Introduction to Screening and Diagnostic Tests

Screening Overview

• Screening is the search for unrecognized disease in apparently healthy individuals using rapidly applied tests or examinations.
• The goal is to identify disease early, allowing for earlier intervention and improved outcomes.
• Screening aims to reduce mortality and suffering from disease.

Objectives of Screening

• Introduce the concept of screening to students.
• Explain the key difference between screening and diagnostic tests.
• Outline the criteria for diseases suitable for screening and for effective screening tests.
• Define sensitivity, specificity, predictive values (positive and negative).
• Describe various types of screening.

Screening vs. Diagnostic Tests

Definition

• Screening is a test for a particular disease in asymptomatic individuals.

Possible Outcomes of Screening

• Apparently healthy: (Screening tests)
• Apparently normal: (Periodic re-screening)
• Apparently abnormal:
• Normal – periodic re-screening
• Intermediate surveillance
• Abnormal – treatment

Uses of Screening

• Case detection
• Disease control
• Research purposes
• Educational opportunities

Types of Screening

• Mass/Population Screening: Screens an entire population without considering risk factors. Examples include:
• Single disease screening (e.g., cervical cancer)
• Multiphasic screening (e.g., biochemical profiles)
• Selective/Targeted Screening: Screens individuals identified as being at high risk for the disease Examples include:
• Single disease screening (e.g., CXR for pneumoconiosis)
• Multiphasic screening (e.g., antenatal examinations)
• Opportunistic Screening: Uses the opportunity to administer a screening test when an individual is already in contact with health services for another reason.

Criteria for Screening

• Disease to be Screened:
• Must be a significant health problem (high prevalence)
• Have recognizable latent or early asymptomatic stages
• Have an understood natural history of development from latent to symptomatic disease.
• Must be detectable before symptoms/signs appear.
• Required facilities should be available for confirmation of the diagnosis.
• Effective treatment should exist.
• Evidence that early detection and treatment reduces morbidity and mortality.
• Expected benefits of early detection to outweigh risks and costs.
• Screening Test to be Applied:
• Acceptable to those screened
• Repeatable results (reliability)
• Intra-observer variation (within observer)
• Inter-observer variation (between observers)
• Biological (subject) variation:
• Changes in observed parameters
• Variations in patient perceptions/answers
• Regression to the mean
• Error relating to technical methods:
• Defective equipment
• Calibration errors
• Faulty reagents
• Inappropriately designed or unreliable tests.

Validity of Screening Tests

• Validity (accuracy): The extent to which the test accurately measures what it is designed to. Measures whether those who have the disease are correctly identified and whether those who do not have the disease are correctly identified. Has two components:
• Sensitivity and specificity.
• Other factors including Simplicity, Safety, Rapidity, Low cost, and administration ease.
• Sensitivity (SN): The ability to correctly identify those who have the disease. Calculated as (true positive/[true positive + false negative]) × 100 = TP.
• Specificity (SP): The ability to correctly identify those who don't have the disease. Calculated as (true negative / [true negative + false positive]) × 100 = TN.
• Predictive values:* Measure whether an individual actually has the disease given the results of the screening test.
• Positive predictive value (PPV): Percentage of truly diseased people among those with positive test results calculated as (true positive / [true positive + false positive]) × 100.
• Negative predictive value (NPV): Percentage of healthy people among those with negative test results calculated as (true negative / [true negative + false negative]) × 100.

Dependence and Calculation

• True Positive (TP) is directly related to Sensitivity (SN).
• False Negative (FN) is inversely related to Sensitivity (SN).
• True Negative (TN) is directly related to Specificity (SP).
• False Positive (FP) is inversely related to Specificity (SP).
• Positive Predictive Value (PPV) depends on sensitivity, specificity, and prevalence of disease.
• Negative Predictive Value (NPV) depends on sensitivity, specificity, and prevalence of disease.

Examples of Screening Programs

• Childhood anemia screening
• Cervical cancer screening

This quiz introduces the concept of screening and how it differs from diagnostic tests. It covers the criteria for effective screening and the key terms related to screening processes such as sensitivity and specificity. Test your knowledge on various types of screening aimed at improving health outcomes.