The Rational Clinical Examination
David L. Simel, Drummond Rennie
A Primer on the Precision and Accuracy of the Clinical Examination
David L. Simel, Sheri Keitz
The diagnostic odds ratio (DOR) creates a single measure of accuracy that tells us which symptom...
Topics Discussed:
diagnostic process, hierarchy of evidence, odds ratios (relative odds)
Excerpt:
"The presence of a "good" symptom or sign creates a
large effect on the probability, convincing the clinician that the
target condition is much more likely to be present than the prior
probability suggests. The suggestion that some prespecified LR threshold
defines a good clinical finding for all disease is a myth so persistent
that it represents a medical urban legend. Some researchers and
clinicians define a "good" test result as that
associated with an LR greater than 10 or an LR less than 0.1, but
these results do not have intrinsic properties that are the sine
qua non of high value. For example, a pretest probability of 10% and
positive test with an LR = 10 generates a posttest probability
of 53%; this is a big increase in the probability of disease
but hardly an increase that clinches the diagnosis. Furthermore,
this is a similar posttest probability that follows from a disease
with a pretest probability of 20% and a positive test with
an LR = 5. Thus, although positive test results are increasingly
powerful as the LR increases and negative results are increasingly
valuable as the LR decreases, the efficiency of the finding in making
a diagnosis depends on the pretest probability...."
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