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Message - August 25, 2001
What is the "Viral Load"?


By Albrecht Ernst 


The first diagnosis of your hepatitis C was probably made due to the result  of an (relatively cheap) antibody test such as ELISA or similar. The result of this test was ' positive ', and that means that your body at some time has come into contact with the hepatitis C virus and that the immune system of your body developed antibodies against this virus to fight this infection.

In order to find out, whether the virus is still active in your body, a different test has to be made: The presence of the virus has to be detected directly.

For this purpose there are two types of tests available,  the qualitative PCR test or a quantitative test.

The qualitative test can have only two results: Either the virus is detected, or the virus is not detected. If the virus is detected (result: positive), you know only that you do have the virus in your body, but you don't know, in which quantity it is in your body. If no virus is detected (result: negative),  you know that you either don't have the virus (any more), or that its concentration is so low that it cannot be detected.

 The quantitative test on the other hand gives you a number for the...

(I) 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'  (abbreviated:  eq ).

The viral load can range from  'not detected'  to hundreds of millions hepC viruses per milliliter, or up to near a hundred million IUs (International Units) per milliliter.

(II) What does "negative", "not detectable" mean ?

The meaning of "not detectable" or "negative" depends on the test used.

The most sensitive test is normally the qualitative PCR test. In the laboratory, to which I go, the detection limit for this test is 100 eq/ml. That means that any viral load below 100eq/ml would give the result  "negative", and any viral load above 100eq/ml would give the result  "positive" . But, in this qualitative test, 'positive' can mean everything above the detection limit,  110eq/ml as well as 11,000,000eq/ml - it doesn't give you a number for the viral load.

The quantitative HCV RNA test via  *PCR*  is often somewhat less sensitive - a detection limit of 200eq/ml  in the case of the laboratory, to which I go. But as the name says: It measures the quantity of viruses in your blood.  It has the advantage to give you a number for the viral load. - There is also a cheaper  *bDNA*  test. But its much less sensitive, with its detection limit being at approximately 200,000eq/ml.

So, when you are " negative ", maybe there is no hepatitis C virus in your blood. But maybe also you do have hepatitis C viruses in your blood, but the number of viruses is lower than the detection limit of the test. -  In clinical trials, ' negative ' nowadays normally is defined as less than 100 Eq/ml.

(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 (ml).  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.
Thanks to the WHO, we now have another way to express the viral load, the IU (International Unit).

1.) 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.

2.) Viral loads given 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 1,730,000 equivalents/ml.  Sometimes also the prefix  "k" (kilo) is used, it means thousands.

the following ways to express the viral load are only rarely seen today:

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. Exponential numbers might show up in a form like 1.73x106 or 1.73x10(exp)6 or 1.73x10^6 which are all the same number 1,730,000.

c.) Logarithmic format

Now, recently some people express these numbers also in  logarithmic form (logarithmic transformed number).


6.24 is the logarithmic transformed number of the viral load of our above example. So, if this  logarithmic number is given to you,  you need a calculator to convert this to a plain number. You have to use the function 10x, where you have to replace x with the logarithmic number, in the above case  6.24.  You have to calculate 106.24 = 1,730,000 , 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 (expressed 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 !

3.) Viral load given 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.

Therefore it is important to have a close look at your lab report and see in what units the result is given !

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 in eq/ml
below 200.000
very low
below detection limit of bDNA test
average viral load at 3,200,000 eq/ml
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.

Please note: I am no MD. I am just an informed layperson. What I have learned and written down here cannot and should not replace the advice from a good hepatologist! And I have written this in English, which is a foreign language for me. So beware....



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