentially at risk population diagnosis or asymptomatic
individuals with positive screening
test
Test method Simple, acceptable to people May be invasive, expensive but
justifiable to establish diagnosis
Positive result Essentially indicate suspicion of Provides a definite diagnosis
disease that warrants
confirmation
Cost Cheap Higher cost
Uses of screening
1. Case detection
2. Control of disease
3. Research purposes
4. Educational opportunities
Criteria for identifying a disease
suitable for screening
• The condition sought is an important health problem
• The natural history of the condition is well
understood
• There is a recognizable latent/early symptomatic
stage
• There is a suitable and acceptable test or
examination
• There is an accepted, cost-effective and affordable
treatment
Contd.
• Facilities and resources for diagnosis and treatment are
available
• There should be an agreed policy on whom to treat as
patients (protocols for diagnosis and treatment)
• The cost of case-finding (including diagnosis and
treatment) is economically balanced in relation to
expenditure on medical care as a whole
• Case-finding is a continuing process and not a ‘once and
for all’ project
Screening: Natural history of diseases
Patient
seeks
Screening medical
attention
Infectious
diseases
Disease Start of
detectable symptoms
Lead Time
Chronic
diseases
Disease Screening Start of Patient
detectable symptoms seeks
medical
attention
Lead Time
Criteria of a suitable screening test
• Simple
• Cheap or inexpensive
• Easy to apply or administer with minimum
discomfort
• Acceptable to the public
• Safe
Types of screening
• Mass screening
• Targeted screening/Selective screening
• Multiple or multiphasic screening
• Case finding or opportunistic screening
Mass screening
• Involves screening of whole population or subset
• Example: Visual defects in children
Targeted screening/Selective screening
• It is applied selectively to specific exposures or high
risk group
• Most economical
• Example : Screening for cervical cancer among low
SES female
Multiple or multiphasic screening
• Use two or more screening tests at the same time for
a single disease
• Example: H/o cough for > 3 weeks, MT test and X-ray
chest sequentially for TB
Case finding or opportunistic screening
• Aimed at patients who consult a health practitioner
for some other
• Example: Advice of screening for cervical cancer
when a women of reproductive age go for some
other health problem
Elements for Evaluating Screening Test
• Validity
• Predictive Value
• Reliability
Validity
• The validity of a test is defined as its ability to
distinguish between who has a disease and
who does not
• It has two component-
»Sensitivity
»Specificity
Sensitivity
Specificity
Comparison of dichotomous test result
with disease status
Test Result Disease
Have the disease Do not have the disease
Positive True Positive (TP): False Positive (FP):
Have the disease Do not have the disease
and test positive but test positive
(a) (b)
Negative False Negative (FN): True Negative (TN):
Have the disease Do not have the disease
but test negative and test negative
(c) (d)
Issues of false positives and false negatives
• The issue of false positives is important. Because-
✓Burden on the health care system will be increased
✓Anxiety and worry induced in persons who have been
told that they have tested positive
✓Labeled “positive” by a screening test never have that
label completely erased and may be handled as
handicapped by parents and school personnel and
limited in regard to employment
Consequence
contd.
• The issue of false negatives is also important.
Because-
✓A false-negative result could represent a virtual
death sentence
Gold Standard
• To quantitatively assess the sensitivity and specificity
of a test, we must have another source of truth with
which to compare the test results
• A sounder indication of the truth often referred to as
the gold standard (or reference standard or criterion
standard).
Test of Continuous Variable
• We often test for a continuous variable, such as blood
pressure or blood glucose level, for which there is no
“positive” or “negative” result
• A decision must be made in establishing a cutoff level above
which a test result is considered positive and below which a
result is considered negative
• The choice of a high or a low cutoff level for screening
depends on the importance we attach to false positives and
false negatives
Trade –Offs between Sensitivity
and Specificity
• The point at which sensitivity and specificity
has minimum difference of gap between them
is said trade-off
Rule-in and Rule-out concept
• A highly sensitive test if negative, rules out the
diagnosis
• A highly specific test if positive, rules in the
diagnosis
Predictive value
It has two component-
➢ Predictive Value of a Positive Test (PPV):
Likelihood that a person with a positive test
has the disease
➢ Predictive Value of a Negative Test (NPV):
Likelihood that a person with a negative test
does not have the disease
Calculating Predictive Value
Test Result Disease Predictive Value
Have the disease Do not have the disease
Positive True Positive (TP): False Positive (FP):
Have the disease Do not have the disease
and test positive but test positive
(b)
(a)
Negative False Negative (FN): True Negative (TN):
Have the disease Do not have the disease
but test negative and test negative
(c ) (d)
Exercise
• Find out the sensitivity, specificity, PPV and
NPV of the following statements:
Diagnosis of Renal Calculus by X-Ray KUB for
screening and CT scan for confirmation
KUB X-Ray Result Renal Calculus
Present Absent
Positive 80 600
Negative 11 500
Solution
KUB X-Ray Result Renal Calculus
Present Absent
Positive 80 600
a b
Negative 11 500
c d
Determinants of positive predictive value
–Prevalence of the disease
–Specificity of the test
Why should we be concerned?
• Higher the prevalence, the higher the predictive
value
• A screening program is most productive and efficient
if it is directed to a high-risk target population
• Screening a total population for infrequent disease
can be very wasteful of resources
• High-risk population may be more motivated to
participate in screening program and more likely to
take recommended action
Relationship between Positive Predictive
Value and Specificity of the Test
Reliability (Repeatability)
• Consistent results when given to same person under
the same conditions
• The factors that contribute to the variation between
test results are-
a. Intrasubject variation (variation within
individual subjects)
b. Intraobserver variation (variation in the
reading of test results by the same reader)
and
c. Interobserver variation (variation between
those reading the test results
Relationship between validity and
reliability
Biases in Screening
• Lead time bias
• Length/time bias
• Volunteer bias
Thank You
