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  • Image not available. The sensitivity of a diagnostic test is the likelihood that persons with the disease of interest will have positive test results.
  • Image not available. The specificity of a diagnostic test is the likelihood that persons who do not have the disease of interest will have negative test results.
  • Image not available.Positive predictive value measures the likelihood of having the disease of interest among those whose diagnostic test results are positive.
  • Image not available.Negative predictive value is the likelihood of not having the disease of interest among those whose diagnostic test results are negative.
  • Image not available.Likelihood ratios can be used to measure the extent to which the likelihood of the disease of interest is changed by the results of a diagnostic test.
  • Image not available. The area under a receiver operating characteristic (ROC) curve can be used to assess the performance of a diagnostic test.
  • Image not available. Screening for a particular disease is conducted in order to detect the disease at an earlier stage than would occur through routine methods.
  • Image not available. An error in the evaluation of a screening test, known as lead time bias, can occur when persons with disease detected by screening appear to live longer simply because of the earlier recognition of their illnesses.
  • Image not available. An error in the evaluation of a screening test, known as length-biased sampling, can occur when persons with disease detected by screening appear to live longer simply because they have more slowly progressing illnesses.

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Image not available. A 54-year-old high school teacher visited her family practitioner for an annual checkup. She reported no illnesses during the preceding year, felt well, and had no complaints. The hot flashes she had experienced a year ago had resolved without treatment. The physician performed a physical examination, comprising breast, pelvic (including a Papanicolaou smear), and rectal examinations; all were unremarkable. The physician recommended that the patient have a mammogram, which was scheduled for 1 week later.

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The results of the mammogram were not normal, and the radiologist suggested that a breast biopsy be performed. The family practitioner notified the patient of the abnormal mammogram and referred her to a surgeon, who concurred that physical examination of the breast was normal. Based on the mammographic abnormality, however, the surgeon and the radiologist agreed that fine-needle aspiration (FNA) of the abnormal breast under radiologic guidance was indicated. Evaluation of the FNA specimen by a pathologist revealed cancer cells, and the patient was scheduled for further surgery the following week.

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The practice of clinical medicine is the artful application of science. A seemingly straightforward chain of decisions by the physicians in the Patient Profile ultimately led to the diagnosis of breast cancer and subsequent treatment. In practice, however, the process of clinical reasoning can be extremely complex. Each decision made by the clinicians in the Patient Profile included the possibility that information was incorrect. Sir William Osler eloquently described the difficulties of clinical decision making in 1921:

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The problems of disease are more complicated and difficult than ...

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