Screening Programs and Test Evaluation

• Screening and diagnostic tests help in identifying diseases and conditions early.
• Dr. Bharat Paul is the presenter of this information.

Contents Overview

• The information includes an introduction to screening and diagnostic tests.
• It also covers why screening is needed, lead time, and the various uses and types of screening.
• The relationship between screening and diagnostic tests, the use of multiple tests, and potential biases in screening are examined.
• An evaluation of screening programs, examples, and references are also included.

Introduction to Prevention

• Primordial prevention focuses on preventing the development of risk factors.
• Primary prevention involves health promotion and specific protections against diseases.
• Secondary prevention emphasizes early diagnosis and treatment of existing conditions.
• Tertiary prevention aims to limit disability and rehabilitate individuals with long-term conditions.

Screening Definitions

• Screening involves searching for unrecognized diseases or defects in apparently healthy individuals.
• Rapidly applied tests and examinations are typically used to screen individuals.
• Breast cancer screening using mammography is an example of screening.
• Case finding involves using clinical or laboratory tests to detect disease in individuals seeking health care for other reasons.
• VDRL testing to detect syphilis in pregnant women is an example of case finding.

Diagnostic Tests Definitions

• Diagnostic tests use clinical and/or laboratory procedures to confirm or refute the existence of disease or true abnormality in patients with signs and symptoms.
• VDRL testing of patients with lesions suggestive of secondary syphilis is an example of diagnostic tests.
• The gold standard test is the best test available, often invasive or expensive.
• Breast biopsy for breast cancer is an example of a gold standard test.

Need for Screening

• Screening helps quarantine cases of infectious diseases.
• Screening is required during an immunization program to screen out those already infected.
• Screening helps detect a disease when treatment will be most successful.
• Screening is important due to the iceberg phenomena of disease, where many cases are invisible.

Lead Time

• Lead time is the advantage gained by screening or the period between diagnosis by early detection versus diagnosis by other means.

Screening Test Outcomes

• Screening tests yield two possibilities.
• Individuals are either apparently healthy or abnormal.
• If apparently healthy, periodic screening is carried out
• If abnormal, the individual proceeds to normal, intermediate or abnormal courses of treatment.

Uses of Screening

Case Detection

• Prescriptive screening.
• The process involves presumptive identification of unrecognized disease
• It doesn't arise from patient's request, but is performed for people's benefit.
• Neonatal screening is one such example.
• Bacteriuria in pregnancy, breast cancer, diabetes mellitus, and iron deficiency anemia are screened for.

Control of Diseases

• This involves prospective screening for the benefit of others
• An example is screening immigrants for syphilis.

Research Purposes

• Used to understand the natural history of a disease like cancer.

Educational Opportunities

• Screening programs can serve as platforms for educational endeavors related to health and disease prevention

Types of Screening

Mass Screening

• Involves screening large populations or subgroups like adults.

High Risk or Selective Screening

• Applied selectively to high-risk groups defined by epidemiological research.
• An example is screening for cervical cancer in lower social groups.

Multi Phasic Screening

• The application of two or more screening tests to a large population at one time.
• Multiphastic screening is very expensive

Screening vs. Diagnostic Tests

Screening Tests

• Done on apparently healthy individuals.
• Applied to groups.
• Results can be arbitrary and are not final.
• Based on one criteria or cut off point.
• Less accurate and less expensive.
• Not a basis for treatment.
• Initiation comes from the investigator.

Diagnostic Tests

• Completed on those with indications for a disease.
• Applied to single patients.
• Diagnosis is not final.
• Based on a number of findings.
• More accurate and more expensive.
• Used as a basis for treatment.
• Initiation comes from a patient.

Criteria for Screening

Important considerations for determining if you should screen for a disease

• The health problem must be important.
• There must be a recognizable latent or early symptomatic stage.
• The natural history of the disease must be known.
• A test should exist that finds the disease prior to the onset of signs and symptoms.
• Confirmation of diagnosis must be available.
• Effective treatment must be accessible.
• There has to be a policy on which patients to treat.
• Early treatment should reduce morbidity and mortality.
• Expected benefits should exceed the risks and costs.

Focus for any screening

• Assessment of acceptability, repeatability, reliability, precision, and reproducibility.
• It also needs high validity and should yield meaningful results.
• Aspects like simplicity, safety, rapidity, and cost-effectiveness remain crucial.

Validity

• Determining the extent to which a test accurately measures what it intends to measure.
• It gauges the test's ability to accurately distinguish individuals those who have the disease from those who do not.
• The term is used to ensure that the measured values closely match the true values.
• The components of validity are sensitivity and specificity.

Acceptability

• A test should be acceptable to the targeted individuals, so they are willing to participate in the screening
• Tests should not be painful, discomforting, or embarrassing.
• Per rectal exams may not be as acceptable as PSA tests for prostate cancer screening.

Repeatability

• It must yield consistent results when conducted multiple times on the same material or individual, maintaining identical conditions.
• It includes reproducibility, precision and reliability.
• Observer variation, biological variation, and technical methods are three major factors of the process.

Observer Variation

• Intra-observer variation refers to the variation between repeated observations by the same observer on the same subject at the same time.
• Inter-observation variation refers to variation between different observers on the same subject.

Biological Variation

• This variation may be due to changes in the parameters observed.
• Cervical smears from a woman on different days is an example.
• Patient's perception of symptoms can result in variation.

Errors

• Erroneous calibration and defective instruments result in errors.

Sensitivity

• Refers to a test`s and can be improved through a high cut-point
• It accurately identifies individuals who have the disease (true positives).
• Defined as a fixed characteristic

Specificity

• Refers to the ability of a test to be accurate and can be improved through testing for a low cut-point
• It correctly identifies individuals who do not have the disease(true negatives).
• Defined as a fixed characteristic.

The Borderline Problem

• Setting the cut-off point is crucial for screening tests.
• If set to high, that is level C, the test will be highly sensitive though will yield many false positives.
• If you set it high to level D, then the test`s specificity increases.

Sensitivity and Specificity

Unimodal Distribution

• The closer the ROC plot is to the upper left corner, the higher the overall accuracy of the test.

Bimodal Distribution

• At area A, Non-glaucomatous eyes, the screening level set has a poor sensitivity and a good specificity.
• At area B, Glaucomatous eyes, the screening level set has a good sensitivity and a poor specificity.

Where the Cut Point is Set:

• A diagnostic test that is expensive or invasive.

Minimize False Positives

• Use a cut-point with high specificity.

Maximize True Positives

• If something will result in death easily, it increases true positives.
• Use a cut-point with high sensitivity.

Sensitivity and Specificity: Principle Screening Programmes

• It's the point to find results that are 100% for both so that only true results appear

Principle of Sensitivity vs. Specificity

• Sensitivity assesses the rate of true positives
• Specificity measures true negatives
• These measures tend to be inversely related.
• Sensitivity and specificity can be varied by altering the criteria determining a positive result.

Test Results - Predictive Value

• Assess test results through the calculation and interpretation of positive and negative predictive values.
• PPV and NPV are not fixed characteristics of tests.

Positive Predictive Value

• The proportion of patients who test positive, and actually have the disease.

Negative Predictive Value

• The proportion of patients who test negative are actually free of the disease.

Yield

• Refers to the amount of previously unrecognized disease that is diagnosed as a result of screening efforts.
• It relies on levels of sensitivity, specificity ad prevalence to work.

ROC Curves

• Receiver operating characteristic curves plot the true positive rate, or sensitivity, in function of the false positive rate or 1- specificity for different cut-off points, which is ideal in diagnosis

Multiple Tests Types

Sequential Testing (Two-Stage Screening)

• After an initial screening test, individuals who test positive are subjected to a second test to reduce false positives.
• Increases specificity but reduces sensitivity.
• Diagnosis requires all tests to be positive.

Simultaneous Testing

• Two or more tests are conducted in parallel to increase sensitivity.
• More false positives are also identified, decreasing specificity.
• Any positive result from the test is considered positive

Bias in Screening Tests

Lead Time Bias

• Involves the overestimation of survival when determining the survival duration among screen detected cases and diagnosis

Length Time Bias

• Overestimation of survival duration among screen-detected cases due to relative excess of slowly progressing cases.
• They are disproportionally identified due to detection being proportional to the amount of them

Over Diagnosis Bias

• It happens when all these people with harmless abnormalities wind up being counted as "lives saved" when it might actually be harming otherwise healthy people through over diagnosis

Selection Bias

• The factors differing between those willing to get tested against those who are not.
• Women for example with a family history of breast cancer would choose to join a mammography a program

Disadvantages of Screening Tests

• Tests are imperfect leading to false positives and false negatives.
• Screening involves costs and use of medical resources for those who do not need treatment.
• Adverse effects from the procedures such as stress, anxiety, radiation, chemical exposure.
• Unnecessary investigations and treatment for these false positive results.
• It can result in stress and anxiety caused by the prolonging knowledge of an illness lacking any improvement in outcome
• A false sense of security caused by the false negatives, which may delay final diagnosis.

Evaluation of Screening Program

• Screening is evaluated through randomized control trials and uncontrolled trials
• It can also be evaluated with: case control trials