Foundations of Epidemiology - Screening: Validity & Reliability

Disease Prevention Strategies

• Types of Disease Prevention

  • Primary Prevention:

    • Goal: Prevent disease occurrence.

    • Examples: Immunizations, tobacco control programs.

  • Secondary Prevention:

    • Goal: Early detection of disease through screenings in clinical settings.

  • Tertiary Prevention:

    • Goal: Improve treatment outcomes and minimize morbidity.

    • Focus on developing new therapies.

Screening Overview

• Definition: Application of tests or procedures to detect disease in asymptomatic individuals (preclinical or sub-clinical diseases).

• Purpose: Early detection for disease control and secondary prevention.

Lung Cancer Screening Insights

• Statistics:

  • Lung cancer is the leading cause of cancer deaths.

  • More screening is required; specifically, with low-dose CT for early detection.

  • 7 of 8 adults fitting the criteria did not report getting screened.

• Screening Criteria: Adults aged 55-80 with a significant smoking history (30 pack-years) who currently smoke or have quit within the past 15 years.

Natural History of Disease

• Stages of Disease:

  • Time progression includes Asymptomatic Phase, Symptomatic Phase, and outcomes that follow.

  • Effectiveness of screening relies on these stages.

Screening Recommendations by USPTF (C grade or better)

• Cancer Screening Modalities:

  • Breast Cancer: Mammography.

  • Cervical Cancer: Cervical cytology (Pap smear) + HPV testing.

  • Prostate Cancer: PSA test.

  • Colorectal Cancer:

    • Invasive: Colonoscopy, Flexible sigmoidoscopy.

    • Non-invasive: FIT, FOBT, DNA tests (e.g., Cologuard).

  • Lung Cancer: Low-dose CT for current or past smokers.

Evaluating Screening Programs

• Key Measures:

  • Validity: Does the test measure what it should?

  • Reliability: Can results be replicated?

    • Assess consistency of results across repeated tests or observers.

Potential Harms of Screening

• Risks associated with screening tests, including complications, false positives leading to unnecessary anxiety, and false negatives that provide false reassurance.

• Overdiagnosis: Diagnosing diseases that would not have caused symptoms or required treatment during a person's lifetime.

Example of Overdiagnosis in Practice

• Hypothetical scenarios depicting how early screening versus no screening affects patient outcomes.

Factors Affecting Screening Validity

• Sensitivity: Proportion of people with the disease who test positive.

• Specificity: Proportion of people without the disease who test negative.

• Positive Predictive Value (PPV): Likelihood of having the disease given a positive test result.

• Negative Predictive Value (NPV): Likelihood of being disease-free given a negative result.

Practical Applications of Evaluating Screening

• Use of 2x2 tables for assessing the effectiveness of new tests against established gold standards.

• Case Study: Evaluating a new molecular test for colorectal cancer screening.

Understanding Predictive Values and Prevalence

• PPV and NPV are influenced by test sensitivity, specificity, and the population prevalence of the disease.

• Adjusting test cut-offs affects sensitivity and specificity, offering trade-offs between false positives and negatives.

Summary of Test Validity Measures

• Summarizes sensitivity, specificity, PPV, and NPV, exploring their importance in the context of public health screening.