The idea
is to see how varies the probability of detecting the illness in the patient
after making him a clinical test in its diagnosis. The main data should be had
the previous probability to the same one, called: Pre-test. This value can be
obtained in several ways: a) an estimate based on the clinical experience, b)
the prevalence of the illness, etc. To make this the Theorem of Bayes is used as
follows:
Positive
Post test Probability: VPP = Positive Predictive Value
Negative
Post test Probability = (1 – Negative Predictive Value) = 1 – VPN
The
traditional calculations were based on the following steps: 1) to transform the
probabi lities pre test to Odds, 2) to obtain the Post test Odds by multiplying
these by the Positive Likelihood ratio; and 3) retransform these Odds to
probabilities post test. A shorter way is to use the Fagan’s Nomogram, where
one enters with the value of the probability pre test, the Likelihood respective
ratio crosses and the probabilities post test are obtained.
By using
the present clinical algorithm both procedures can be improved. Only is needed
the values of sensitivity and specificity of the clinical method, then they are
introduced and two curves that show the variability of the probabilities post
test are obtained (in function of the probabilities pre test). Therefore, the
clinician can introduce his estimated value of the pre test probability for
obtaining the post test probability, just by looking the correspon ding charts.
Also given the respective numerical tables are presented.
See
Example 1, in page 6.13 (for better explanation to read the whole topic 6.5)
Just as
it was explained in the Topic 4, one can see like the variability of the
extrinsic clinical indexes with the prevalence of the illness - that is a way to
consider to the previous probabilities to the test, or pre test probability.
By using
the same algorithm, the intrinsic clinical indexes can be obtained at the
beginning and to see the curves of the extrinsic ones at the bottom of the page
developed in Excel.
The
application examples can take of those developed in the text.