WASHINGTON — The Trump administration is hoping to transform Medicaid by allowing states to test work requirements, premiums and other conservative policies, but a new government report says federal and state officials do not properly evaluate whether such experiments improve patient care or reduce costs.

Evaluations “generally lacked rigor,” and the findings were often kept secret for years, so they were of little use to policymakers, the Government Accountability Office, a nonpartisan investigative arm of Congress, said in the report issued last month.

Under federal law, the secretary of health and human services can waive certain Medicaid requirements so states can use federal money for the program in ways that would not ordinarily be allowed — for example, to cover people and services that would not otherwise be covered.

These waivers have been used for demonstration projects testing innovative ideas and involving millions of people and tens of billions of federal dollars. The Government Accountability Office said the number and scope of Medicaid waivers had grown in recent years, with demonstration projects now accounting for one-third of all federal Medicaid spending — and more than 75 percent in 10 states, including Texas.