Q. What is viral load and how is it measured?
A. Viral load or viral burden is the quantity of HIV-RNA (HIV virus) that is in the blood. RNA is the genetic material of HIV that contains the information needed to make more virus. Viral load tests measure the amount of HIV-RNA in a small amount of blood (one milliliter, ml).
Q. When should viral load be measured?
A. An expert panel for the International AIDS Society has issued guidelines (9) indicating when viral load should be measured. The following is the panel's timetable for testing:
- Take two different viral load measurements 2-3 weeks apart to determine a baseline measurement.
- Repeat every 3-6 months thereafter in conjunction with CD4 counts to monitor viral load and T-cell count.
- Repeat the test 4-6 weeks after starting or changing antiretroviral therapy to determine the effect on viral load.
Individuals should avoid having viral load measurements done in the 3-4 weeks following an immunization (including flu shots) or within one month of an infection. Temporary increases in viral load have been seen in these instances. To minimize misleading results, it's best to avoid testing at these times.
Q. How is viral load measured?
A. Changes in viral load are often reported as logarithmic or "log changes." This mathematical term indicates a change in the value of what is being measured by a factor of 10. For example, if the baseline viral load by PCR were 20,000 copies of RNA/ml of blood, then a 1 log increase equals a ten-fold (ten times) increase or 200,000 copies of RNA/ml of blood. A 2 log increase equals 2,000,000 copies of RNA/ml of blood, or a 100-fold increase.
Using the same starting point of 20,000 copies or RNA/ml of blood, a 1 log decrease means that the viral load has dropped to 2,000 copies of RNA/ml. A 2 log decrease equals a viral load of 200 copies of RNA/ml of blood. An easy way to figure out log changes is to either drop the last "0"or add "0" to the original number.
Any change of less than one-half log is insignificant. More simply, if the viral load measurement has not tripled or dropped to 1/3 of its previous level, the difference might be unimportant. For example, if the baseline viral load were 20,000 copies of RNA, a rise to 60,000 or a fall to 7,000 copies might just be the result of transient changes. Repeat testing of a single specimen may give two quite different results and natural biological day-to-day variability of samples from the same person may cause measurements to vary slightly. Researchers believe that clinical decisions made on the basis of changes in viral load ideally should be based on separate measurements taken 2-3 weeks apart.
For additional information about viral load testing or other treatment issues, contact:
HIV/AIDS Treatment Information Service