Viral Load and Log Numbers

Viral Load is one of the numbers that folks with Hepatitis C take very seriously. We take it more seriously than we probably should given that the viral load numbers do not directly correlate with whether you are symptomatic, the amount of damage to your liver or the seriousness of the side effects that you may be experiencing. It is, however, the number that is measured to determine the ongoing success of your treatment regimen and to determine in the long run whether you have cleared your body of the virus. Given that, it is followed with a great deal of attention.

The viral load is expressed in the number of copies of the Hep C Virus RNA that are contained in a milliliter (ml) of blood. This is expressed as the number international units (IU) of Hep C RNA per ml. In my case, my viral load number ranged from 3,000,000 IU per ml when I was diagnosed to just slightly below 13,000,000 IU/ml at the onset of treatment.

The changes in viral load that we track during treatment are expressed as logarithmic or log numbers. Log number differences in the amount of virus are differences in amount that are expressed as factors of 10. These log number differences are the numbers that are considered significant in Hepatitis C treatment. Using my case as an example, if 13,000,000 IU/ml is my viral load at the start of treatment, then a drop in viral load to 1,300,000 is a log 1 change. A drop to 130,000 is a log 2 change, to 13,000 is a log 3 change, 1,300 is log 4, 130 is log 5 and going to undetectable, or under 15, is right about a log 6 change in viral load. In treatment, the doctors want to see a log 2 drop in viral load by week 12 or the patient is considered to be non-responsive.

Once we understand that, the changes in viral load that we see at various times during our disease and during treatment and the significance of those changes become easier to understand. For example, the changes in my viral load as I progressed from 3,000,000 to 13,000,000 before starting treatment are actually not significant changes despite the fact that they look like large changes. In order to have a log 1 increase in my viral load, I would have had to see it increase to 30,000,000 and a log 2 increase would mean that I would have had to see a viral load number of 300,000,000 IU/ml. Now that would be a high viral load indeed. Likewise if you had a viral load of 250,000 and saw it jump to 500,000 it would be considered a not significant change in amount even though your viral load doubled.

The same thing applies to watching viral load as it drops. If you have that same 250,000 IU/ml at the beginning of treatment, a log 1 drop would require a change to 25,000 and to achieve the log 2 reduction your doctors will want to see by week 12 you need a drop to 2,500 IU/ml. In order to reach the undetectable level, you would need a drop of somewhat over log 4. In my case, when I went from 13,000,000 to 4,000 after one week of treatment, that was over a log 3.5 drop in viral load. Likewise when I had my viral breakthrough and went from under 15 to 17,000 it was about a log 3.1 increase in my viral load.

Both of these numbers were significant because of the size of the logarithmic change in the amount. When my confirmation breakthrough test came back with a number of 40,000 that was not a significant change from the 17,000 number that signaled my viral breakthrough despite the fact that it doubled. When the first test results I got after going on treatment outside the study came back at 10,000 that was also not a significant change. I was happy to see my number going down, but just to get to a simple log 1 change I would have to drop to 4,000 and the magic log 2 change means I have to drop to 400 IU/ml.

So don’t panic over fluctuations in your viral load numbers that might appear to be quite large if they don’t reach the level of a ten-fold (log 1) change or greater. Even then, it may not be signaling a major change in your illness, but if it doesn’t even reach that level, it probably means little or nothing at all…