HIV PCR -- screening

Shan Putnam sputnam at
Fri Jun 9 22:45:27 EST 1995

Don Lee <don_lee at> wrote:

>yelnif at (Yelnif) wrote:
>> Basically, PCR amplifies DNA.  The only requirement for this is that one
>> molecule of target DNA be present and the terminal 15-20 nucleotide known
>> (on each end of the target DNA).  PCR amplifies the DNA between the known
>> sequences.  Because HIV integrates its genetic material into the host
>> chromosome and the entire sequence of the HIV genome is known, it is
>> possible to identify contaminate blood samples.  So, there is no "window,"
>> if the HIV DNA is present it will be identified.
>> -Jim Finley, grad student-virology, UAB

>Not having followed the original thread, I think the "window period" refers to time when the amount of HIV RNA or DNA can be picked up by PCR.
>This should depend on the lower limit of detection of the PCR and how many copies of HIV genetic information is being circulated around.  For example, if you suppose that the PCR can detect to even just one copy of RNA or DNA, then could it be possible that there is still enough HIV genome circulating in the body but just not sampled in your test tube.

I agree about the windows, but the question of PCR sensitivity comes
down to how much "clinical sample" are you going to collect to test
and confirm as a true negative.  For example, lets say PCR can detect
10 genomes, that means we would have to have at least 10 genomes for
whatever volume of clinical sample (blood) we collected.  When we test
via PCR we don't collect all the person's blood for testing.  So if
someone is infected with 100 virus particles, (100 vp/total volume
blood), and we sample 7 mls of blood, the probability of collecting 10
genomes is very slim (I guess some Poisson Distribution).  However, if
we would sample all the blood (applying some type of concentration
step), then we would be able to detect the vp's right away.  

I attended a Kary Mullis lecture in which he brought up HIV testing
via PCR of blood products (blood banking).  The bottom line was that
to call a Unit of blood absolutely negative, one would have to test
the entire unit, thereby destroying it, which is no good to the blood
bank industry.


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