Klazien Matter-Walstra escribió:
> Dear All,
>
> I am working on the subject "diagnostic tests" and the idea of the
> "goldstandart". Now I sometimes cannot find out to "what" sensitivity and
> specivity of a diagnostic test relate to. What I mean is the following: When
> you perform a test say to find out if somebody has a flu virus, what should
> the test measure ?
> 1. How many people with a positive result will get ill and have a flu ?
> (sensitivity and specivity relate to the outcome "Illness" yes or no), or
> 2. In how many cases with a positive test result can the flu virus be
> isolated (Goldstandart ?)
Sensitivity related to how many positive test you will find among patients with
true flu ? But this is not our usual clinical question but how many patients
with true flu are there among those with a positive test ? that is the positive
predictive value. Your questions are of this type, you are asking about positive
predictive value. It can be calculated applying Bayes theorem if you give any
aproximation of how much is the probability of having flu without knowing the
test result (the a priori probability), PPV = Sensitivity · Probability /
(Sensitivity · Probability + (1-Sensibility) · (1- Specificity)). A related
concept is positive likelihood ratio (how much will change the prior probability
of having the disease if we know that the test is positive).
Another problem is how we define "to have flu" (the gold standard) clinically
(question 1) or microbiologically (question 2).
> As i see it sensitivity and specivity in both cases may be very different,
> but also the two cases may be of different clinical value, as many people
> whith a positive test in 2. may not become ill at all.
>
> How is this problem handled and can somebody help me with literature
> concerning this problem, specially what is the "Goldstandart" in 1 ?
I can't give you others reference in english than evidence-based medicine book,
in the web site of the book you will find more extensive explanations and
calculator http://hiru.mcmaster.ca/ebm.htm
Best wishes
------------------------------------
Luis Ruiz del Fresno
F.E.A. Unidad de Medicina Intensiva
H.U. Virgen de la Victoria
Campus Universitario Teatinos sn
Málaga 29010 Spain
------------------------------------
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|