Screening vs Diagnostic Tests & Concept of lead Time

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