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Polymerase chain reaction (PCR) is a
technique which is used to amplify the number of copies of RNA viral
particles and, unlike
hepatitis C antibody tests that look for the body response to the virus,
a PCR (polymerase chain reaction) test looks for actual presence of the
virus. There are 3 types of PCR tests:
-
HCV PCR viral detection test —
looks for the virus, sometimes called 'qualitative test';
-
HCV PCR viral load test —
looks for the virus and estimates how many HCV viruses per mL of
blood, sometimes called 'quantitative test';
-
HCV PCR genotype test — looks
for the virus, and determines the particular type/s of HCV.
PCR testing may be of benefit in
cases of needle-stick and other sharps injury in healthcare settings or
for exposure-prone sexual practices.
When
an individual is found to be positive for antibodies to HCV during blood
donation or a health screening of at-risk populations, detection
of HCV by RNA/PCR confirms chronic HCV infection.
Despite improvements in the sensitivity of PCR technology, it’s
important to assess HCV viral status on the basis of a minimum of 2 PCR
tests — over a 6-month period — rather than on the basis of a single PCR
test result. This is because it is possible for levels of hepatitis C
virus in the bloodstream to fluctuate such that the level of virus may
fall so low that the PCR test won’t pick it up. Thus, someone who tests
PCR negative may still be infectious.
HCV RNA is detectable in serum
within one to two weeks when accidental parenteral exposure results in
infection.
Detection of HCV RNA without antibodies for HCV is strongly indicative
of acute hepatitis C; this will be confirmed by subsequent
seroconversion. Acute hepatitis C is unlikely if both markers are
absent.
Even if people with HCV know their PCR status they should still avoid
all blood-to-blood contact. In any situation involving potential
blood-to-blood contact, it’s important for everyone to assume that they,
and all others, are potentially infectious. If an HCV antibody positive
person finds out they are PCR negative, they’d be making a big mistake
in sharing any drug injecting equipment, snorting straws, razor blades
or razors, tattooing equipment, toothbrushes or other equipment that
could transfer blood.
When HCV RNA is undetectable on at least
two occasions 6 months apart, it is difficult to distinguish patients
who still harbor antibodies after spontaneously resolving HCV infection
in the past from patients with false-positive reactivity.
Acute HCV infection is also unlikely if anti-HCV antibodies are present
and HCV RNA absent; such cases generally correspond to patients whose
liver disorders are due to another cause and who encountered and cleared
HCV at some time in the past. These subjects should nonetheless be
retested for HCV RNA a few weeks later, as HCV RNA may disappear
transiently before chronic replication becomes detectable.
Finally, when both anti-HCV antibodies and HCV RNA are detected, it is
difficult to distinguish acute hepatitis C from an acute exacerbation of
chronic hepatitis C, and from acute hepatitis of another cause in a
patient who also has chronic hepatitis C.
The diagnosis of acute infection should
be based on HCV RNA testing with a sensitive technique. This can be done
at any time starting one week after exposure. Antiviral treatment is not
urgent in this setting, and can be initiated when symptoms or an
increase in serum aminotransferase activity occurs.
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