Scientists investigating a mystery contaminant in the blood thinner heparin are closing in on what they believe is a counterfeit substance, most likely made in China from animal cartilage, that was chemically altered to act like the real drug.
The United States Food and Drug Administration declined to confirm that statement and a spokeswoman said the agency would have no comment.
But in interviews, heparin experts in China and the United States — including one researcher involved in the inquiry — said that a chemically altered substance called over-sulfated chondroitin sulfate is probably the contaminant that the F.D.A. says is linked to hundreds of allergic reactions and 19 deaths.
Even so, researchers said they were not certain that the contaminant, constituting between 5 percent and 20 percent of the drug, is what is causing the allergic reactions, nor do they know precisely how or when it was mixed into the active ingredient.
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