Newsgroup: it.salute.aids
Titolo: calcolo delle probabilità

ho trovato questo documento scritto da un medico riguardo le
probabilità di contrarre l'hiv dopo un test elisa a 4 settimane
risultato negativo.
lo posto così come è. poi ditemi voi cosa ne pensate.
1) An ELISA result that is below the cut-off is absolutely negative. A
result of, say 0.05, is every bit as negative as a result of 0.00 or
0.01. It has to do with the physical chemistry of ELISA technology,
not that some people have "a little bit" of HIV antibody.
2) The longer cut-offs for reliable results (3 months, 6 months) are
old news, but when knowledge moves as fast as in this field, "old" is
only a few years. Providers who are not AIDS experts should be
forgiven if they are unaware of the latest information. Many providers
tend to worry about lawsuits, so they lean toward conservative advice--
and the longer you go, the more reliable a negative HIV test. On top
of that, a lot of docs simply don't understand the concept of
"predictive value"--about which more below.
3) As I have said many times, there is no time cut-off for the last
HIV result that applies with equal usefulness for all patients. For
most of the very low risk exposures that generate the giant number of
irrational, unrealistic fears on this forum, a 4 week test is fine.
But some people at especially high risk need testing at 8 weeks or
even 3 months.
Why? I have gone through the following calculations several times, but
it bears repeating. This is the concept of the "predictive value" of a
test.
Situation A: Low-risk, worried well, panicked person after a
heterosexual exposure, say with a commercial sex worker. Odds the
worker had HIV, say 1%. Odds of transmission by unprotected vaginal
sex, average 1 in 1000 (tops). Likelihood of a positive test at 4
weeks, 90%. Even before the test is done, the odds that the exposed
person acquired HIV are 0.01 x 0.001 = 0.00001. That's 1 chance in
100,000. Add a test that is 90% reliable (the 4 week test) and the
chance someone has HIV, after getting the negative test result, is 1
in a million. Think how low a risk that is: Equivalent to sex with an
infected person once a day for 2,800 years before catching HIV. In
other words, essentially 100% assurance.
Situation B: High risk, really at risk person, a gay man who has
receptive anal sex with a known HIV infected person. The risk of
infection in such a person is 1%, i.e. 1 in 100. Now test at 4 weeks,
same test, i.e. 90% reliable. After the negative test, the risk of HIV
is 0.01 x 0.1 = 0.001. That means there still is a 1 in 1000 chance
the exposed person has HIV. Although most rational people can live
happily with a 1 in a million chance, most of us would lose a lot of
sleep at 1 in 1000. So this person needs additional testing, out to 8
weeks or even 3 months after exposure. Exact same test, same test
peformance--but difference levels of assurance. That is, different
predictive values.

 lo posto così come è. poi ditemi voi cosa ne pensate.
a me sembra un modo pseudo-scientifico di ritirare in ballo le categorie a
rischio, per di più con un calcolo di comparazione palesemente truccato tra
il caso A: eterosessuale che scopa senza preservativo, hai una probabilità
su un milione, ossia un rapporto ogni 2000 e rotti anni (quindi non rischi
un cazzo e quindi vai e dacci dentro!) e il caso B gay che ci dà dentro che
guarda caso ha una probabilità su 1000.
peccato che per arrivare ad attribuire l'uno su un milione all'etero ha
usato un fattore moltiplicativo per due volte, mentre nel caso del gay
questo fattore moltiplicativo non è stato usato.
il messaggio subliminale è chiaro: se sei etero, tromba pure liberamente che
non rischi praticamente nulla, se sei gay invece sei mille volte di più a
rischio.
trash
il gatto grigio
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