Instructor: Mitch Heflin, MD
Email: hefli001@mc.duke.edu

Chapter 18: Diagnostic Tests (required)

Chapter 16: Process of Diagnosis (optional)

Learning goals:

Describe the “diagnostic process,” including pretest & posttest probability and test and treatment thresholds.

Define, calculate and demonstrate the use likelihood ratios.

Explain the advantage of using a likelihood ratio over its individual components, sensitivity and specificity.


An 80 year old retired textile worker comes to you for a routine follow-up visit. He has been generally healthy, although he smoked 1 pack per day of cigarettes until he quit 5 years ago and still drinks two mixed drinks every evening. He complains of fatigue and increased dyspnea on exertion. He denies chest pain, fever, chills, cough, orthopnea, PND or edema. He has noted occasional palpitations associated with these symptoms.

On exam, he is pleasant but disoriented. His abdomen is soft, except for some very mild suprapubic fullness and tenderness. On rectal exam, his prostate is diffusely enlarged and nontender. He has a copious amount of firm, heme negative stool in his rectum.

His clinical exam reveals that he has lost 5 lbs in the last 6 months. His BP is 130/80 with a regular HR of 92. His conjunctiva are pale, lungs clear, and heart sounds normal.

He has no edema, a normal abdominal exam, and a rectal exam reveals soft heme negative brown stool.

      A CBC reveals a Hgb of 9.0 with a Hct of 30 (down from 13.5 and 39 at last visit).

      • What is your differential diagnosis of this problem?
      • What is your pretest probability for each item on the differential?
      • Are there any other tests that you would like to do to help determine the cause of this problem?
      • Read the methods and results of the paper by Guyatt et al.
      • Calculate the following (try not to look at the LR’s offered in table III):
        The likelihood ratio for iron deficiency anemia if his ferritin=10.
        The likelihood ratio for iron deficiency anemia if his ferritin=40.
        The likelihood ratio for anemia due to iron deficiency if his ferritin is >100.