PCR Undetectable -Testing for HCV
by Darlene Morrow, BSc
Step One: The initial test for HCV is called an
antibody test. When your body gets an infection, it
produces antibodies (anti-HCV) to help destroy the
foreign invader. By testing for anti-HCV, the physician
knows that you have been exposed to HCV. Eighty percent
of all people infected with HCV go on to develop chronic
liver disease. A positive test result, coupled with an
elevation in liver enzymes (ALT and AST) showing active
liver tissue degradation, more than likely means that the
HCV is still present, but the test is not conclusive.
Step Two: To confirm the diagnosis of HCV a PCR
(polymerase chain reaction) test is done. This test looks
for the genetic material of the virus in your blood. The
Hepatitis C virus uses RNA instead of DNA to replicate
itself. The HCV RNA PCR can be qualitative (meaning only
that it is looking for a positive or negative sign of the
HCV RNA) or quantitative (counting the actual number of
copies of the virus present in a milliliter of blood).
Limits of the PCR: Here in British Columbia the
upper limit of the viral load test is over 750,000
copies, and the lower limit is 200 copies. What does this
mean? It means that the test cannot measure a number
below 200 copies of HCV RNA. Anything below that number
will be classed as undetectable. If you have 199 copies
of HCV RNA in your blood, the result could be
"undetectable."
Variability in the PCR: There are further
complications and uncertainties with the PCR. We can look
at three main areas:
- How the sample is collected and stored.
- When the sample is collected (time of day)
- Where the sample is collected from.
How the sample is collected and stored: After
the initial blood is drawn, it is centrifuged to separate
the white (plasma) and red cells. The PCR test must now
be performed within two hours, or else the plasma sample
must be frozen at -70C. Any deviation from these steps
results in a PCR of questionable value.
When the sample is collected: A study presently
underway is looking at the PCR results from samples taken
at various times of the day. Early results suggest that
the HCV RNA level may well vary during the day. Therefore
it could be important to collect samples at the same time
for the duration of the study.
Where the sample is collected from: At present
the test is performed using your plasma. Recent studies
suggest that whole blood may be a better place to look.
[1] In addition HCV resides in your liver cells where it
disguises itself using your own cell membrane as its
outer layer (it is now no longer recognizable as a
foreign product). Once in the liver, replication of the
virus occurs. More recent studies have shown that as many
as 60 percent of those patients that tested PCR (HCV RNA)
undetectable in plasma showed HCV RNA positive when the
test was performed in liver tissue. [2] But not all
studies have confirmed these results and more research is
necessary before a more definitive answer is possible.
How the PCR is used: Initially it was thought
that the PCR could be used to help guide the selection of
people infected with HCV who would most likely respond to
interferon. Could we tie PCR levels to successful
treatments - e.g., would a low PCR mean a better
response? Does an early response mean a better chance of
success? Data from subsequent studies have not fully
supported these hypotheses. And in fact, it has been
difficult to find a correlation to relate to the PCR. And
we must be VERY, VERY careful about the significance we
place on these numbers. It is a double-edged sword. These
numbers can be used by the government to deny equal
access to drugs and Pharmacare coverage. For most people
the cost of interferon is prohibitive enough even with
Pharmacare. At the very best, all we can say is that the
PCR test is a research tool. The proper use of that tool
will be determined through future research.
For the present: The PCR is used to determine
response to treatments such as interferon and ribavirin.
The PCR is performed at the beginning to give a baseline.
It is repeated in 6 months. The hope is that the PCR will
be undetectable at this time. If it is undetectable, the
individual is classed as an initial responder to the
therapy. But the really hard part is remaining PCR
undetectable. The full treatment course for the combo is
12 months. Six months after completion the PCR is
repeated. If the individual is still PCR undetectable,
they are classed as a sustained responder. And it looks
like there is a really good chance of remaining PCR
undetectable IF you make it past this point.
References:
[1]. Schmidt, W.N. ; Wu, P. ; Han, J.Q. ; Perino, M.J.
; Labrecque, D.R. ; Stapleton,J.T. "Distribution of
Hepatitis C Virus (HCV) RNA in Whole Blood and Blood Cell
Fractions: Plasma HCV RNA Analysis Underestimates
Circulating Virus Load." Journal of Infectious
Diseases, July 1997;176(1):20-26.
[2]. Detection of Hepatitis C Virus (HCV) in Liver
Tissue of HCV RNA Seronegative Patients By Means of
anti-HCV-Antibodies and RT-PCR. V Dries ; I von Both ; M
M=FCller ; G Gerken ; P Schirmacher ; M Odenthal ; KH
Meyer zum BFCschenfelde ; HP Dienes. AASLD Annual Meeting
97.
Get your own Free
Home Page