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PREVENTION

Screening

Screening is meant to identify individuals with a disease before clinical symptoms occur.  Screening is effective if intervention at an early stage of the disease will be beneficial, and if the disease can be detected prior to this turning point.  Collecting information on patient medical history and habits, while not considered screening, can prove valuable in the decision to screen a patient.  Physicians must be attentive to the types of screening procedures and the criteria and factors that determine use and effectiveness of each procedure.  A test with high sensitivity will yield many false positives resulting in unnecessary follow up screenings, which can be emotionally and financially costly for patients.  A test with low sensitivity will not identify many with disease, while low specificity will fail to rule out many disease free individuals.  This is an important factor to consider in regards to disease severity, as it may result in treatment delay.  Another important factor to consider is the number needed to treat in order to have a positive outcome. How effective is the intervention and what are the side effects?

 Guide to Clinical Preventive Services: Second Edition (1996)

Introduction - Methodology
pp xxli – xlvi (pp 39 – 47 of 93 *)

Read the following source for a review of these concepts and terms:

Guide to Clinical Preventive Services
pp xliii - liii (42-54 0f 93*)

      Prevalence

      Incidence

These terms are used to describe the burden of suffering.  Know the difference between these terms

What are the criteria of effectiveness for the three categories of preventive services – counseling interventions, immunizations and chemoprophylaxis?

      Sensitivity

      Specificity

These relate to the accuracy of the screening test. 

How are they related to false negatives, false positives, true negatives and true positives? 

When is it better to use a test with a high sensitivity and low specificity? - high specificity and low sensitivity?

What are the ethical concerns of a test with low sensitivity?  - low specificity?

      Positive predictive value

How can the prevalence of disease affect the PPV?

      Reliability

        Inter-observer variation

        Intra-observer variation

These terms relate to the reproducibility of a test.  Know the difference between these two terms.

What are the different ways in which to test reliability?

      Treatment efficacy

How does this affect screening for disease?

      Lead time bias

      Length bias

These determine the apparent affect of screening on outcome

How would lead time bias be an issue in the case of mammograms and breast cancer?

How can length bias affect the apparent survival time of a patient with prostate cancer?

      Relative risk

      Attributable risk

Describe the focus of a highly effective intervention in terms of these risks.  Imagine different scenarios – high prevalence, low morbidity and mortality vs.  low prevalence, high morbidity and mortality.

Review the following:

      Randomized controlled trials

      Blinded trial

      Cohort study

      Case control study

      Ecologic study

   Ecologic fallacy

   Selection bias

   Observer bias

   Recall bias

   Confounding

   Statistical power

These refer to the various study types and factors that influence the quality of the evidence presented in these studies.

What are the differences between the study designs?

What factors may influence the choice in study designs?

What is the measurement of risk for cohort studies? - case control studies?

Which these studies can be done prospectively?

How can you avoid the ecologic fallacy?

Will a confounding factor strengthen or weaken your apparent measure of risk?

What are some methods to control for confounding variables?

* Adobe Acrobat page numbers

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