THURSDAY, Dec. 1, 2016 (HealthDay News) -- A significant number of people with HIV have strains of the AIDS-causing virus that are resistant to both older and newer drugs, researchers report.

The researchers looked at 712 HIV patients worldwide whose infection was not controlled by antiretroviral drugs. They found that 16 percent of patients whose infection was resistant to modern drugs had HIV mutations linked with resistance to older drugs called thymidine analogues.

Among patients whose HIV had this mutation, 80 percent were also resistant to tenofovir, the main drug in most modern HIV treatment and prevention programs, the researchers reported.

The findings were published in the Nov. 30 issue of The Lancet Infectious Diseases journal.

"We were very surprised to see that so many people were resistant to both drugs, as we didn't think this was possible," study lead author Ravi Gupta, of University College London, said in a school news release.

"Mutations for thymidine analogue resistance were previously thought to be incompatible with mutations for tenofovir resistance, but we now see that HIV can be resistant to both at once. This emphasizes the need to check the genetic profile of patient's virus before prescribing first-line treatments, as they may have already developed resistance to other treatments that they did not mention having taken," Gupta said.

Drug resistance typically occurs when patients fail to take their medications as directed by their doctor.

"To prevent these multi-resistant strains from developing, we need cheap, reliable systems to assess people before treatment," he said.

What's needed, Gupta said, are easy-to-use resistance-testing kits to help screen for drug resistance before giving treatment. This would also help doctors to "monitor HIV drug resistance globally more effectively," he said.

"However, until such kits are widely available, we could test the amount of virus in the bloodstream before and after giving treatment. Although not as precise as resistance testing, this could help us to detect treatment failure earlier and switch patients to second-line drugs," he added.

If a patient's HIV becomes resistant to first-line drugs, they're given second-line drugs that cause more side effects. But many rural patients don't have access to second-line drugs, so trying to preserve the effectiveness of first-line treatments is crucial, Gupta explained.

More information

The U.S. National Institute of Allergy and Infectious Diseases has more on HIV treatment.