Criteria for screening test.

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I now have thorough content from Park's Textbook and Creasy & Resnik's Maternal-Fetal Medicine. Here is the complete answer:

Criteria for a Screening Test

Screening is defined as the presumptive identification of an unrecognized disease or defect by applying tests or examinations rapidly to apparently healthy individuals. A screening test is not diagnostic - positive findings require referral for confirmatory diagnosis and treatment.

The criteria are classically divided into two groups: those pertaining to the disease and those pertaining to the test itself (Wilson & Jungner criteria, as elaborated in Park's Textbook).

A. Criteria Related to the Disease

Important health problem - The condition should be a significant public health problem (generally with high prevalence).
Recognizable latent/early asymptomatic stage - There must be a detectable pre-symptomatic phase during which intervention is possible.
Adequately understood natural history - The progression from latent to declared disease must be well known, including the point at which the process becomes irreversible.
Detectable before symptoms - A test must be able to identify the disease prior to the onset of signs and symptoms.
Facilities for confirmatory diagnosis - Resources must exist to follow up screen-positive individuals with definitive diagnostic tests.
Effective treatment available - There must be an accepted, effective treatment for the condition detected.
Agreed policy on treatment thresholds - There should be consensus on whom to treat (e.g., borderline hypertension, impaired fasting glucose).
Early detection reduces morbidity/mortality - Good evidence must exist that early intervention improves outcomes.
Benefits outweigh risks and costs - The expected benefits (lives saved, disability prevented) must exceed the harms and financial costs of the program.

B. Criteria Related to the Screening Test

The test must satisfy the following properties:

1. Acceptability

The test must be acceptable to the target population. Tests that are painful, embarrassing, or discomforting (e.g., rectal or vaginal examinations) are unlikely to achieve high participation rates in mass campaigns.

2. Repeatability (Reliability / Reproducibility / Precision)

The test must give consistent results when repeated on the same individual under the same conditions. Repeatability is affected by:

Intra-observer variation - same observer, two different occasions
Inter-observer variation - two different observers measuring the same subject
Biological (subject) variation - natural fluctuation in the parameter being measured
Technical/method errors - lab or equipment variability

3. Validity (Accuracy)

Validity is the ability of a test to measure what it purports to measure. It has two components:

Parameter	Definition	Formula
Sensitivity	Ability to correctly identify those WITH the disease (true-positive rate)	a / (a + c) × 100
Specificity	Ability to correctly identify those WITHOUT the disease (true-negative rate)	d / (b + d) × 100
Positive Predictive Value (PPV)	Probability that a positive test truly has the disease	a / (a + b) × 100
Negative Predictive Value (NPV)	Probability that a negative test truly does not have the disease	d / (c + d) × 100

PPV and NPV depend on disease prevalence - the same test has a lower PPV in a low-prevalence population and a higher PPV in a high-prevalence population. Sensitivity and specificity are inherent properties of the test and are independent of prevalence.

4. Additional Practical Criteria

Simple - Easy to perform, ideally by non-specialist personnel
Safe - Minimal risk to the person being screened
Rapid - Results should be available quickly
Inexpensive - Cost must be feasible for wide application
Cost-effective - The overall program should be economically justifiable
High yield - Should detect a sufficient number of new cases to justify the effort

The Sensitivity-Specificity Trade-off

A higher cut-off point for a test increases specificity (fewer false positives) but reduces sensitivity (more false negatives), and vice versa. In practice, a trade-off is required:

For serious but treatable diseases (e.g., PKU in newborns), high sensitivity is preferred (missing a case is more harmful than a false alarm).
For conditions where treatment has significant side effects, high specificity is preferred.

Classic Example

Cervical cytology (Pap smear) screening for pre-malignant cervical lesions satisfies all criteria: cervical cancer is an important health problem, there is a detectable latent phase (CIN), treatment is effective, the test is acceptable and inexpensive, and early detection clearly reduces mortality.

In contrast, cytomegalovirus (CMV) screening in pregnancy fails the criterion of effective intervention - even though CMV causes significant fetal disease, no successful treatment can be applied to screen-positive women, so the program has limited value.

Sources: Park's Textbook of Preventive and Social Medicine, pp. 156-158; Creasy & Resnik's Maternal-Fetal Medicine, p. 517.

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