The U.S. Food and Drug Administration said Wednesday that the agency found a contaminant in some lots of the recalled blood-thinner heparin sold by Baxter International Inc.

The contaminant found in the drug acts like heparin, the FDA said. Therefore, it would not show up on traditional tests. Samples that tested positive contained 5 percent to 20 percent of the contaminant. The agency said it could not yet identify how the substance got into Baxter’s product and whether its introduction was deliberate.

“It reacts like heparin in some of the conventional tests used for heparin, which is why conventional acceptance tests of this ingredient might not detect this contaminant. We still don’t know whether this inadvertently got into the supply or whether it was actually added,” said Janet Woodcock, acting director of the FDA’s Center for Drug Evaluation and Research, according to the Washington Post.

Last month, Baxter recalled the bulk of its heparin products after receiving hundreds of reports of allergic reactions possibly linked to heparin, as well as reports of four deaths. Last week, the company recalled all remaining heparin products.

The FDA said it had since received reports of 19 deaths that had occurred since January 1, 2007 that fit the description of being caused by an allergic reaction, and 785 serious events possibly linked to heparin. Agency officials estimated last week that about 200 of the reports were allergic reactions.

Peter Arduini, President of Baxter’s medication delivery business, said the company had received about 450 adverse event reports since December 15, 2007. Of those, four deaths were possibly related to an allergic reaction to heparin, he said, adding that the company noticed a spike in adverse event reports from mid-December through early January.

“It's too early to conclude that the deaths were caused by the allergic reaction. Just because there's a report that a patient took heparin doesn't mean that the heparin caused the event,” Arduini said.

Arduini added that investigators were also not certain whether all of those deaths were associated with Baxter's heparin.

“We don't yet have direct causal link between the contaminant and adverse events. Some of the batches of heparin causing the effect have this contaminant in them. So there is an association between the heparin-like compound and adverse events, but no causal relation yet,” Woodcock said.

Baxter said in a statement that it found the contaminant in samples from the Chinese plant and samples processed at its Wisconsin factory from Chinese ingredients.

“These results suggest that the root cause may be associated with the crude heparin, sourced from China, or from the subsequent processing of that product before it reached Baxter,” the company said, according to the Post.

The other major U.S. heparin supplier, APP Pharmaceuticals Inc, ramped up production to avoid a drug shortage after Baxter recalled most of its heparin products last week.

Tests on APP's heparin have not turned up any contamination, Woodcock said.