Medicaid enrollment jump spurred by Obamacare, but how much?

Enrollment in Medicaid spiked in December, aided by Obamacare exchanges and an expansion of the government-run health coverage program for the poor in 25 states.

But it was far from clear just how many of the Medicaid enrollees are new people drawn by Affordable Care Act-related initiatives, as opposed to re-enrollments, according to a leading health-care analyst who called the data released Wednesday "confusing."

By the end of December, more than 6.3 million people were determined to be eligible for Medicaid or CHIP, the program covering children, through state-run agencies and state-based Obamacare exchanges, according to a Centers for Medicare & Medicaid Services report released Wednesday.

That tally does not include the 750,000 or so people who were determined eligible in Medicaid through the federally-run Obamacare exchange HealthCare.gov.

Adding the two enrollment numbers together equals more than 7 million Medicaid-eligible determinations. But some of the determinations made by HealthCare.gov may be duplicative of state-based decisions.

Another 2.2 million or so people had purchased private Obamacare insurance through a federally run or state-run exchange by the end of December.

Medicaid enrollment in December through the state-based entities alone 2.3 million people, according to CMS, which noted that represented a 20 percent hike in the rate of sign ups over November's tally.

The enrollments came nationally, both in 25 states which are not expanding Medicare eligibility to include most poor adults, as well as states that are expanding. The sign-ups included the newly eligible, re-enrollments, and people who previously qualified for Medicaid but did not enroll for some reason.

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But CMS noted that the enrollment spike was particularly pronounced in states that are expanding the long-time joint, federal-state-run health program.

"Compared to the average for July through September, the number of December determinations were 73 percent higher in states that chose to expand Medicaid compared to 3 percent in states that rejected the expansion," a CMS blog post said.

Health care analyst Timothy Jost, a professor at the Washington and Lee University School of Law, said that high percentage is "the key number" and suggests that a big chunk of the Medicaid sign-up reported Wednesday are due to new enrollees.

But how big a chunk is difficult, if not impossible, to figure out from the CMS data, Jost said.

"The numbers are just so confusing," he said.

"I think it's safe to say that more people are getting Medicaid now than would have before the Affordable Care Act," Jost said. "I can't tell from these numbers how many new ernollees we have, hove many of these are new determinations, how many are under the Medicaid expansion, and how many are currently eligible."

"At least, I can't," he added. "I would assume that if they [CMS] could figure it out from this data, they would tell us."

"We don't have 6 million new enrollees," he said flatly.

CMS noted that 14 states, including Texas, Pennsylvania, Virginia and Michigan, and the District of Columbia are including in their enrollment data people who previously had Medicaid. Four states, Massachusetts, Ohio, Pennsylvania and Wisconsin did not provide eligibility determination date, according to CMS.

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Medicaid expansion has been the subject of heated debate in a number of states, some of whose Republican governors have favored expansion in opposition to legislatures run by the GOP.

In states that expand coverage, most people with incomes up to 133 percent of the Federal Poverty Level, or $15,282 for an individual and $31,322 for a family of four, will be eligible for Medicaid coverage beginning Jan. 1.

In those states, the federal government is paying 100 percent of the costs of expansion for the first several years, and beginning in 2020 and thereafter will cover 90 percent of the costs from expansion.

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—By CNBC's Dan Mangan. Follow him on Twitter @_DanMangan