Differentiate between screening test and then diagnostic test.

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Screening Test vs. Diagnostic Test

Definitions

Screening test: The presumptive identification of an unrecognized disease or defect by the rapid application of tests, examinations, or other procedures in apparently healthy individuals. It is not meant to be diagnostic - persons with positive or suspicious findings must be referred for further diagnostic work-up and treatment. - Park's Textbook of Preventive and Social Medicine

Diagnostic test: A test applied to symptomatic individuals or those who have screened positive, aimed at confirming or ruling out a disease with greater accuracy. It forms the basis for treatment decisions. - Park's Textbook of Preventive and Social Medicine

Key Differences (Tabular)

Feature	Screening Test	Diagnostic Test
Population	Done on apparently healthy individuals	Done on those with symptoms, signs, or a positive screen
Application	Applied to groups/populations	Applied to individual patients; all diseases are considered
Result finality	Results are arbitrary - not final; just a flag	Diagnosis can be modified in light of new evidence; it is the sum of all evidence
Basis of decision	Based on one criterion or a cut-off point	Based on evaluation of multiple symptoms, signs, and lab findings
Accuracy	Less accurate	More accurate
Cost	Less expensive	More expensive
Basis for treatment	Not a basis for treatment	Used as a direct basis for treatment
Initiative	Comes from the investigator or health agency	Comes from a patient with a complaint
Sensitivity/Specificity	High sensitivity preferred (catch all positives; false positives acceptable)	High specificity preferred (minimize false positives; confirm true disease)

(Source: Park's Textbook of Preventive and Social Medicine, Table 1)

Elaboration on Key Points

1. Purpose

A screening test casts a wide net over a large population to separate those who probably have the disease from those who probably do not.
A diagnostic test is used to verify whether a screened-positive (or symptomatic) individual truly has the disease.

2. Sensitivity vs. Specificity Trade-off

Screening tests prioritize high sensitivity - they must not miss true cases (low false-negative rate). Missing a true case at the screening stage is the bigger harm.
Diagnostic tests prioritize high specificity - they must not label a healthy person as diseased (low false-positive rate), since a wrong diagnosis leads to unnecessary, potentially harmful treatment.

3. Criteria for a Good Screening Program

As outlined in Creasy & Resnik's Maternal-Fetal Medicine:

The condition must have a significant health burden
Effective early treatment must exist for those who screen positive
The test must be valid (accurate) and reliable (reproducible)
The test must be inexpensive and easy to perform
The test must be safe and acceptable to patients
The program must be cost-effective

Example: Cervical cytology (Pap smear) for cervical cancer is a successful screening program because it fulfills all these criteria. In contrast, cytomegalovirus screening has limited value despite disease burden because no effective intervention exists for screen-positive patients.

4. Relationship Between the Two

Screening and diagnosis are not competing - they are sequential. A positive screening result triggers a diagnostic workup. For example:

Glycosuria (screening) → Glucose tolerance test (diagnosis) for diabetes
Elevated fundal height (screening) → Ultrasound (diagnosis) for fetal growth restriction
Abnormal cell-free DNA result (screening) → Amniocentesis (diagnosis) for chromosomal anomaly

5. Some Tests Serve Both Roles

Certain tests can function as both. The glucose tolerance test and haemoglobin estimation for anaemia are used both as screening tests in population surveys and as diagnostic confirmatory tests in clinical practice. - Park's Textbook of Preventive and Social Medicine

Concept of "Lead Time"

Screening is most valuable when there is a significant lead time - the time between when a disease can be detected by screening and when it would normally present clinically. Screening is not worthwhile for diseases with an obvious, rapid onset where no preventive action is possible in the pre-symptomatic phase.

Sources:

Park's Textbook of Preventive and Social Medicine - Section: Screening and Diagnostic Tests, Table 1
Creasy & Resnik's Maternal-Fetal Medicine: Principles and Practice - Section: Screening Versus Diagnosis, p. 311

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