Variability of Viral Load Measurements

Viral Load Testing

By Jules Levin, Executive Director of NATAP

Dr. Coombs displayed a graph showing only one individual's variability in RNA measures over the course of one week. Although anecdotal, it is intended to illustrate a point. This person's RNA was measured three times (morning, noon, and night) on Monday, Wednesday, Friday and the following Monday. There was a good amount of variability in the scores, both within given days and between different days. (These measures are approx.) On the 1st Monday, the measures were 25,000, 28,000 and 37,000; on Wednesday: 35,000, 37,000 and 35,000; on Friday-- 40,000, 47,000 and 84,000; on the last Monday-- 50,000, 57,000, 65,000, 67,000 and 71,000. There are rises of 2 to 3 fold, all within 1 week. The first measure (25,000) on Monday is more than 3-fold less than the highest measure on Friday (84,000).

There are two points, Dr. Coombs is trying to illustrate.

There is variability within individual measurements of the virus. It is
generally believed that the biological variability is 3-fold (or 0.5 log). This needs to be taken into consideration when sequentially assessing a patient's RNA measures;
Various factors perturb virus level: we are learning that concurrent infection, immunizations, and unexplained events can cause the virus load to jump quite dramatically 2, 3 or more fold. For example, reactivation of genital herpes by itself, which can be clinically innocuous, can result in a significant short-term rise in viral load.

A doctor and the patient need to understand there is variability in measurement and generally it is considered that a 3-fold difference in measures falls with the window of variability we may expect to see.

Commentary: If a 3-fold change is sustained repeatedly over time (sustained means repeating the test a few times with the same result), that difference may not be merely a variability, but may reflect a real change in viral load.

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About the author: Jules Levin is the Executive Director of NATAP, based in New York City.

The National AIDS Treatment Advocacy Project (NATAP) is a New York State non-profit corporation dedicated to facilitating the effort for development of effective treatment for HIV.

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