The contaminant linked to dozens of deaths and hundreds of illnesses from tainted batches of the blood thinner heparin manufactured in China was chemically related to a dietary supplement used to treat joint pain, researchers confirmed on Wednesday.

The U.S. Food and Drug Administration announced in March that it believed the contaminant to be over-sulfated chondroitin sulfate or OSCS, a modified version of the joint supplement chondroitin sulfate.

Wednesday's announcement was confirmation of that finding and means that new tests put in place to scan for OSCS contamination should be effective.

Maker Baxter International Inc. launched two recalls of its heparin products — used in heart surgery and kidney dialysis — on January 17 and February 28 after 113 patients in 13 states suffered adverse reactions such as low blood pressure, nausea and shortness of breath.

Ram Sasisekharan of the Massachusetts Institute of technology and colleagues at the Centers for Disease Control and Prevention found that more than 98 percent of the 152 reactions they studied were at health centers where the heparin was contaminated by OSCS.

"Of 54 reactions for which the lot number of administered heparin was known, 52 (96.3 percent) occurred after the administration of OSCS-contaminated heparin," they wrote in the New England Journal of Medicine.

OSCS can thin blood, passes the quality control screening test used at the time and is far cheaper to acquire than getting real heparin from pig intestines.

Sasisekharan said in a telephone interview that the study made no attempt to determine how the heparin became contaminated, but the chondroitin was purified. "It is hard to explain a natural biological source for this," he said.

In the samples his group examined, up to 35 percent of the heparin, by dry weight, was actually OSCS.

Sasisekharan said the contamination coincided with the spread of a virus that had created a shortage of pigs in China.

"There weren't enough pigs around, you're not able to get enough heparin, and the over-sulfated chondroitin passes the test that is in place to screen heparin," he said.

A key unanswered question is whether patients who survived exposure to the tainted heparin still may have health problems.

"There were countries that were still using the contaminated heparin because of an acute shortage, so they took the risk," said Sasisekharan. "We have no idea what the long-term exposure implications are."