Screening vs Diagnostic Tests & Concept of lead Time

1. SCREENING Vs DIAGNOSTIC
TESTS and LEAD TIME
P.PRATHYUSHA
ROLL:90
Guided by Dr. Sipra mam

2. • Under WHO guidelines....
• Important health problem.
• Treatment.
• Facilities for diagnosis and treatment.
• Latent stage of disease.
• Test or examination for the condition.
• Should be acceptable to the population.
• Case finding should be a continuous process.. ,not just
a once and for all” project.
Principles of screening

3. Diagnostic test
• It is to establish presence or absence of
disease as a basis for treatment decisions in
symptomatic or screening positive
individuals.

4. Screening Vs
diagnostic tests

5. • Screening test
• Done on apparently
healthy.
• Applied to groups.
• Test results are arbitrary
and final.
• Based on one criteria.
• Not a basis for treatment.
• Simple and acceptable to
patients.
• Diagnostic test
• Done on those with sick.
• Applied to single patient.
• Diagnosis is not final ,it is
sum of all evidences.
• Based on evaluation of no.
Of signs and symptoms.
• Used as a basis for
treatment.
• Expensive but justifiable to
establish diagnosis.

6. • Initiative come from
the investigator or any
agency providing care.
• Less accurate.
• Ex ;serological test for
AIDS virus in blood
donors, neonatal
screening etc..,
• Initiative come from
patient with a
complaint.
• More accurate.
• Ex; VDRL testing of
patient with lesions
suggestive of
secondary syphilis
etc.,

7. •
•
•
Screening test
Positive Negative
Diagnostic test
Positive
Negative
Rescreening
Treatment

9. CONCEPT OF LEAD TIME
• LEAD TIME:
• It is the length of time between the
detection of disease and its usual clinical
presentation and diagnosis .
• OR it is the time between early diagnosis with
screening ,and diagnosis without screening.

10. Example.......
Advantage gained by screening ..,i.e., the period b/w
diagnosis by early detection and diagnosis by other
means.

11. Contd......
• Lead time bias: is the factor when
comparing , in detection of disease ,a
traditional test with a new or experimental test
but the out come of disease is unchanged.
• The new or experimental test merely identifies
the disease earlier than the traditional.

12. Example :
Early detection of cancer by screening may
prolong the perceived survival time without
affecting the course of disease. It relatively
depends on their socioeconomic conditions or
access to medical care.
It can affect interpretation of survival rate.

13. •HUNTINGTON’S DISEASE is a genetic disorder.
diagnosed when symptoms appear around 50
yrs and person dies around 65 yrs. therefore Pt
lives about 15 yrs after diagnosis. A genetic test
at birth may diagnose the disease early but pt
survives 65 yrs after the diagnosis with
knowledge of disease.

15. THANK
YOU