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Understanding Viral Load Assays for Cytomegalovirus and Epstein-Barr Virus



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What is Significant Change?

Slide 27

March 2010

Now let's return again to the issue of variability. Two recent studies sent the same samples to 30 laboratories and only about half of the results agreed within 0.5 log (remember, this is a 3-fold change!). This lack of precision between laboratories is not unexpected, since different labs are calibrating in different ways and using different assays.

What about precision within a laboratory? Let us imagine that a laboratory can test the same specimen over and over with a level of precision where the standard deviation is about 0.2 log. In statistics, 2 standard deviations encompass roughly 95% of the population. In other words, testing the same sample multiple times would produce values that are generally no greater than 0.4 log from the mean.

To put this variability into perspective, let's think about the significant change in HIV. For HIV, we are looking for an initial 1 log drop in response to treatment, and generally changes of less than 0.5 log are not considered to be not clinically significant, or different, than a previous value. If we borrow the same 0.5 log concept for CMV and EBV, then the laboratory needs to be producing results that have less variability than 0.5 log. In the example here, this means that the controls should be running within a range of 2 standard deviations, otherwise, the run is not valid. If a laboratory has an assay with an unacceptable level of precision, then the results may not be reflective of true changes in patient viral loads.

Significant Change

 


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