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Every week, usually on Tuesday, the Centers for Medicare & Medicaid Services sends state Medicaid departments something called a "flat file."

These files are sort of similar to the much-discussed 834 transmissions, which the exchange sends to an insurance plan when someone signs up. Except the flat files are for the Medicaid program, and lists people that the exchange thinks -- but hasn't officially determined -- will be eligible for the Medicaid program.

And, much like the those 834 transmissions, Medicaid officials say, these flat files are riddled with errors and incomplete information.

"They are really incomplete with lots of data cells missing," said Matt Salo, who runs the National Association of Medicaid Directors. "Sometimes the immigration status is missing, or their town is in a different state. A quick glance sometimes shows they're not Medicaid eligible because they earn too much, or they're already in the system."

The flat file was not initially meant to be an enrollment document. It was meant, instead, to give states a sense of enrollment volume, so they could beef up their staffing, if necessary. A separate account transfer function was supposed to be the actual enrollment vehicle.

That separate account transfer function, however, has not yet gotten off the ground. So the federal government told Medicaid directors Friday that it let states rely on a cleaned-up version of these flat files as an enrollment document. The Centers for Medicare & Medicaid Services will not penalize states that accidentally enroll a non-eligible resident based on information in these flat files.

"This new, transitional opportunity for states to enroll individuals assessed or determined eligible by the [exchange] using the information provided through the AT flat files will ensure that enrollment can be completed in a timely way without regard to temporary file transfer system issues at either the federal or state level," Cindy Mann, director of the Center for Medicaid and CHIP Services, wrote in the Friday letter.

Medicaid enrollment has been one of the more robust elements of the health law's early insurance expansion. About four out of every five people who gained coverage through the health law in October did so through this public program, which is expanding to cover more Americans who earn less than 138 percent of the poverty line (about $15,000 for an individual).

The 26 states that signed up for the Medicaid expansion saw a 15 percent increase in applications in October, according to data released this week by the Center for Medicaid and CHIP Services. Non-expansion states saw an increase too, with 5 percent more applicants than they averaged in the six months leading up to the health law's start.

Salo cautions against reading too much into these numbers, as they represent one month's worth of data in a much longer sign-up campaign. Colorado, for example, saw a 102 percent increase in enrollment in October. Those numbers won't turn up every month. "Any time you see something that has got huge swings, you've got to be a bit cautious," Salo said.

Much like the private insurance side, Medicaid has also dealt with technical challenges. In states on the federal marketplace, officials had initially envisioned a seamless enrollment process, one where the exchange would use an individual's information to guide them to either Medicaid or private insurance coverage.

Right now though, the system to transfer files from the federal marketplace to state Medicaid offices has not launched. So while states have estimates from flat files of how many people might be eligible for Medicaid, they don't have actual enrollment information.

The federal government plans to improve the "flat files" before it begins releasing them for enrollment purposes.

"The expanded AT flat file will contain sufficient information in order for states to complete enrollment and assign the individual to the appropriate Medicaid or CHIP group," Mann wrote in her letter. "The file will include data elements such as: date of birth, social security number, eligibility category used for assessment or determination, and verification inconsistencies."

Salo said there's a bit of a wait-and-see attitude from Medicaid directors, given the errors in the past. While the federal government has said that they won't penalize states that enroll someone via these flat files who doesn't actually qualify, states have their own concerns about doing so.

"That's not really how we run a program," Salo said."We don't enroll people who aren't eligible. From a financial perspective, we want to make sure we're enrolling the people who are eligible."

As for when the account transfer ability will come into effect, Salo says Medicaid directors have little idea of what to expect. Just as the federal government isn't ready to send these files, some states aren't totally ready to receive them.

"We're talking about a lot of systems involved in getting people covered and enrolled," Salo says."Sometimes, it doesn't work on the state side."

KLIFF NOTES: Top  health policy reads from around the Web.

Former prisoners will be a large part of the Medicaid expansion. "The Department of Justice estimates former inmates and detainees will comprise about 35 percent of the people who will qualify for Medicaid coverage in the states expanding their programs to anyone earning less than 138 percent of the federal poverty level, or about $15,000 for an individual in 2013. The Congressional Budget Office estimated earlier this year  that 9 million people will get that new coverage next year." Guy Gugliotta in The Washington Post.

Of course President Obama will enroll in the exchange. "Even from a practical standpoint, the president doesn't need insurance under the law; the White House medical team tends to the First Family. But given the level of political polarization surrounding the law, if Obama opted out of the exchanges it would provide more fodder for his opponents. It's the same sort of political calculation that prompted House Minority Leader Nancy Pelosi (D-Calif.) to declare all her aides would sign up under the law, even though she could have exempted her leadership staff." Juliet Eilperin in The Washington Post.