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Ideal Screening Test - Park's Textbook of Preventive and Social Medicine

Criteria for Screening (Two Main Considerations)

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A. Criteria Related to the Disease

1. The condition must be an important health problem (generally, prevalence should be high)
2. There must be a recognizable latent or early asymptomatic stage
3. The natural history of the condition (including progression from latent to declared disease) must be adequately understood
4. There must be a test that can detect the disease prior to the onset of signs and symptoms
5. Facilities must be available for confirmation of the diagnosis
6. There must be an effective treatment available
7. There must be an agreed-upon policy about whom to treat (e.g., borderline diabetes, lower ranges of blood pressure)
8. There must be good evidence that early detection and treatment reduces morbidity and mortality
9. The expected benefits (lives saved) must exceed the risks and costs

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B. Criteria Related to the Screening Test

1. Acceptability

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

2. Repeatability (Reliability / Reproducibility / Precision)

• The test must give consistent results when repeated more than once on the same individual under the same conditions
• Repeatability depends on three major factors:

• Intra-observer variation: Same observer reads the same material differently at two times
• Inter-observer variation: Different observers read the same material differently (e.g., two radiologists reading the same chest X-ray - one calls it positive, the other negative)
• Blood pressure is a classic example of a poorly reproducible measurement because it is affected by all three factors

• Kappa statistic (κ): Measures inter-observer agreement beyond chance. Values:
  • 0.0 = agreement = chance alone

  • 0.75 = excellent agreement

  • 0.40 - 0.75 = fair to good agreement
  • < 0.40 = poor agreement

3. Validity

a. Sensitivity

• The ability of a test to identify correctly all those who have the disease (i.e., true positives)
• = TP / (TP + FN) × 100
• Also called "positivity in disease"
• A test with high sensitivity has few false negatives
• A highly sensitive test is used to rule OUT a disease (if negative, disease unlikely)

b. Specificity

• The ability of a test to identify correctly all those who do NOT have the disease (i.e., true negatives)
• = TN / (TN + FP) × 100
• Also called "negativity in health"
• A test with high specificity has few false positives
• A highly specific test is used to rule IN a disease (if positive, disease likely)

• Sensitivity = a / (a + c) × 100
• Specificity = d / (b + d) × 100
• False positive rate = b / (b + d) × 100 = 100 - Specificity
• False negative rate = c / (a + c) × 100 = 100 - Sensitivity

• Increasing sensitivity decreases specificity and vice versa
• An ideal screening test should be 100% sensitive AND 100% specific - in practice, this rarely occurs

• PPV increases with higher disease prevalence
• PPV is more useful clinically than sensitivity/specificity because it reflects the actual population being tested

4. Yield

• Yield is the amount of previously unrecognized disease that is diagnosed and brought to treatment as a result of screening
• Yield depends on: sensitivity of the test, prevalence of unrecognized disease in the population, and the proportion of the population who accept screening
• Yield increases if the test is applied to high-risk groups

5. Other Criteria for an Ideal Screening Test

• Simple - easy to perform, even by paramedical staff
• Safe - no harm to the person being screened
• Rapid - results available quickly
• Easy to administer - suitable for mass application
• Inexpensive (low cost) - economically feasible for large-scale use

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Summary: Properties of an Ideal Screening Test (Mnemonic: A R V Y S S R E C)

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