Criteria for screening

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Criteria for screening in psm

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Criteria for Screening (PSM)

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

Before starting any screening programme, it must have ethical, scientific, and financial justification. The criteria are divided into two broad categories: the Disease and the Screening Test.

A. Criteria Related to the DISEASE

The disease must fulfil all of the following before screening is considered worthwhile:

#	Criterion
1	The condition should be an important health problem (high prevalence)
2	There should be a recognizable latent or early asymptomatic stage
3	The natural history of the disease (latent to declared) should be adequately understood
4	There must be a test that can detect the disease before signs and symptoms appear
5	Facilities for confirmation of diagnosis must be available
6	There must be an effective treatment available
7	There must be an agreed-on policy about whom to treat (e.g., borderline blood pressure, borderline diabetes)
8	Early detection and treatment must reduce morbidity and mortality (good evidence required)
9	Expected benefits (lives saved) must exceed the risks and costs

Only when the above disease criteria are satisfied is it appropriate to consider a suitable screening test.

B. Criteria Related to the SCREENING TEST

The test must satisfy all of the following:

1. Acceptability

The test must be acceptable to the population
Tests that are painful, discomforting, or embarrassing (e.g., rectal/vaginal exams) have poor uptake in mass campaigns
High cooperation rate is necessary

2. Repeatability (Reliability / Precision)

The test must give consistent results on repeated application. Variability can arise from three sources:

Observer variation - intra-observer (same observer, different times) and inter-observer (different observers) variability
Subject/biological variation - e.g., blood pressure fluctuations, regression to the mean
Technical/method errors - defective instruments, faulty reagents, calibration errors

3. Validity (Accuracy)

The test must accurately distinguish diseased from non-diseased individuals. Validity has two components:

Sensitivity - ability to correctly identify those WITH the disease (true positive rate)
Specificity - ability to correctly identify those WITHOUT the disease (true negative rate)

4. Yield

The number of previously unrecognized cases brought to treatment as a result of screening
Influenced by: sensitivity of the test, prevalence of the disease, proportion of the population screened

5. Simplicity

Should be simple enough to be performed by paramedical/auxiliary staff

6. Safety

Must be safe with minimal or no risk to the individual

7. Rapidity

Should be rapid to screen large numbers in a short time

8. Ease of Administration

Can be easily organized and delivered in a community setting

9. Cost

Cost must be economically balanced in relation to benefits (cost-effectiveness)

Classic Reference: Wilson & Jungner Criteria (WHO, 1968)

These Park's criteria are derived from the 10 original Wilson & Jungner principles published by the WHO. The key additions from W&J are:

The treatment for cases found should be acceptable to the population
Case-finding should be a continuous process, not a once-and-for-all project

Quick Memory Aid

For Disease criteria: "INN LEAP"

Important health problem
Natural history understood
No symptoms yet (latent stage exists)
Lab/test available
Effective treatment
Agreed policy on treatment
Profit (benefit > risk/cost) + Proof of benefit from early Rx

For Test criteria: "A²R²V + SSeC"

Acceptability, Accuracy (validity), Repeatability, Rapidity, Validity - Simplicity, Safety, economic/Cost, Coverage/yield

- Park's Textbook of Preventive and Social Medicine, pp. 157-160 (Criteria for Screening section)

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