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The sensitivity of a diagnostic test is the likelihood
that persons with the disease of interest will have positive test
results.
-
The specificity of a diagnostic test is the likelihood that
persons who do not have the disease of interest will have negative
test results.
- Positive predictive value measures the likelihood of having
the disease of interest among those whose diagnostic test results
are positive.
- Negative predictive value is the likelihood of not having
the disease of interest among those whose diagnostic test results
are negative.
- 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.
-
The area under a receiver operating characteristic (ROC) curve
can be used to assess the performance of a diagnostic test.
- Screening for a particular disease is conducted in order to
detect the disease at an earlier stage than would occur through
routine methods.
-
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.
-
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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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 ...