The US Food and Drug Administration (FDA) has received hundreds of reports of serious injuries and/or deaths in patients who have been administered heparin made by Baxter containing this contaminant - Baxter had sourced its API from SPL, who had sourced it from China.
The lots in question were recalled in February.
SPL said it voluntarily initiated the nationwide withdrawal of the API as a precaution following the recent health scare involving Baxter's heparin, although as yet, no new adverse events related to this latest issue have been reported.
The voluntary recall also affects 23 finished product lots of heparin sodium in 5% dextrose and 0.9% sodium chloride injection solution manufactured and distributed by B. Braun Medical within the US and Canada.
The company pulled these products from the market after it was notified by SPL of the API contaminant.
After weeks of investigations, the 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.
It has been revealed as oversulfated chondroitin sulphate, which 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.
"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 API used in a finished heparin product.
SPL supplies a refined form of the heparin API to Baxter and other firms from a plant it partly owns in China, called Changzhou SPL, along with a facility it owns in Wisconsin, US.
The contamination was present in the powdered form of raw heparin that was purchased by SPL's suppliers in China , according to Robert Rhoades, a pharmaceutical consultant for Becker & Associates, speaking on behalf of SPL.
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.
Meanwhile, the made-in-China heparin scare has also spread beyond the US, with Germany and Japan both recalling the drug two weeks ago.
German manufacturer Rotexmedica pulled its heparin product from the market, after around 80 severe allergic reactions were reported by patients who were given the drug since 14 February - a much higher number than usual.
Although the product in question was manufactured locally in Germany, the drug's API has also been associated with two Chinese suppliers.
Following on from this, as a precautionary measure only, three unidentified pharmaceutical firms in Japan have also taken a zealous approach to the situation and recalled their products from the market, due to a possible association with China-made API, even though there have been no adverse events reported in the country relating to heparin produced by SPL.
"The three Japanese companies made clear that the recall was simply a precaution, as there has not been a pattern of adverse reactions to heparin reported in Japan similar to what has been observed in the US and Germany," SPL said in a statement.