(Adds additional Baxter comment, FDA comment, second lawmaker letter to FDA)

By Lisa Richwine

WASHINGTON, Feb 21 (Reuters) - Two U.S. House of Representatives Democrats asked Baxter International Inc <BAX.N> on Thursday to provide records about the manufacturing of its blood-thinning drug, heparin, as well as recent reports of serious reactions.

Baxter suspended production of multi-dose vials of the drug, heparin, last week while officials search for the cause of four deaths and hundreds of health problems reported in patients.

The Food and Drug Administration has admitted it never inspected a Chinese plant that supplies an active ingredient for Baxter's heparin before approving it.

"The key question that Baxter must answer is this: Was your firm misled by FDA into believing that the Chinese firm was an approved supplier of an active ingredient for heparin? If not, then we are concerned that your company was knowingly distributing an unapproved drug," Reps. John Dingell and Bart Stupak said in a letter to Baxter Chief Executive Robert Parkinson.

Dingell chairs the House Energy and Commerce Committee, and Stupak heads the subcommittee on oversight and investigations. Both are Michigan Democrats.

The letter asks Baxter to provide records relating to suppliers of active ingredients for heparin, due diligence performed on manufacturing sites, promotional materials and documents that discuss pricing, plus other information. The lawmakers also asked for correspondence related to heparin between Baxter and the FDA or other regulatory bodies.

FDA officials say it is unclear if the Chinese site is linked to about 350 reports of potential reactions, including breathing difficulty, vomiting and rapid drops in blood pressure.

Baxter spokeswoman Erin Gardiner said the company would respond to the letter from the lawmakers.

"Baxter has fully complied with all regulatory and legal requirements with regard to working with" its supplier of the active ingredient of heparin, she added.

The FDA said on Monday it planned to start an inspection this week of the plant in Changzhou, China, which is owned by Wisconsin-based Scientific Protein Laboratories LLC (SPL).

Gardiner said Baxter routinely inspects ingredient suppliers and visited SPL's Chinese and Wisconsin facilities last year.

An FDA official said earlier this week the agency failed to inspect the Chinese site before approving Baxter's product because it mixed up the name with another company that had been checked recently.

Dingell and Stupak wrote separately to the FDA and its parent agency, the Department of Health and Human Services, asking about pre-approval inspection policies.

FDA officials have said it is agency policy to inspect all manufacturing sites before granting a drug's approval and that the heparin plant mix-up was an isolated case.

The lawmakers said they were considering if a requirement for a pre-approval inspection needs to be written into law.

"Since it appears that FDA feels free to ignore this long- standing policy, we are now considering whether such safeguard should be codified into law," the lawmakers wrote.

FDA spokeswoman Karen Riley said the agency would respond to the lawmakers.

"Protecting the American people by determining the cause of the problem with the Baxter heparin and addressing that problem is our highest priority," she added.

Sen. Charles Grassley, an Iowa Republican, also has requested information about the heparin case from the FDA and Baxter.

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