© 2010 Jones and Bartlett Publishers, LLC

Chapter 12 Clinical Epidemiology

© 2010 Jones and Bartlett Publishers, LLC 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

© 2010 Jones and Bartlett Publishers, LLC 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.

© 2010 Jones and Bartlett Publishers, LLC Some questions relevant to clinical epidemiology 1. Who is most likely to participate in screening and diagnostic testing? 2. How accurate is the screening or diagnostic test? 3. If a treatment is efficacious, what proportion of patients benefit from the treatment? 4. What characterizes those who benefit and those who do not benefit from the treatment? 5. How much do patients benefit from a treatment? 6. What are the risks associated with screening or diagnostic testing or treatment?

© 2010 Jones and Bartlett Publishers, LLC Medical Screening  Used to suggest or detect disease among individuals in a population without signs or symptoms of the health problem.  Examples: Tuberculin skin test Beck Depression Inventory Papanicolaou test (Pap smear, Pap test, cervical smear, or smear test) Prostate-specific antigen (PSA) test

© 2010 Jones and Bartlett Publishers, LLC Screening Guidelines  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 health care 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

© 2010 Jones and Bartlett Publishers, LLC Screening Guidelines Cont.  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

© 2010 Jones and Bartlett Publishers, LLC 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.

© 2010 Jones and Bartlett Publishers, LLC Sensitivity and Specificity  Sensitivity – The ability of the test to correctly identify those with the disease  Specificity – The ability of the test to correctly identify those without the disease  Test’s Worth – The ability of a test to predict the presence or absence of a disease

© 2010 Jones and Bartlett Publishers, LLC Likelihood Ratios  Two additional measures for appraising screening and diagnostic evidence are Likelihood Ratio Positive (LR+) Likelihood Ratio Negative (LR-)

© 2010 Jones and Bartlett Publishers, LLC Likelihood ratios  Positive – The level of confidence we can have that a person who obtains a score in the affected range truly does have the health problem  Negative – The confidence that a score in the unaffected range comes from a person who truly does not have the health problem

© 2010 Jones and Bartlett Publishers, LLC 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

© 2010 Jones and Bartlett Publishers, LLC Measures of Prognosis  Case fatality rate – the proportion of newly diagnosed cases that die from a given disease in a specified time period.  Survival rate – Proportion of persons surviving regardless of cause of death.

© 2010 Jones and Bartlett Publishers, LLC Cox proportional hazards regression model  Useful for analyzing survival data  Indicates the probability that a person will experience an event  Allows you to estimate the relative risk, adjusted for potential prognostic factors, thereby minimizing the threat of confounding.

© 2010 Jones and Bartlett Publishers, LLC Lead Time Bias  Lead Time – The 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.

© 2010 Jones and Bartlett Publishers, LLC 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

© 2010 Jones and Bartlett Publishers, LLC 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

© 2010 Jones and Bartlett Publishers, LLC Avoiding bias Randomized Controlled Trial  Through randomization, different prognostic factors are balanced out between groups, and the “true” screening or treatment effect can be determined

© 2010 Jones and Bartlett Publishers, LLC 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

© 2010 Jones and Bartlett Publishers, LLC 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.