Introduction to Basic Diagnostic Test Interpretation

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This unit will acquaint you with the basics of assessing the value of a diagostic test.  Here, "diagnostic test" is used in the broadest sense to mean any type of information that could be helpful in making a diagnosis.  I am not only talking about laboratory tests but also about symptoms, risk factors, and physical exam findings.   The most common use of this approach will be when a patient has an abnormal lab test result and you wonder "What does this really mean?"  That is, "How likely is it that this patient really has the disease in question."

Here is a sample problem:

A 40 year-old woman has a routine mammogram that is interpreted as "positive."  The probability of breast cancer is 1% for a woman at age 40 who participates in routine screening.  If a woman has breast cancer, the probability is 80% that she will have a positive mammogram.  If a woman does not have breast cancer, the probability is 10% that she will also have a positive mammogram.  What is the probability that this woman with a positive mammogram actually has breast cancer?

See if you can estimate the answer.  Be warned though, the answers to these types of problems are not intuitive.  After you finsh the section on the basics, you can return to this problem and compute the correct answer.  You may be surprised how different it is from your intuition.

At this point, you may be thinking:  "This stuff seems too mathematical.  No busy physician would have the time to mess with formal calculations every time a lab test comes back."  That is true, but there are still several good reasons to learn the basic methods.

  1. Problems like this will appear on board examinations.
  2. These concepts appear frequently in the medical literature.   Formal analyses of test characteristics form the basis of recommended testing strategies and clinical practice guidelines.
  3. Learning the basic concepts will help you make better decisions about test ordering and interpretation even if you don't go through the math each time.

The remaining sections cover the terminology of diagnostic test interpretation and present a step-by-step  approach to evaluating these types of problems.  You will need to practice these skills on your own to attain even minimal mastery.  You will know that you have mastered the material when you no longer have trouble solving the practice problems.
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