Screening methods
Dr K N Prasad
Community Medicine
Diseases are having iceberg phenomenon
Screening is the early detection of disease
precursors to disease. Or susceptibility to
diseases in individuals who do not show any
signs of the disease
Tests
Questions
Examinations
Laboratory tests
X rays
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 refers to the application of a medical
procedure or test to people who as yet have no
symptoms of a particular disease, for the
purpose of determining their likelihood of having
the disease.
The screening procedure itself does not
diagnose the illness.
Screening test will need further evaluation with
subsequent diagnostic tests or procedures.
Differences
 Screening test
 Diagnostic test
 Done on apparently
healthy
 Applied to groups
 Based on one or cut off
criteria
 Less accurate
 Not a basis for treatment
 Initiative comes from the
health agency
 Done on those with sick
or indications
 Applied to single patient
 Based on evaluation of
number of symptoms,
signs or lab tests
 More accurate
 Basis for treatment
 Initiative comes from
patient with complaint
Lead time concept
• Time lag present between disease
onset and usual time of diagnosis
Disease
onset
First possible
detection
Final critical
point
Usual time of
diagnosis
outcome
A
Screening time
B
Lead time
Lead time is the period between diagnosis by early detection and
diagnosis by other means
Screening pathways
Healthy
Disease or precursor Screening possible
Detectable
Symptoms develop
Advance disease
Death
Intervention to avert
disease development
or its consequences
Life prolonged
Objective
The goal of screening is to reduce morbidity or
mortality from the disease by detecting diseases in
their earliest stages, when treatment is usually more
successful.
Sort out the population who has risk from others
Examinations
Pap smear, Mammogram, Clinical breast exam, Blood
pressure determination, Cholesterol level, Eye
examination/vision test, and Urinalysis.
Uses of screening tests
1. Cases detection (Prescriptive )
1. Screened primarily for their own benefit Ex.
Neonatal, cancers
2. Control of diseases (Prospective)
1. Screened for benefit of others
2. Ex. STD, TB
3. Research purpose
1. May aid in obtaining basic knowledge about
natural history of disease
4. Education opportunity
1. To create public awareness and educating health
professionals
Types of screening
1. Mass screening
1. Screening afford to all irrespective of the
particular risk Ex. TB
2. High risk or selection screening
1. Applied to selective high risk group on the
basis of epidemiological research
2. Ex. Cervical Cancer, heart diseases
3. Multi purpose screening
1. Application of two or more tests in
combination to a large number at one time
2. Ex. Questionnaire, examination, laboratory
tests etc.
Criteria for population based
screening
1. Knowledge of disease
1. Condition must be an important problem
2. Presence of recognisable latent or early
symptomatic stage
3. Natural history of the condition must be
adequately understood
2. Feasibility of screening procedures
1. Suitable test or examination
2. Acceptable to the population
3. Should be a continuous process
Contd.
 Diagnosis and treatment


acceptable treatment with recognisable disease
Facilities for diagnosis & treatment should be
available

agreed upon policy to treat as patients
 Cost considerations

adequate resources
 Expected benefit
Principles
1. Condition should be an important public health
problem
2. There should be a recognizable early or latent
stage
3. There should be an accepted treatment for
persons with condition
4. The screening test is valid, reliable and acceptable
yield
5. The test should be acceptable to the population to
be screened
6. The cost of screening and case finding should be
economically balanced in relation to medical care
as a whole
Screening tests
Acceptability :
Simple, minimum pain or discomfort, easy,
safety, rapidity , acceptable procedure, etc.
Repeatability
Must give consistent result when repeated
several times on the same subject or
individual
Observer variation
Biological variation
Observer variation
Intra observer or within observer variation
Measurements should be same( subject or
material) if recorded several times at that time or
event
Ex. Ht, wt, lab tests. etc..
Inter observer variation or between observer
variation
Differences in measurements on the same (
subject or material) by different investigators
Ex. X ray, blood smears, organisms, etc..
Biological variation
1. Variation in physiological variables. Ex. BP,
Rates, etc.
2. Changes in parameters observed
1. Ex. MI, Cancers, etc
3. Variations in their symptoms /signs
1. Memory, pain, quality of care, etc.
4. Regression to the mean
1. Tendency of values merge at the mean.
This is tested by repeated observation over time
Thought for the day
Every job is a self portrait of the
person who did it.
Autograph your work with
excellence.
Thank you
Difference
• Screening test is intended for initial
examination.
• Diagnosis involves confirmation of
presence or absence of disease in
someone suspected or at risk for
having a disease.
• Case finding measures