Most of the Medicare Part D prescription plan drug list changes that insurers made in the middle of 2008 and 2009 expanded enrollees’ access to drugs, according to the U.S. Government Accountability Office (GAO).

Medicare Part D gives private insurers a vehicle for selling prescription coverage to Medicare enrollees.

The GAO looked at the Part D plans’ mid-year changes to drug lists, or “formularies,” in response to a request from House Democrats.

In 2008, mid-year formulary changes had the potential to reduce access to drugsfor just 4.9% of enrollees who filled a prescription for a drug that year, Kathleen King, a GAO director, writes in a report summarizing the GAO’s mid-year Part D formulary change review.

In those cases, the plan providers, or “sponsors,” removed a drug from a formulary or tightened utilization management requirements.

But the percentage of enrollees who lost access to a drug was probably lower than 4.9%, because Part D program rules often require plans to let enrollees who are already on a drug continue to take the drug when the formulary changes, King says.

All but 4 of the 4,238 Part D plans in existence in 2008 made 2 or more mid-year formulary changes in 2008, and all but 5 of the 4,207 plans in existence in 2009 made 2 or mid-year change in 2009.

The percentage of plans making 100 or more mid-year formulary changes increased to 60% in 2009, from 47% in 2008.

The percentage of the changes that involved adding a drug to a formulary or easing utilization management requirements increased to 89% in 2009, from 87% in 2007, King says.

Other Medicare Part D prescription drug coverage from National Underwriter Life & Health: