Screening of diseases

1. Screening

2. Screening
• Screening is the process to detect among healthy people
disorders or risk factors of which they are unaware
• Screening is the process in which we use a test to determine
whether an individual likely has a particular health indicator
or not or is likely to develop a particular health indicator or
not

3. Screening
• Screening is not the same as diagnosis; screening tests give
us information about whether the disease is likely to be
present while diagnosis is confirmation of presence of the
disease e.g. Urine Pregnancy test (screening) vs Serum HCG
levels (diagnostic)
• A screening test assesses the presence of an underlying
marker that is associated with outcome of interest e.g.
Screening = Breast CA, Screening Test = Mammography
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4. Flow diagram for
a screening program
Population
Test -ve Test +ve
Unaffected Affected
Intervention
Diagnostic procedures
Screening test
Re-screen Re-screen

6. Population screening
• Immediate objective of a screening test is to classify people
as being likely or unlikely of having the disease.
• Ultimate objective of a screening test is to reduce mortality
and morbidity.

7. Principles for Screening Programs
1. Condition should be an important 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, with 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
7. Long latent period (the interval between exposure to a
carcinogen, toxin, or causing organism and development of a
consequent disease
8. Early detection improves prognosis

8. Feasibility
• Acceptability
Quick
Easy
Safe
• Cost effectiveness
Screening
Diagnosis
Follow-up
Intervention

9. Effectiveness
• Outcome measures:
Morbidity
Disability
Mortality

10. Criteria for Evaluating a Screening
Test
Validity: provide a good indication of who does and does not
have disease
-Sensitivity of the test
-Specificity of the test
Reliability (precision): gives consistent results when given to
same person under the same conditions
Yield: Amount of disease detected in the population, relative
to the effort. Prevalence of disease/predictive value.

11. Validity
• To assess the validity of a screening tool in establishing the presence
of disease we compare with a gold standard.
• Sensitivity: True positives
measures the proportion of positives that are correctly identified as
such. Ideal is 100% ; 100% of cases are detected.
(e.g., the percentage of sick people who are correctly identified as
having the condition)
• Specificity: True negatives
measures the proportion of negatives that are correctly identified as
such. Ideal is 100% ; 100% of non-cases are negative.
(e.g., the percentage of healthy people who are correctly identified as
not having the condition)

12. Types of Screening
• Mass screening : Mass screening means, the screening of a
whole population or a subgroup. It is offered to all,
irrespective of the risk status of the individual.
• Selective screening : High risk screening is conducted among
risk populations only.
• Multiphasic screening : It is the application of two or more
screening tests to a large population at one time instead of
carrying out separate screening tests for single diseases.

13. Examples of screening
• Women receive regular screening tests beginning in young
adulthood for cervical cancer (Pap smear)
• Physicians assess blood pressure and cholesterol as screening
tools for the development of cardiovascular disease
• Mammography for Breast CA
• Women use home pregnancy tests to screen for presence of
an embryo or fetus

14. Example Case:
Screening vs. symptom diagnosis
• Person 1’s cancer detected through screening
• Person 2’s cancer detected due to symptoms
• Person 1 and person 2 have equal time from cancer onset to
death
• Time from cancer detection to death is longer in Person 1
• Lead time: The time from detection from screening and
detection through symptoms afforded by screening test.