Massive fraud in Medicare's prescription drug programs may be going untracked, according to a government watchdog.

"Because the" Centers for Medicare and Medicaid "does not have data on potential fraud and abuse for more than half of Part D plans, it cannot assess the efficacy throughout the Part D program of sponsors' efforts to control fraud and abuse," said the inspector general for the Department of Health and Human Services.

Medicare's Part D program provides prescription drug coverage for more than 36 million seniors and disabled people. Projections for Medicare Part D expenditures in 2013 totaled $68 billion.

Only 46 percent of insurers reported fraud and abuse between 2010 and 2012, while 293 of the 573 insurers with a CMS contract for all three years failed to report any data on fraud or abuse from the 14.5 million covered.

"Therefore, CMS does not have data on incidents of potential fraud and abuse for plans covering almost half of the beneficiaries enrolled in Part D," according to the IG report.

Though not required, insurers can also "refer incidents of potential fraud and abuse to CMS, Federal and local law enforcement, and State agencies," the IG said. "Sixty-one percent of the 261 sponsors did not refer any identified incidents to these entities."

"CMS could strengthen the usefulness of reported data on fraud and abuse by clarifying terms and requesting more specific information about incidents," the IG said.