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Understanding Viral Load Assays for Cytomegalovirus and Epstein-Barr Virus


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Reality Check

Slide 28

March 2010

What level of change is significant, or "real," in viral load assays? It is thought that variation within the patient is about 0.2 or 0.3 log. This means that 2 samples drawn at different times on the same day could vary by 2-fold. Within the laboratory, as we just discussed on the previous slide, the variation is another 0.2 or 0.3 log.

As you can see, a patient's viral load must change substantially to be outside the variability inherent in the biological and laboratory processes.

Let's pause for a moment to compare the state of the art for viral load testing in HIV and Hepatitis C versus CMV and EBV. You can see that for HIV and HCV, there are defined reporting units, international standard reference materials, and defined treatment goals. It is also generally regarded that in order for a viral load to be significantly different than the previous one, HIV should change by about 0.5 log or more (at least 3 fold), whereas HCV should change by about 1 log (or 10 fold). Any differences less than that are to be expected because of the variability of the viremia in the patient and the variation of the laboratory assays.

When tracking viral loads in patients with HIV or hepatitis C, the treatment goals are becoming well-established. Initiation of effective treatment for HIV should result in decreasing viral loads at a rate of 1 log per month (or 90% lower than each previous month). For HCV, demonstration of a 2 log, or 100 fold drop in viral load, in the first 12 weeks is an indication that the patient stands a good chance of successful treatment in the long term.

For CMV & EBV, there are a multitude of assays and reporting units, no international reference material, no defined treatment goals and very loose concepts of significant change.

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