welcome to another MedCram lecture and board vitals question this one comes
from the internal medicine and pathology question banks here's the question in a
move that could increase the nation's blood supply by at least four percent a
year the FDA has recommended an end to a lifetime ban on blood donations for gay
men the FDA issued draft guidance in 2015 regarding the acceptance of men who
have sex with men or MSM to donate blood the window for HIV - na t which is
nucleic acid testing is approximately nine days and the incidence of HIV in
the MSM population is about 1% per year in other words that's one per hundred
persons in one year based on this information what is the likelihood that
an MSM individual would be within the window to be able to transmit HIV via
blood components and we have our answer choices down below here we've got a is
one in one-and-a-half million one in four thousand fifty five which is the
correct answer one in three hundred and fifty seven thousand one in a hundred
thousand or one in five hundred and seven so let's take a look at this and
see what the right answer is and why that's the right answer
so the thing that you've got to realize is that when someone gets HIV infection
there was a period of time of about twenty-two days until antibodies would
show up and then of course you've got antibodies from that point on and then
it's pretty easy to be able to tell whether the donated blood has HIV in it
that's a no-brainer that's no problem the problem was is that if you checked
for antibodies within 22 days of the person getting an HIV infection you
would not detect it and you would not know whether or not this blood was
tainted with the HIV virus well go ahead now and enter in something called na T
well na T actually looks for something different than antibodies remember
antibody are the body's response to the virus so
instead of looking for antibodies we did something called nucleic acid testing
and that brought it all the way down to nine days what this did was it shortened
the window period so in other words now we could do nucleic acid testing on
blood and it would be able to detect far more infections but we're still left
with this nine day period of time after an infection whenever it happens that
you will not be able to tell whether or not this blood is tainted with the HIV
virus so the question is really what are the chances that if you use na T testing
that you're gonna spread HIV into the population because we cannot detect it
again what we're looking at here in the question is specifically that the window
period as we talked about is nine days that's nine days out of 365 days the
question is is what is the chances of somebody having HIV during the nine day
window period well that's very simple it's simply the chances or the
probability of having HIV times the probability of being during the window
period and so what is that well the chances of having HIV as they said in
the MSM population is going to be one over 100
multiply that by being in the window period and that's simply going to be
nine divided by 365 this equals nine divided by three six five zero zero
which equals you guessed it one over forty 55 I hope that answers your
question thanks for joining us