Screening Tests Overview

Screening Tests

• Screening for diseases, or risk factors predicting diseases, utilizes tests on a large scale to detect disease in seemingly healthy individuals.
• This is done to facilitate early detection and treatment, a component of secondary prevention.
• Screening tests typically assess risk factors rather than establish a firm diagnosis, requiring follow-up and treatment tailored to each individual.

Types of Screening

• Mass Screening: Targets the entire population (or a subset), regardless of individual risk. Examples include visual defects in schoolchildren, mammograms for women under 40, and newborn screenings. Mass screening is useful for early treatment to reduce the duration or severity of an illness.
• Multipurpose/Multiphasic Screening: Involves using several tests simultaneously to detect multiple diseases. An example is screening pregnant women for VDRL, HIV, and HBV using serological tests.
• Targeted Screening: Focuses on specific high-risk groups, increasing the probability of identifying cases. Examples include screening pregnant women for Down syndrome in high-risk cases, cancers with a family history, and hypertension and diabetes.
• Case-finding/Opportunistic Screening: Identifies diseases in patients already seeking healthcare for other reasons. Testing for Rhesus disease (RHD) in children is an example.

Criteria for Choosing Screening Tests

• Significant Burden of Disease: The disease should significantly impact the population.
• Detectable Preclinical Stage: The disease's early stages can be detected.
• Well-Understood Disease History: The disease's progression is well understood.
• Appropriate Test Availability: Valid, reliable, inexpensive tests for early disease detection.
• Effective Treatment: Treatment options should be available for confirmed cases to benefit the disease outcome.
• Appropriate Policy: A clear policy for the screening program.

Screening Tests: Characteristics

• Inexpensive: Affordability is crucial.
• Acceptable to the public: The test should not cause undue or unnecessary distress to the population.
• Reliable: Produces consistent results.
• Valid: Measures what it's intended to, separating people with and without disease accurately via specificity and sensitivity indicators.

Indicators for Validating Screening Tests

• Sensitivity: Accuracy in identifying true cases among those with the disease.
• Specificity: Accuracy in identifying those without the disease among those without the disease.
• Predictive Value: The accuracy of positive or negative test results in predicting the presence or absence of a disease.

Screening Test Results

• True Positives: Correctly identifying affected individuals.
• False Positives: Healthy individuals incorrectly identified as sick.
• True Negatives: Correctly identifying healthy individuals.
• False Negatives: Affected individuals incorrectly identified as healthy.

Diagnostic Tests

• Purpose: Confirms suspected diagnoses using laboratory investigation methods (genetic, microbiological, biochemical, physiological investigations).

Screening vs. Diagnosis

Surveillance

• Purpose: Ongoing collection, analysis, and interpretation of health data to inform, implement and evaluate public health practices with timely distribution to those who need the information.
• Aims: Monitor disease trends; describe disease history; identify epidemics; monitor infectious agents; evaluate hypotheses; plan policy; and evaluate public health interventions.

Types of Surveillance

• Passive Surveillance: Relying on health care providers/laboratories to report cases.
• Active Surveillance: Health departments actively request information to identify possible cases.
• Sentinel Surveillance: Focused on a specific subgroup to track disease patterns.
• Scanning Surveillance: Quickly identifying cases before signs become prominent for early disease interventions.

Sources of Surveillance Data

• Mortality Data: Death registries
• Morbidity Data: Disease reports from routine services/individual case reports
• Data for Detecting Epidemics: Data from field investigations, community-based surveillance, etc.