DR. SHAILAJA DARAL
MODERATOR: DR. ANITA VERMA
DEPT. OF COMMUNITY MEDICINE
VMMC AND SAFDARJUNG HOSPITAL
DEFINITION OF SCREENING
The search for unrecognized disease or defect by means of rapidly
applied tests, examinations or other procedures in apparently healthy
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.*
Tests done in individuals with no symptom or sign of an illness are
referred to as screening tests.*
(* K. Park Textbook of PSM 21st edition;
* J M Last Dictionary of Epidemiology 4th edition, WHO Public Health
* J H Abramson and Z H Abramson Survey methods in Community
Medicine 5th edition)
BASIS OF SCREENING
Iceberg phenomenon of
tip of the iceberg
HIDDEN BURDEN OF DISEASE
BASIS OF SCREENING
Screening is a form of secondary prevention.
It detects disease in its early asymptomatic phase whereby early
treatment can be given and disease can be cured or its
progression can be delayed.
It has both diagnostic (?) and therapeutic components.
SCREENING TEST VERSUS DIAGNOSTIC TEST
1. Done on apparently
2. Applied to groups
3. Results are arbitrary and
4. Based on one criteria
5. Less accurate
6. Less expensive
8. Initiative comes from
1. Done on sick or ill
2. Applied on single patient
3. Diagnosis is not final
4. Based on evaluation of a
no. of signs/symptoms &
5. More accurate
6. More expensive
7. Used as a basis for
8. Initiative comes from a
COMMON SCREENING TESTS
FASTING BLOOD GLUCOSE FOR DIABETES
BLOOD PRESSURE FOR HYPERTENSION
PSA TEST FOR PROSTATE CANCER
PAP SMEAR FOR CERVICAL CANCER
MAMMOGRAPHY FOR BREAST CANCER
FECAL OCCULT BLOOD FOR COLON CANCER
TYPES OF SCREENING
1. MASS SCREENING
Application of screening test to large, unselected population.
Everyone in the group is screened regardless of the probability of
having the disease or condition.
a) visual defects in school children
b) mammography in women aged 40 years or less
c) newborn screening program in Japan
2. HIGH RISK / SELECTIVE / TARGETED SCREENING
The screening of selected high-risk groups in the population.
a) screening fetus for Down’s syndrome in a
mother who already has a baby with Down’s
b) screening for familial cancers, HTN and DM
c) screening for CA Cervix in low SES women
d) screening for HIV in risk groups
3. MULTIPURPOSE SCREENING
The screening of a population by more than one test done
simultaneously to detect more than one disease
a) screening of pregnant women for VDRL, HIV,
HBV by serological tests
4. MULTIPHASIC SCREENING
The screening in which various diagnostic procedures are
employed during the same screening program.
a) DM – FBS, Glucose tolerance test
b) Sickle cell anemia – CBC, Hb electrophoresis
5. OPPORTUNISTIC / CASE FINDING SCREENING
There is no accurate or precise diagnostic test for the disease
and where the frequency of its occurrence in the population is
small. The main objective is to detect disease and bring patients
a) RHD in children
USES OF SCREENING
1. CASE DETECTION
prescriptive screening, people are
screened for their own benefit
(cancer, diabetes, hypertension)
2. CONTROL OF DISEASE
prospective screening, people are
screened for the benefit of others
to know the natural history of a
CRITERIA FOR CHOOSING A SCREENING TEST
a) Significant burden of disease
b) Detectable and long preclinical stage of disease
c) Adequately understood natural history of disease
d) Appropriate test available for early detection of disease
e) Facilities for diagnosis of disease
f) Early detection of disease has outcome benefit
g) Effective treatment available for disease
h) Policy of screening program for disease
2. SCREENING TEST
• GOLD STANDARD: an external source of truth regarding the
disease status of each individual in the population. Gold
standard is a benchmark and its results are considered
• Infections – CULTURE
• Cancers – BIOPSY
• Drug testing – RANDOMIZED CONTROLLED TRIAL
• Cause of death – AUTOPSY
WHAT IS VALID AND RELIABLE?
VALIDITY IS THE ACCURACY OF A TEST.
RELIABILITY IS THE PRECISION OF A TEST.
ACCURACY: “how close is result of a test to its true value?”
PRECISION: “how close are the results of a test on repetition?”
An expression of the degree to which a test measures what it purports
How to understand validity?
ESTIMATE: a measurement or statement about the value of a quantity
An epidemiological estimate is the end product of the study design, the
study conduct and the data analysis.
Error is a false or mistaken result obtained in a study or experiment.
Systematic error is one sided variation of measurements from the true
value. It has a recognizable source.
A study that has little systematic error is described as VALID.
COMPONENTS OF VALIDITY
The ability of a test to correctly identify those who have the
a/ (a + c) expressed as percentage.
Problem of FP.
The ability of a test to correctly identify those who do not have
the disease. TN
d/ (b + d) expressed as percentage.
Problem of FN.
PROBLEM OF FN AND FP
ignoring of disease signs and symptoms
postponement of treatment
detrimental to overall health
discomfort, inconvenience, anxiety
burden on health facilities
difficulty in “de-labeling”
USE OF MULTIPLE TESTS
•Sequential Testing (Two-Stage Screening)
After the first (screening) test is conducted, those who tested
positive are brought back for the second test to further reduce
Consequently, the overall process will increase specificity but
with reduced sensitivity.
The percentage of sick people with a given test result divided by
the percentage of well individuals with the same result.
The relative probability of a positive test in a
diseased individual in comparison to a diseasefree individual.
TP / FP = SENSITIVITY / 1 – SPECIFICITY
Negative LR: The relative probability of a negative test in a
diseased individual in comparison to a diseasefree individual.
FN / TN = 1 – SENSITIVITY / SPECIFICITY
WHY DO WE NEED LIKELIHOOD RATIO?
• Many tests in clinical medicine have continuous results or
multiple ordinal levels. Putting multiple categories into either
positive or negative test causes loss of information. Likelihood
ratios enable clinicians to interpret and use the full range of
diagnostic test results.
• While predictive values relate test characteristics to
populations, likelihood ratios can be applied to a specific
• Likelihood ratios refine clinical judgment. Application of a
likelihood ratio to a working diagnosis generally changes the
diagnostic probability—sometimes radically.
EXAMPLE OF INTER-OBSERVER AND INTRA-OBSERVER
• INTRA-OBSERVER VARIATION:
two readings of blood pressure measurement
• INTER-OBSERVER VARIATION:
chest x-ray films by different radiologists
Type of inter-observer agreement.
Takes into account agreement between two observers purely by
It is the inter-observer agreement over and above agreement
that is purely by chance.
It is standardized to lie between -1 to 1.
-1 = no agreement
1 = perfect agreement
0 = agreement purely due to chance
EXAMPLE OF KAPPA STATISTIC
(15 + 70) / 100 = 0.85 or 85%
KAPPA statistic = PO - PE / 1 – PE
(a + b) / (a + b + c + d)
(a + c) / (a + b + c + d)
(b + d) / (a + b + c + d)
So here, KAPPA = 0.57
(c + d) / (a + b + c + d)
YIELD is the amount of unrecognized disease that is detected and
brought to treatment as a result of screening.
YIELD = TP + FP / TP + FP + TN + FN
It depends on prevalence of the disease and sensitivity of the
Hence, yield of a screening test is high in high – risk screening.
RECEIVER OPERATING CHARACTERISTICS
For a given test, the true positive rate (TPR) or SENSITIVITY
against false positive rate (FPR) or 1 – SPECIFICITY can be
measured. All possible combinations of TPR and FPR compose a
One TPR and one FPR together determine a single point in the
ROC space, and the position of a point in the ROC space shows
the trade – off between sensitivity and specificity, i.e. the
increase in sensitivity is accompanied by a decrease in specificity.
Thus the location of the point in the ROC space depicts whether
the screening test classification is good or not.
Plotting SENSITIVITY and 1 – SPECIFICITY at different values of a
screening test gives the ROC curve.
The area under the curve (AUC) gives a way to measure the
accuracy of the screening test.
In an ideal situation, a point determined by both TPR and FPF
yields coordinates (0, 1). This ideal point indicates that the
screening test has a sensitivity of 100% and specificity of 100%.
Screening test with 50% sensitivity and 50% specificity lies on the
diagonal determined by coordinate (0, 0) and coordinates (1, 0).
Theoretically, random error would give a point along this
A point predicted by a screening test that falls into the area
above the diagonal represents a good screening classification.
METHOD TO FIND THE OPTIMUM CUT – OFF POINT OF A SCREENING
YOUDEN INDEX AND COST CONSIDERATIONS
EVALUATION OF SCREENING TEST
conduct an RCT of the screening test to
compare the disease specific cumulative
mortality rate between the intervention
and control group.
this also eliminates confounding.
allows study of distribution of lead time,
effects of early treatment and
identification of prognostic factors.
b) Non – experimental:
cohort study (comparison of advanced
disease or death rates in those who
choose to screen and those who do not)
case - control study (comparison of
screening history in those who have
advanced disease and those who are
ecological study (correlation of screening
pattern and disease experience of several
2. MEASURES OF EFFECT
a) Comparison of survival experience
b) Comparison of disease specific mortality rate
ISSUES WITH SCREENING
1. Lead time bias - the systematic error of apparent increased
survival from detecting disease in an early stage.
2. LENGTH TIME BIAS
Diseases with a long pre-clinical phase are more likely to be
detected during screening. Moreover, pre-clinical phase for the
same disease may be variable in different individuals.
3. SELECTION BIAS
Not everyone will take part in a screening program. There are
factors that differ between those willing to get tested and those
who are not.
Willingness depends on perceived risk of disease and intelligence
a) breast cancer screening – more positive
outcome if only intelligent people participate. More negative
outcomes if only high risk patients participate.
Screening may identify abnormalities that would never cause a
problem in a person's lifetime. Causes overestimate of disease as
well as survival.
a) PAP testing and Cervical CIS
b) PSA testing and low grade prostate cancer
c) mammography and DCIS
A prior (pre-test) probability is an initial probability value originally
obtained before any additional information is obtained.
A posterior (post-test) probability is a probability value that has
been revised by using additional information that is later obtained.
Road traffic accident patients in a hospital are
listed and one of the patients is randomly selected.
a) what is the probability that he will have
b) during the later analysis of this patient it was
found that he has CT scan positive for cerebral
hemorrhage. Now, what is the probability that he
actually has cerebral hemorrhage?
For the first question, we know that
Probability of cerebral hemorrhage in a RTA patient is 28%.
For the second question, we know that
CT scan has a sensitivity of 90% and specificity of 95% for
detecting cerebral hemorrhage.
Bayes theorem gives us a way of calculating probability of an
event in the light of presence of a second event, which itself
occurs in the presence of the first one.
Probability that our patient has cerebral hemorrhage, in view of
an already positive CT scan for cerebral hemorrhage, is…
Pre-test probability of event (P1) X sensitivity of test (Sn)
--------------------------------------------------------------------------- X 100
(P1 X Sn)
(1 – P1) X (1 – Sp)
P1 = 0.28
Sn = 0.90
Sp = 0.95
Post-test probability (P2) = 0.875 or 87.5%
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