Preventive Medicine and Screening Programs

Screening for Disease

• Screening involves actively searching for diseases in apparently healthy people.
• Epidemiological studies show that a large portion of diseases are hidden in the community, unlike the pattern seen in hospitals.
• The iceberg phenomenon analogy illustrates the hidden mass of disease (subclinical cases, carriers, undiagnosed cases) compared to the apparent disease seen by physicians.

Introduction

• Screening is a fundamental aspect of disease prevention.
• Screening identifies unrecognized disease or defects in apparently healthy individuals through rapid examinations and tests.
• Historically, annual health checkups were designed for early detection of hidden diseases.
• Approaches like using technicians for simple, inexpensive laboratory tests and reducing physician time are used for wider screening access.

Concept of Screening

• Screening programs have evolved from focusing on individual diseases to targeting selected groups such as antenatal mothers, school children, and occupational groups.
• Screening tests have significantly expanded.
• Screening programs can be seen as a logical extension of healthcare.
• Screening programs differ from periodic health exams in being more applicable, less expensive, and needing less physician time.

Screening and Diagnostic Tests

• Screening tests are not diagnostic and provide an initial examination.
• Positive results lead to further diagnostic work-up by physicians.
• Some tests are used for both screening and diagnosis, but different criteria are used.

Screening and Diagnostic Tests Contrasted

Concept of Lead Time

• Screening is not beneficial for diseases with obvious onsets.
• Limited to conditions with a significant lag between disease onset and diagnosis.
• Valuable to catch diseases before reaching a critical point where treatment becomes ineffective or permanent damage is done.
• Screening efforts should concentrate on conditions where the time lag between onset and critical point is long enough for effective public screening.
• Benefits of screening programs must outweigh the complexities and costs.

Aims and Objectives

• Screening aims to identify individuals with a disease or increased risk, directing them to medical supervision and treatment.
• Screening emphasizes earlier diagnosis and treatment to influence disease progression favorably.

Explanation of Terms

• Screening: Testing for infection or disease in populations not seeking health care. Examples include serological testing for AIDS, neonatal/premarital screenings.
• Case Finding: Using tests to detect disease in individuals seeking health care for other reasons. This includes screening for diseases in pregnant women, etc.
• Diagnostic Tests: Confirming or refuting a disease or abnormality in patients with symptoms. Examples include VDRL testing, endocervical cultures for infections.

Uses of Screening

• Case Detection: Screening for conditions not arising from patient requests (e.g., neonatal screening).
• Disease Control: Screening to benefit others, such as screening immigrants or for infections like rheumatic fever.
• Research Purposes: Screening can help to understand the natural history of chronic diseases and estimate prevalence/incidence.
• Educational Opportunities: Screening can raise public awareness, educate health professionals.

Types of Screening

• Mass Screening: Screening a whole population or subgroup, like all adults (e.g., tuberculosis). This is less useful unless combined with treatment.
• High-Risk/Selective Screening: Targeting high-risk groups (e.g., cancer cervix screening in lower social groups).
• Multiphasic Screening: Combining multiple tests (e.g., chemical and hematological tests, lung function assessment, and visual acuity tests). It is not as beneficial as originally thought.

Criteria for Screening

• Before initiating a screening program, a decision on its worthiness needs ethical, scientific, and possible financial justification.
• Screening criteria depend on two factors:
  • The disease to be screened (recognizable latent or early stage, important health problem, understood natural history).
  • The test to be applied (acceptable, repeatable, valid).

Disease (to be screened)

• Screening should be performed for conditions fulfilling criteria for screening.
• The disease to be screened must be an important health problem.
• The disease must exhibit a recognizable latent (asymptomatic) stage.
• The natural history of the disease must be understood, and the disease progression process known to be possibly reversible.
• The test to be used must be able to detect the disease before it shows symptoms.
• Treatment must be available.
• Benefits of early detection must outweigh the risks and costs.

Screening Test (specific criteria)

• Test must meet criteria for acceptability, repeatability, and validity, plus other criteria (yield, simplicity, safety, rapidity, ease of administration, cost). Tests for one criterion might not meet another criterion.
• A testing choice might need to compromise certain factors.
• Important Considerations include repeatability, validity (accuracy) and how well the test measures what is intended to measure.

Validity (Accuracy)

• Validity refers to test accuracy and separation of diseased and non-diseased persons.
• An example is the glycosuria test for diabetes.
• A better test would be the Glucose tolerance test.
• The measure accuracy refers to agreement of measured values with true values.
• Includes components of sensitivity and specificity for validity assessments.
• Sensitivity and specificity measurements are expressed in percentages.

Evaluation of a Screening Test

• Yield: The number of previously undiagnosed cases found. Depends on sensitivity/specificity, prevalence, participation.
• Combination of Tests: Using multiple tests (especially to enhance specificity or sensitivity); For example, screening for syphilis can involve two tests RPR (nonspecific) and FTA-ABS (more specific).
• Problem of Borderlines: Difficulty of determining a cut-off point. Depends on the prevalence of the disease and mortality when using screening tests.
• Importance of follow-up: Following up on patients with positive screening test results is important.

Evaluation of Screening Programs

• Randomized Controlled Trials (RCTs): Ideal for evaluating a program's effect on disease but are often impractical due to large numbers, long duration, and costs.
• Uncontrolled Trials: May show improved survival from disease.
• Other Methods: Involve case control studies and comparing trends across areas using different screening degrees of coverage. This helps determine whether screening is effective compared to conventional care approaches.