A majority of adults drug tested by Quest Diagnostics in 2015 were misusing their prescription drugs, according to a new study of patient results released by the clinical laboratory services company.

The 54 percent rate of misuse is an improvement from 2011 results in the same study, when the rate stood at 63 percent. Misuse among children has dropped sharply in the same time period, from 70 percent in 2011 to 44 percent last year.

The study defines misuse as taking either too much, too little or none of the recommended dosage. It also includes the 45 percent of test results where patients were found to be taking illegal drugs.

The Quest study said patients were at a high risk of misuse even when results were examined by age, payer type, location and gender, but credited new prescription drug policies in reducing the rate of misuse.

“The overall decline since 2011 in prescription drug misuse suggests better provision of information of prescribed drugs (better matching between what we are told and what we detect), less misuse of additional drugs other than those prescribed, better compliance with prescriptions, or a combination of these factors,” the study said. “This likely represents improved education and use of available tools by healthcare providers and patients as awareness of this epidemic increases. Additionally, more states have required continuing medical education in prescription control and have developed programs to combat drug misuse.”

The study also examined which drugs were most often misused based on the patient’s age. Amphetamines were the most common group associated with inconsistent test results in patients up to age 17. Marijuana was most common among patients 18 to 24, while benzodiazepines were the top drug class among patients 25 and up.

Benzodiazepines being mixed with heroin were the focus of another part of the study, which for the first time included more than 155,000 heroin test results. In those tests, 28.6 percent of patients above the age of 18 tested positive for both drugs, and 92.3 percent of the benzodiazepines in those combined results weren’t prescribed, which may be due to prescription drugs being sold illicitly.

“Physicians and other health professionals need to consider these patterns of diversion and drug abuse when prescribing and monitoring both prescription and illicit drugs,” the study said.

Mixing benzodiazepines and heroin can be deadly, due to both drugs lowering respiratory rates, potentially leading to slowed or stopped breathing and increasing the risk of heart attack.