Living + Magazine Issue 1 - Positive Living BC

DIAGNOSTICS
Ultrasensitive viral load
test now available in BC
by DAN O’NEILL, TREATMENT
INFORMATION PROGRAM
A new viral load test available in BC will
now allow your doctor to measure the
presence of HIV even when it reaches
extremely low levels. Until now, when
someone’s viral load dropped below 400
copies/ML, the current viral load tests
would indicate that the HIV virus was
“detectable but unquantifiable”.
With the introduction of the Roche
Amplicor Ultrasensitivetest, HIV viral
loads as low as 50 copies/mL can be
measured. The new test may also give a
count that is “detectable but unquantifiable”
which would presumably be
some number below 50 copies/mL.
The greater sensitivity of
the new test may give people
an earlier indication of
how effectively their drug
regimen is reducing virus in
the blood. Even if the viral
load is below detection in the blood, it
does not mean the virus has been eliminated
nor does it tell you that the virus
has been cleared from other sites such
as lymph nodes.
The overall accuracy of the count is
the same as for the old test, so changes
of less than 0.5 log (a three-fold difference)
should not be considered significant.
As with any test in HIV where there
are few medical decisions that are timecritical,
so you should be careful about
making a significant change in therapy
on the basis of one test result. Rather,
look for a trend over several tests.
If you had several counts below 400
with the old test and then get a detectable
count of, say, 200 with the
Ultrasensitive test, what does that mean?
Dr. Richard Harrigan who heads the
Virology Laboratory at the Centre for
Excellence cautions against reading too
much into one detectable reading between
50 to 400 after have several tests
below quantification with the old test.
There are several possibilities: (1) your
viral load hasn’t changed but the test
can now count it because it’s between
50 and 400; (2) with the test being accurate
only to within 0.5 log (a 3-fold
difference), it may reflect the test variability
rather than a reduction in drug
efficacy; (3) your viral load might have
been higher on the day you took the test
because you’d forgotten to take all your
medicines or diarrhea and vomiting had
reduced the amount of drug in your
The greater sensitivity of the
new test may give people an earlier
indication of how effectively their drug
regimen is reducing the virus in the blood.
body; or (4) it is an early warning of a
reduction of efficacy of your drug regimen.
Dr. Harrigan says there are three reasons
why viral loads become detectable
on therapy or never reach undetectability
on therapy. If the pretreatment
viral load was high, the combination of
three drugs might not have the potency
to reduce the count to below 50. For
example, if your viral load was 200,000
and each drug you took reduced the
virus in your plasma by ten fold, three
drugs would reduce it to 200, detectable
on the new test but below quantification
on the old. The second reason is
pharmacokinetic – the way the drug is
absorbed and metabolized in your body.
If the dose is too low, if the drug is poorly
absorbed due to drug or food interactions
or to vomiting and diarrhea, if the
drug is cleared from your body too
quickly due to drug interactions or idiosyncratic
metabolism, or if you forget
to take your drugs, then there will be
inadequate levels of drug in your body
to inhibit adequately the growth of the
virus. This situation may be important
in the development of the third reason
for viralogical failure of a regimen,
which is viral drug resistance. If the virus
has become drug resistance, then
viral load is likely to increase over time,
whereas pharmacokinetic and drug potency
issues causing viralogical failure
probably won’t show a trend to significantly
increased viral load until resistance
sets in. Selection for drug resistance
occurs when the virus is replicating
in the presence of one or more
drugs.
Resistance to one of the three drugs
in a regimen could cause viral load to
increase by 1 log (ten fold) without a
return to pretreatment virus levels.
Drug resistance occurs quickly with the
non-nucleoside reverse transcriptase
inhibitors and 3TC and slowly with the
other reverse transcriptase inhibitors
and the protease inhibitors. If your viral
load goes from 100 to 400 to 1500
and adherence has not been a problem,
then you may want to talk to your doctor
about switching drugs. If your viral
load consistently hovers in the 50 – 400
range and you’ve taken your drugs as
prescribed, the advantage to switching
to try to achieve an “undetectable” result
would depend on factors such as
pretreatment viral load, previous use of
other antiretrovirals and other issues
such as dosing regimens and side effects
of the new regimen. On the positive
side, people show good clinical results
on therapy for quite some time even if
their viral load has not gone below detection.
JULY/AUGUST 1999 • LIVING + 25