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Understanding “Log” Changes

Changes in viral load—the number of copies of HIV’s genetic material found in a sample of blood—are often expressed as “log” changes. This term refers to scientific notation, which uses powers of 10 to keep large numbers manageable.

For example, a 1-log reduction is a reduction by 10 times the original amount. For example, if a person starts antiretroviral therapy with a viral load of 100,000 copies/mL and has a viral load of 10,000 copies/mL at the next visit, that’s a 1-log reduction.

Here’s a handy cheat sheet:

  • 1-log reduction = 10 times smaller
  • 2-log reduction = 100 times smaller
  • 3-log reduction = 1,000 times smaller
  • 4-log reduction = 10,000 times smaller
  • 5-log reduction = 100,000 times smaller