The US Food and Drug Administration (FDA) has now identified the contaminant found in samples of Baxter's recalled heparin and confirmed that it is indeed linked to a Chinese manufacturing plant.
In its latest press briefing the FDA revealed the contaminant's identity as oversulfated chondroitin sulphate.
"It doesn't appear to be a natural contaminant that got in there," said the agency, but a man-made compound that had been chemically modified to mimic raw heparin, the active pharmaceutical ingredient (API) used in a finished heparin product.
Oversulfated chondroitin sulphate is made from animal cartilage and is a cheaper alternative to raw heparin, which is normally derived from pig's intestines. It is not approved use in medicine, said Dr Janet Woodcock, director of the FDA Center for Drug Evaluation and Research.
The contamination was present in the powdered form of raw heparin that was purchased by Scientific Protein Laboratories' (SPLs) suppliers in China, according to Robert Rhoades, a pharmaceutical consultant for Becker & Associates, speaking on behalf of SPL.
SPL supplies a refined form of the heparin API to Baxter from a plant it partly owns in China, called Changzhou SPL, along with a facility it owns in Wisconsin, US.
The FDA again stressed that its investigations into the source of the contamination are still ongoing: "We don't know how it was introduced or why," said Woodcock, adding that investigators "cannot rule in or out" whether the contaminant was accidentally or deliberately put into the product in an act of counterfeiting.
The agency also reiterated that it is still yet to clarify that the contaminant is actually the "root cause" of the hundreds of severe adverse events and 19 deaths that forced Baxter to pull the product from the market.
Commenting on the latest developments, Baxter's corporate vice president and chief scientific officer, Norbert Riedel, said in a statement that the contaminant was very likely "introduced at the workshop or consolidator level,'' before the heparin reached SPL and Baxter.
"We're at a critical juncture in the investigation and further progress can be accelerated with the cooperation of the consolidators and workshops," he said.
China's State Food and Drug Administration (SFDA) has stressed that it is working closely with the FDA to get to the bottom of the matter.
However, the task at hand is a challenge, as the production chain for heparin is very complicated, according to Wu Zhen, the SFDA's deputy commissioner.