screening vs diagnostic tests & concept of lead time

SCREENING Vs DIAGNOSTIC TESTS and

LEAD TIME

P.PRATHYUSHA ROLL:90

Guided by Dr. Sipra mam

• 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

Diagnostic test• It is to establish presence or absence of

disease as a basis for treatment decisions in symptomatic or screening positive individuals.

Screening Vs

diagnostic tests

• 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.

• 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.,

•

• •

Screening test

Positive Negative

Diagnostic test

Positive Negative

Rescreening

Treatment

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.

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

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.

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.

•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.

THANK YOU