Researchers have developed a new genetic test that uses polymerase chain reaction (PCR) technology to measure the size of the HIV reservoir—the collection of latently infected cells that are not replicating and thus evade antiretroviral treatment. Typically, the reservoir is measured with a PCR test that indicates the amount of viral DNA inside latently infected CD4 cells. But such a test cannot determine which cells contain HIV that can produce viable new copies of the virus and which ones contain defective virus that poses no threat, the latter of which account for perhaps 98 percent of the HIV reservoir. The new PCR test, which can distinguish between functional and defective viral DNA inside resting cells, is 10 to 100 times more accurate and much faster than a commonly used assay. It will therefore improve scientists’ ability to measure how experimental cure therapies impact the viral reservoir.