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Biggest Challenge in Preventive Medicine is to distinguish between
people who have the disease and those who do not..
This gives an idea of
progress of a disease from its
subclinical stages to overt
The Search for unrecognized disease or defect by means of
rapidly applied tests, examinations or the other procedures in
apparently healthy individuals.
But Today, Screening is
considered a form of secondary
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.`
Earlier it was to conserve physicians time for diagnosis,
administer inexpensive lab tests etc,.
The advantage gained by screening. (The period between
diagnosis by early detection and diagnosis by other means.)
Screening programmes work better where the time lag between
the disease’s onset and its final critical point are sufficiently long.
1. Done on apparently
2. Applied to groups
3. Results are arbitrary
4. Based on one criteria
5. Less accurate
6. Less expensive
7. Not a basis for
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
SCREENING TEST vs DIAGNOSTIC TEST
1. CASE DETECTION:
Defined as “The presumptive identification of unrecognized
disease, which does not arise from a patients request”.
The people are screened
primarily for their own benefit.
Heel Prick Blood Sample
2. CONTROL OF DISEASE:
People are examined for the benefit of others.
- Screening of Immigrants from infectious diseases like
Ebola, Tb & Syphilis to protect the home population.
- Screening for HIV, STD’s etc,.
may, by leading to early
diagnosis permit more
effective treatment and
reduce the spread of
infectious disease and
Ebola Check at Airports
3. RESEARCH PURPOSES:
- To know the history of many chronic diseases like
cancer, HTN etc.
- Screening may aid in obtaining more basic knowledge about
the natural history of such diseases.
provides a prevalence
provides an incidence
4. EDUCATIONAL OPPORTUNITIES:
Screening programmes help in
- Acquisition of information of public health
- Providing opportunities for creating public
- For educating health professionals.
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 all school
b) Mammography in women
c) Colonoscopy for occult
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
Before initiating a Screening Programme, a decision must be
made whether it abides to all the ethical, scientific and
The principles that should govern the introduction of screening
programmes were first enunciated by Wilson and Junger (1968)
- SCREENING TEST.
The Criteria for Screening is based on two considerations:
The Disease should be important Health problem (High
Disease should have Long & Detectable Preclinical stage.
The Natural history of disease should be adequately
Appropriate test must be available for early detection of
disease (before signs and symptoms appear)
Facilities must be available for diagnosis of disease
(Confirmation/ Gold standard)
Early detection of disease and treatment should be able to
reduce mortality & Morbidity.
The disease should be treatable, and there should be a
recognized treatment for lesions identified following screening.
Expected benefits must exceed risks and costs.
A Policy should be agreed on, concerning whom to treat as
2. SCREENING TEST
a) Inexpensive & Easy to Apply- (Simplicity)
The test should be simple to perform, easy to interpret and,
where possible, capable of use by paramedics and other
Ex: Blood and urine tests
and ECG for early
• Since participation in screening is voluntary, the test must be
acceptable to those undergoing it.
• In general tests that are painful, discomforting or
embarrassing are not likely to be acceptable.
Ex: Screening for prostrate cancer might not be acceptable to a
large proportion of the community.
VALIDITY IS THE ACCURACY OF A TEST.
RELIABILITY IS THE PRECISION OF A TEST.
WHAT IS VALID AND RELIABLE?
ACCURACY: “how close is result of a test to its true value?”
PRECISION: “how close are the results of a test on repetition?”
Validity determines the Accuracy of the Test.
- It expresses the ability of a test to separate those who have the
disease from those who do not.
- A test with little systematic error is a valid test.
The ability of a test to correctly identify those who have the
disease (True Positives)-
“Proportion of Truly Ill Population”
Expressed as percentage….. TP/ TP+FN.
Ds present Ds absent
The ability of a test to correctly identify those who do not have the
disease. (True Negatives)
Proportion of Truly Healthy Population.
Ds present Ds absent
An Ideal Screening Test should have 100% Sensitivity, and 100%
Specificity. (Not Practically Possible)
If a Person with disease is labeled Negative:
- False reassurance
- Ignores any disease signs and symptoms
- Postponement of treatment.
- Detrimental to overall health
If a Person without disease is labeled Positive:
- Further testing with long, expensive tests.
- Discomfort, inconvenience, anxiety
- Burden on health facilities
- Emotional trauma
- Difficulty in “de-labeling”
CONCEPT OF CUT-OFF POINT
- Unlike in Bimodal Distribution(Dichotomous), Some diseases comes
in Continuous Variables (Ex: Diabetes, HTN). In these Cases, It is
difficult to calculate Sensitivity & Specificity.
-So, A Cut Off Point must be set to distinguish between Positive
and Negative Result.
Consider 20 diabetics and
20 Non-diabetics screened using
a blood sugar test – Vertical axis
From Low to High.
Low Cut-Off Point
- False Positives originate (More Non-diabetics are diagnosed
High Cut-Off Point
- False Negatives originate (More diabetics are not diagnosed
Different Cut-off points yield different sensitivities and
The cut off point that identifies more true negatives will also
identify more false negatives.
The cut off point that identifies more true positives will also
identify more false positives.
The choice of a high or low cut off level for screening therefore
depends on the importance we attach to FPs or FNs.
In case of Lethal diseases (Early Intervention possible) Cut off point
must be set at Low level , as Greater sensitivity is required.
(False Positives can be tolerated)
SEQUENTIAL TESTING (Two stage)
After the first (screening) test is conducted, those who
tested positive are brought back for the second test to further
reduce false positives.
Consequently, the overall process will increase specificity
but with reduced sensitivity.
-Net Sensitivity and Net Specificity can be calculated for both
the tests in sequence.
Net sensitivity falls, but Net Specificity will be gained.
Two or More tests are conducted in parallel.
The goal is to maximize the probability that subjects with
the disease (True Positives) are identified.- High Sensitivity.
Consequently more False Positives are also identified.
Net sensitivity is Gained, but Net Specificity will be lost- when
Compared to either of the tests.
Positive Predictive Value:
The Proportion of the people who is screened positive
that actually have the disease. (Are the people with disease
Negative Predictive Value:
The Proportion of the people who is screened negative
that are actually FREE of the disease. (Are the people without
disease correctly identified?)
These Values are not fixated for a particular test.
Predictive accuracy depends on-
Prevalence of the Disease.
Specificity of the Test.
Increase in Sensitivity causes a
modest increase in PPV, but
increase in Specificity raises PPV
More prevalent diseases has high PPV, that’s why SCREENING
is more efficient & productive, If done in High risk population.
Reliability determines the Precision of the Test. (Repeatability)
It means that all the results of the test should be similar
(Cluster at one place), when conducted each and every time.
This is not possible because of the Variations that cause the
test to not yield same results every time. (like
Lab equipment failure etc.)
- Intrasubject Variation
- Intraobserver Variation
- Intraobserver Variation
3 types of Variation
1. INTRA-SUBJECT VARIATION
This is the Variation in the results of the test conducted
over time (Short periods) on the same individual.
The difference is due to the changes that occur to the individual
over that time period.
Variation in BP during a 24 hour period.
2. INTRA-OBSERVER VARIATION
This is the Variation in the results of the test due to the same
observer examining the result at different times.
EX: Two readings of Blood pressure by the Same observer.
3. INTER-OBSERVER VARIATION
This is the Variation in the results of the test due to the
multiple observers examining the result.
EX: Chest X ray read by two different Radiologists.
Yield is the amount of previously 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 screening test, participation in the programme.
Hence, yield of a screening test is high in high – risk
EVALUATION OF SCREENING PROGRAMMES
Conduct an RCT of the screening test to compare the
disease specific cumulative mortality rate between the
intervention and control group.
Problems include- Long follow up, Costs and
Allows study of distribution of lead time,
effects of early treatment and
identification of prognostic factors.
2. 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 healthy)
-Ecological study (correlation of screening pattern and
disease experience of several populations)
Screening, despite its flaws, is a major public Health determinant,
measured by its effect on Mortality, Morbidity & Disability.
Establishing appropriate criteria requires considerable
knowledge of the Natural history of disease, adequate facilities
for follow up & Rx.
It is necessary to ensure that the program is continuously
monitored to confirm that effectiveness is maintained.
Newer fields such as genetic screening are on the rise which
would help the cause.