|
| |
Jump to : Latest Research on Viral Loads
2008-2004
|
| |
|
| |
|
HCV Viral Load Tests
Alan Franciscus, Editor-in-Chief
Liz Highleyman
Viral load tests are blood tests that measure HCV
ribonucleic acid (RNA, or genetic material) in the blood. The presence of viral
RNA indicates that the virus is actively replicating (reproducing and infecting
new cells). A viral load test is usually first done after a person has tested
positive for exposure to HCV based on an antibody test. A blood sample is taken
and the amount of HCV RNA in a milliliter of blood is measured. Viral load tests
confirm whether an individual is actively infected with HCV. Viral load test
results were previously measured in number of copies, but are now typically
reported in terms of International Units per milliliter (IU/mL).
Types of HCV Viral Load Tests
There are two categories of HCV viral load tests:
Qualitative viral load tests — These tests
determine the presence of HCV RNA in the blood. This type of test is usually
used to confirm chronic infection with HCV. If viral RNA is detected, a positive
result is reported; if viral RNA is not detected, the test result is negative.
Quantitative viral load tests — These tests measure the amount
of virus in one milliliter of blood. They are often used to assess whether or
not treatment with interferon or interferon plus ribavirin is likely to be
successful and, later, if treatment is working.
There are currently three tests commonly used for HCV
viral load testing:
Polymerase chain reaction (PCR) — PCR tests detect HCV RNA in
the blood, which indicates current active infection. This type of quantitative
PCR test is very sensitive, and can measure as few as 50 IU/mL.
Branched-chain DNA (bDNA) — The bDNA method quantitative viral
load testing is easier (and cheaper) to use for a large number of samples, but
only measures viral loads greater than 500 IU/mL. This means that if a person
has a viral load below 500 IU/mL, HCV could be present in the blood but not
detected by the test.
Transcription-mediated amplification (TMA) — TMA technology
allows for the amplification and detection of nucleic acids (components of
genetic material) in the blood. This test can measure as few as 5-10 IU/mL. This
newer test appears easier and cheaper to use, streamlining test processing and
producing consistent, reliable, and more rapid results.
Interpreting Viral Load Test
Results
HCV viral load is often reported as low or high.
Expressed as copies/mL:
·Low: less than 2 million copies
·High: more than 2 million copies
Expressed as International Units (IU/mL):
·Low:less than 800,000 IU/mL
·High:more than 800,000 IU/mL
If no HCV RNA is found by a test, a person’s viral load is
said to be undetectable. Note that whether viral load is undetectable depends on
which test is used. PCR and TMA tests can measure viral loads much lower than
those a bDNA test can detect. Importantly, the blood of an individual with a
very low viral load may still contain HCV even though the current tests cannot
measure it; that is, the virus may not have been truly eradicated from the body.
Viral load test results can vary depending on how a blood sample is handled and
stored. Furthermore, results may vary from lab to lab. For this reason, most
experts recommend that people should get their viral load testing done by the
same laboratory each time, so that results are more comparable.
Changes in viral load are sometimes expressed in terms of logs. A log change is
a 10-fold increase or decrease. For example, a change from 1,000,000 IU/mL to
10,000 IU/mL is a 2-log decrease.
Converting copies per milliliter
to Inter-national Units
There is no standard conversion formula for converting the amount of HCV RNA
reported in copies per milliliter to the amount reported in International Units.
The conversion factor ranges from about one to about five HCV RNA copies per IU.
Usually the lab report will list the conversion from IU/mL to copies/mL.
See Table 1 for a conversion of common viral load tests from IUs to
copies.
Table 1: Conversion Chart
| Assay |
Conversion Factor |
Amplicor HCV Monitor v2.0
(manual procedure) |
1 IU/mL = 0.9 copies/ml |
Cobas Amplicor HCV Monitor v2.0
(semi-automated procedure) |
1 IU/mL = 2.7 copies/ml |
| Versant HCV RNA 3.0 Quantitative Assay |
1 IU/mL = 5.2 copies/ml |
| LCx HCV RNA Quantitatiive Assay |
1 IU/mL = 3.8 copies/ml |
| SuperQuant |
1 IU/mL = 3.4 copies/ml |
Viral load given in IU
Giving the viral load in IU probably soon will replace all other ways to
express the viral load - until recently it was expressed most frequently in eq
or Meq. But at the moment and in old lab reports a wide variety of ways to give
the viral load still can be found.
The IU = International Unit for the hepatitis C viral load is a unit more or
less arbitrarily fixed. Labs now can take part in international comparison tests
using a calibrated sample and thereby normalize their results to an
international standard. So, in the future results from different laboratories
should be directly comparable.
For converting numbers from eq to IU and vice versa, different labs use
different conversion factors, in the range from 2 to 5 viruses per IU. If you do
not know the factor that your lab uses, using a factor of three might be
reasonable. That means: Viral loads given in eq/ml have to be divided by three
to get the viral load in IU/ml. And, viral loads given in IU/ml have to be
multiplied by three to get the result in eq/ml.
Uses of Viral Load Test Results
Viral load test results have many uses, such as confirming active HCV infection,
and predicting and measuring HCV treatment response before, during, and after
therapy. Higher HCV viral loads may be associated with a greater risk of HCV
transmission, particularly transmission from mothers to infants during pregnancy
or birth. Viral load has not been correlated with the risk of sexual
transmission. Furthermore, a correlation between HCV viral load and disease
progression has not been shown.
Confirming active HCV infection — After a person has tested
positive for HCV antibodies, an HCV viral load test is usually performed to
confirm active HCV infection. This test is necessary because in up to 25% of
people exposed to HCV, the virus can be cleared on its own.
Before treatment — Viral load measurement can help predict how
well HCV treatment will work.
The lower the pre-treatment viral load, the more likely it is that a person will
respond to current HCV therapies.
During treatment — A decrease in viral load while on therapy
indicates that treatment is working. A treatment is said to produce a complete
virological response if it reduces viral load to an undetectable level. After 12
weeks of antiviral treatment, a 2-log drop in viral load or elimination of
detectable HCV is an indication that the medications are working. If a person
does not achieve a 2-log drop in viral load or elimination of detectable HCV
after 12 weeks, it is unlikely that he or she will be able to eradicate HCV from
his or her body. Viral load tests during treatment can also detect viral
breakthrough, or increases in viral load that occur after a previous
undetectable test result.
Note: A log drop in viral load is measured by decreasing the number by one
zero. For instance, a one log drop in a viral load of 1,000,000 International
Units is 100,000 International Units; a two log drop in a viral load of
1,000,000 International Units is 10,000 International Units.
After treatment — Viral load measurements can be used after
cessation of therapy to monitor for relapse—that is, to see if the virus becomes
detectable again after being undetectable when treatment was completed.
http://www.hcvadvocate.org/news/newsLetter/advocate1003.html
Viral Load Tests
The viral load test measures the amount of HCV virus
in your blood. There are different techniques for doing this:
PCR
- Polymerase Chain Reaction. Is the most accurate
test available at present. It involves the amplification of the
nucleic acid associated with the virus several million times, by using the
"chain reaction", in order to bring it up to measurable levels.
As the amplification process is fully controlled, the quantity of
original material present in the sample (the viral load) can be calculated
with a great degree of accuracy.bDNA
- Also tests for the presence of the virus in the blood, but is less
sensitive than the PCR test, picking up only levels over 200,000 virus
equivalents/ml.
Viral Load
I.) What is
Viral Load ?
Your viral load is
the amount of viruses present in a given volume of your blood
(usually 1 millilitre = 1 cubic centimeter). More precisely, it
means that the amount of hepC genetic material found in your blood
corresponds to as many hep C viruses as the given number says.
Therefore the given number denotes 'viral equivalents'.
And now, of
course, you would like to know whether your viral load is low or
high....
Although there is no general
consensus on the definition of 'low' and 'high' with respect to
the viral load, data from the Web pages of the National
Genetics Institute give you an idea: As can be seen there, the
average viral load is at 3.2 Million eq/ml. - So, to make a simple
scheme I would propose the following classification:
Viral Load Chart
|
Viral Load in eq/ml
|
Classification
|
Remarks
|
|
below 200.000
|
very low
|
below detection limit of
bDNA test
|
|
200,000-1,000,000
|
low
|
|
|
1,000,000-5,000,000
|
medium
|
average viral load at
3,200,000 eq/ml
|
|
5,000,000-25,000,000
|
high
|
|
|
above 25,000,000
|
very high
|
|
Expressed in IU, the average
viral load is at 1 Million IU/ml. All these classifications of
viral load of course make sense only for patients that are not
being treated against HCV.
II.)
"Not detected"
The viral load can
range from "not detected" to hundreds of millions. The
meaning of "not detected" or "negative"
differs, depending on the test used. In my lab, the
detection limit for the "quantitative" HCV RNA
test by "PCR" is 200 virus equivalents/ml (and
with the "qualitative" test they can detect down
100 virus equivalents/ml ). The less expensive quantitative "bDNA"
test has a detection limit of about 200,000 virus equivalents/ml,
which makes it less sensitive, but above its detection limit it is
more accurate than the PCR test.
So, when you are
"negative", maybe you have no hepatitis C virus in your
blood. But maybe also, you do have hepatitis C virus in your
blood, but the number of viruses is lower than the detection
limit.
III.)
"Positive" - What's important to note, besides the pure
numbers
When you get back
the result of your HCV RNA quantitative test, and when the lab was
able to determine the amount of virus in your blood, then it is
important to write down not only the number, but also in what
units this number is given.
A) Volume
The volume of blood that the number refers to is usually one
millilitre. But some labs give the number for 20 microlitres
= 1/50 millilitre. So in these cases you have to multiply the
result of the viral load by 50 to get the number for 1 millilitre.
B) Amount of
Virus
Unfortunately, there are several ways to express the viral load.
So, in order to be able to compare different results, you have to
know how to convert these numbers to some standard format, which I
would say is just the plain number of viruses per millilitre, like
1.5 Million/ml, or 1,500,000/ml.
1.) By weight
Sometimes, the lab reports the amount of genetic material found by
its weight. 1 pg (pico-gram) of genetic material corresponds to
about 1 million virus equivalents, so, if your lab result is given
in picograms, just multiply the lab result by 1,000,000, and you
have the number of viruses.
2.) By virus
count
a.) Plain numbers
Often the virus count is expressed as a plain number, like 1.73
million, or 1,730,000 or 1730000. Millions sometime are
abbreviated by the prefix "M" (Mega). So when you see
1.73 Meq/ml, it means 1.73 Mega-equivalents/ml or again 1730000
equivalents/ml.
b.) Exponential
format
Large numbers are often expressed in exponential form, that means
a number, multiplied by 10 with an exponent. To convert this to
normal numbers, append as many zeroes to a "1" as the
exponent says, and multiply this with the number. In some lab
report, the viral load was "Hep C RNA Quant 17.3 x 10(exp) 5
equivalents/ml". So, with 5 as exponent, you have to append 5
zeroes to an "1", that gives 100000, and multiply this
with the number 17.3, that gives 1730000 as the viral load.
Normally this would be written 1.73x10(exp)6, or 1.73x106
, which are the same number. At the same time, 17.3x105
= 1.73x106 = 1,730,000
c.) Logarithmic
format
Now, recently some people express these numbers also in
logarithmic form (logarithmic transformed number).
log(1730000)=6.24
6.24 is the
logarithmic transformed number of the viral load of our above
example.
A result of 3.5 for
a viral load, that someone reported, seems to be such a number
(unless he forgot to write down a "10" and an exponent).
You need a calculator to convert this. You have to use the
function 10x, where you have to replace x with the
logarithmic number, in the above case 3.5. The result would be 103.5
= 3162 virus equivalents per milliliter. When you take the
logarithmic number from the first example, 6.24, you have to
calculate 106.24 = 1730000 , and here we have the
original number of virus equivalents again.
If you don't have a
calculator, you can estimate the order of magnitude of a viral
load expressed as a logarithmic number. From the logarithmic
number, you take the first digit (to the left of the point ) and
add 1 to this number. This gives you the number of digits that
your viral load has (expresses as a plain number).
Example: Logarithmic
number 6.24
Left of the point is "6". 6+1 = 7
The number that gives the viral load is 7 digits long, that means
it is between 1,000,000 and 9,999,999 (digit # 1 234 567).
The next digit (
right of the point of the logarithmic number ) shows whether you
are high or low in the range.
In case ( but I have
never seen that ) you have a logarithmic number and a blood volume
other than 1 ml, you have to convert the logarithmic number to a
plain number *first*, and then correct it to correspond to 1 ml !
Therefore it is
important to have a close look at your lab report and see in what
units the result is given !
Viral Load Chart
Viral Load Equivalents per Milliliter (EQ/ML)
Conversion to Logarithmic Form
(Viral Load is how many viral particles per ML of blood)
*[HCV-RNA
(qPCR)-negative] is defined as less than 100 copies/ml of
hepatitis C viral RNA as measured by the National Genetics
Institute assay. Keep in mind, different labs do the PCR test
differently. There is no set protocol for this test as of yet so
results vary from lab to lab.
| |
|
Viral Load in eq/ml
|
Classification
|
Remarks
|
|
below 200.000
|
very low
|
below detection limit of
bDNA test
|
|
200,000-1,000,000
|
low
|
|
|
1,000,000-5,000,000
|
medium
|
average viral load at
3,200,000 eq/ml
|
|
5,000,000-25,000,000
|
high
|
|
|
above 25,000,000
|
very high
|
|
|
|
|
Conversion to Logarithmic Form
|
|
|
EQ/ML |
Log (EQ/ML) |
EQ/ML |
Log
(EQ/ML) |
|
100,000 |
5.000 |
5,250,000 |
6.720 |
|
150,000 |
5.176 |
5,500,000 |
6.740 |
|
200,000 |
5.301 |
6,000,000 |
6.778 |
|
250,000 |
5.398 |
6,250,000 |
6.796 |
|
300,000 |
5.477 |
6,500,000 |
6.813 |
|
350,000 |
5.544 |
7,000,000 |
6.843 |
|
400,000 |
5.602 |
7,250,000 |
6,860 |
|
450,000 |
5.653 |
7,500,000 |
6,875 |
|
500,000 |
5.699 |
8,000,000 |
6.903 |
|
550,000 |
5.740 |
8,250,000 |
6.916 |
|
600,000 |
5.778 |
8,500,000 |
6.929 |
|
700,000 |
5.584 |
9,000,000 |
6.954 |
|
750,000 |
5.875 |
10,000,000 |
7.000 |
|
800,000 |
5.903 |
10,500,000 |
7.021 |
|
850,000 |
5.929 |
11,000,000 |
7.041 |
|
900,000 |
5.954 |
11,500,000 |
7.060 |
|
950,000 |
5.978 |
12,000,000 |
7.079 |
|
1,000,000 |
6.000 |
12,500,000 |
7.097 |
|
1,050,000 |
6.021 |
13,000,000 |
7.113 |
|
1,100,000 |
6.041 |
13,500,000 |
7.130 |
|
1,150,000 |
6.060 |
14,000,000 |
7.146 |
|
1,200,000 |
6.079 |
15,000,000 |
7.176 |
|
1,250,000 |
6.097 |
16,000,000 |
7.204 |
|
1,300,000 |
6.113 |
17,000,000 |
7.230 |
|
1,350,000 |
6.130 |
18,000,000 |
7.255 |
|
1,400,000 |
6.146 |
19,000,000 |
7.278 |
|
1,450,000 |
6.161 |
20,000,000 |
7.301 |
|
1,500,000 |
6.176 |
21,000,000 |
7.322 |
|
1,550,000 |
6.190 |
22,000,000 |
7.342 |
|
1,600,000 |
6.204 |
23,000,000 |
7.361 |
|
1,650,000 |
6.217 |
24,000,000 |
7.380 |
|
1,700,000 |
6.230 |
25,000,000 |
7.398 |
|
1,750,000 |
6.243 |
26,000,000 |
7.414 |
|
1,800,000 |
6.255 |
27,000,000 |
7.431 |
|
1,850,000 |
6.267 |
28,000,000 |
7.447 |
|
1,900,000 |
6.278 |
29,0500,000 |
7.362 |
|
1,950,000 |
6.290 |
30,000,000 |
7.477 |
|
2,000,000 |
6.301 |
35,000,000 |
7.544 |
|
2,200,000 |
6.342 |
40,000,000 |
7.602 |
|
2,250,000 |
6.352 |
45,000,000 |
7.653 |
|
2,500,000 |
6.398 |
50,000,000 |
7.699 |
|
3,000,000 |
6.477 |
55,000,000 |
7.740 |
|
3,250,000 |
6.511 |
60,000,000 |
7.778 |
|
3,500,000 |
6.544 |
70,000,000 |
7.845 |
|
4,000,000 |
6.602 |
75,000,000 |
7.875 |
|
4,250,000 |
6.628 |
80,000,000 |
7.903 |
|
4,500,000 |
6.653 |
90,000,000 |
7.954 |
|
5,000,000 |
6.699 |
99,000,000 |
7.995 |
Albrecht Ernst
ernst@infp.fzk.de
Hepatitis C Viral
Load Is About More Than Numbers
A DGReview of :"Hepatitis c viral
load does not predict disease outcome:
going beyond numbers"
Revista do Instituto de Medicina Tropical de São Paulo
06/05/2002
By Anne MacLennan
Hepatitis C viral (HCV) load does not correlate with the
histological
evolution of the disease, and use of viral RNA quantification as a
predictor
or determinant of severity of this disease is incorrect and of
relative
value.
These are the views of Evaldo Stanislau Affonso de Araujo and
colleagues
following an analysis of 58 patients at the Hepatitis Outpatient
Clinic,
Department of Infectious and Parasitic Diseases, Faculty of
Medicine,
| |