1. Clinical Epidemiology
Chapter 12
Clinical Epidemiology

2. Chapter Objectives Define clinical epidemiology
Understand various aspects of screening and diagnostic tests
Describe various measures for evaluating prognosis
Discuss potential biases and ways to avoid bias in evaluation of tests and treatments

3. Clinical Epidemiology
Clinical epidemiology focuses on patients and the application of epidemiologic methods to assess the efficacy of screening, diagnosis, and treatment in clinical settings
Used to identify the health consequences of employing a test or administering a treatment

4. Questions About Clinical Epidemiology
Who is most likely to participate in screening and diagnostic testing?
How accurate is the screening or diagnostic test?
If a treatment is efficacious, what proportion of patients benefit from the treatment?
What characterizes those who benefit and those who do not benefit from the treatment?
How much do patients benefit from a treatment?
What are the risks associated with screening, diagnostic testing, or treatment?

5. Secondary Prevention
Health screening and detection activities used to identify disease

6. Screening vs. Diagnostic Tests in Cancer
Examples of screening methods
Pap test
Mammograms
Blood tests (e.g., PSA)
Fecal occult blood test (FOBT)
Biopsy
Small blood sample – leukemia

7. Medical Screening
Used to suggest or detect disease among individuals in a population without signs or symptoms of the health problem

8. Why do we do screenings?
Screening is used to promote detection of diseases in their earliest stages, when treatment has the greatest chance of working, in order to reduce morbidity and mortality

9. Common Screening Tests
Papanicolaou test (Pap smear, Pap test, cervical smear, or smear test)
Mammogram
Clinical breast exam
Fecal occult blood test
Prostate-specific antigen test
Blood pressure determination
Cholesterol level
Eye examination
Urinalysis
Tuberculin skin test
Beck Depression Inventory

10. How does mass screening compare with selective screening?
Mass screening is not selective but involves application of screening tests to the total population
Selective screening involves applying the screening tests to high risk groups

11. Screening Guidelines 1968 World Health Organization
The disease or condition being screened for should be a major medical problem
Acceptable treatment should be available for individuals with diseases discovered in the screening process
Access to healthcare facilities and services for follow-up diagnosis and treatment for the discovered disease should be available
The disease should have a recognizable course, with identifiable early and latent stages

12. Screening Guidelines (cont’d)
A suitable and effective test or examination for the disease(s) should be available
The test and the testing process should be acceptable to the general population
The natural history of the disease or condition should be adequately understood, including the regular phases and course of the disease
Policies, procedures, and threshold levels on tests should be determined in advance to establish who should be referred for further testing, diagnostics, and possible treatment
The process should be simple enough to encourage large groups of persons to participate

13. Evaluating Screening Guidelines
How can the application of epidemiology provide a means for evaluating screening guidelines?
Descriptive and analytic epidemiologic methods are useful for establishing and understanding
Extent of public health problem
Efficacy of treatment
Access to health care
Natural course of disease
Efficacy of a screening or diagnostic test

14. Validity, Reliability, and Yield
Validity – How well the test actually measures what it is supposed to measure
Reliability – How well the test performs in use over time(its repeatability)
Yield – The amount of screening the test can accomplish in a time period

15. Validity
Refers to the accuracy and trustworthiness of instruments, data, and findings in research
Are the instruments that were used to measure something valid?
GRE
SAT
ACT

16. Two ways to view and assess validity
Content validity – Achieved when an instrument has appropriate content for measuring a complex concept or construct
If you walk out of a test and feel that it was unfair because it tapped too narrow a band of knowledge
Construct validity – The measures “get at” or actually measure what you are actually studying
Does the Wechsler IQ test measure intelligence?

17. Accuracy
The degree to which the measurement represents what it is intended to represent
Important influence on the validity
Spring on a scale not calibrated correctly; the scale is off.

18. Measures of Validity
Sensitivity – Ability of the test to correctly identify those with the disease
Specificity – Ability of the test to correctly identify those without the disease
Overall accuracy – Ability of a test to predict the presence or absence of a disease
PV+ – Proportion of people with a positive test result who have the disease
PV- – Proportion of people with a negative test result who do not have the disease

19. Likelihood Ratios
Two additional measures for appraising screening and diagnostic evidence
Likelihood Ratio Positive (LR+)
Likelihood Ratio Negative (LR-)

20. Likelihood Ratios
Positive – Level of confidence that a person who obtains a score in the affected range truly does have the health problem
Negative – Confidence that a score in the unaffected range comes from a person who truly does not have the health problem

21. Prognosis
Prediction or forecast of the course of a disease based on anticipation from the usual natural history of the disease or peculiarities unique to the case
Prognostic indicators tell the doctor the likely behavior of the cancer and its responsiveness to treatment

22. Measures of Prognosis
Case fatality rate – Proportion of newly diagnosed cases that die from a given disease in a specified period
Survival rate – Proportion of persons surviving, regardless of cause of death

23. Cox Proportional Hazards Regression Model
Useful for analyzing survival data
Indicates the probability that a person will experience an event
Estimate the relative risk, adjusted for potential prognostic factors, thereby minimizing the threat of confounding

24. Lead Time Bias
Lead Time – Difference in time between the date of diagnosis with screening and the date of diagnosis without screening
Lead Time Bias – When lead time is counted in the survival time of patients, it gives a misleading picture of the benefit of treatment

25. Selection Bias Choosing data that distorts the outcome of a test
May make a test look better or worse than it really is, in terms of survival

26. Over-Diagnosis Bias
Occurs when screening identifies an illness that would not have shown clinical signs before a person’s death from other causes
Makes screening efforts look good because of increased identification of abnormalities
Individual may undergo unnecessary treatment, with its accompanying risk

27. Avoiding Bias Randomized controlled trial
Through randomization, different prognostic factors are balanced between groups, and the “true” screening or treatment effect can be determined

28. Outcomes Research
A relatively new field that seeks to understand the end results of clinical practices and interventions
Combines information about the care people are getting
Important in developing better ways to monitor and improve clinical care

29. Conclusion Clinical epidemiology
Involves the application of epidemiologic methods to improve the quality and value of patient care
Involves assessment of the efficacy of screening, diagnosis, and treatment strategies in clinical settings