Chinese shipments of Baxter International blood-thinner heparin contained large amounts of a contaminant that might explain hundreds of serious side effects, the U.S. Food and Drug Administration (FDA) said Wednesday. The federal agency believes that the shipment may have been tampered with before it arrived at the drug company's U.S. processing and distribution centers.

The government says that the contaminants used in blood-thinning drugs are under investigation but it could be a foreign substance that is similar in chemical make-up to heparin. The product has been linked to 19 deaths and 800 illnesses

Dr. Janet Woodcock, acting director of the FDA's center for drug evaluation and research said they are still investigating the source of contamination but suspect it to have originated in from Scientific Protein's plant in Changzhou, China.

Deerfield-based Baxter and its supplier, Scientific Protein Laboratories of Waunakee, Wis., said they could not detect the contamination owing to the similarity of the foreign substance to heparin.

The contaminant accounted for between 5 percent and 20 percent of some of the samples tested. Remaining U.S. supplies of heparin, made by Baxter competitor APP Pharmaceuticals, do not show contamination, the FDA added.

In mid-January Baxter recalled nine lots of the heparin and now made a full-scale recall of its product last week. Heparin is an anticoagulant which prevents the formation of new blood clots. Heparin works quickly after administration. Since it takes a few days for anticoagulant pills (warfarin) to become effective, heparin is usually administered for about 5 days.

Bleeding complications are potential side effects of heparin therapy and may be caused by heparin interactions with platelets. However, reducing the size of the molecule (low-molecular weight heparins) and varying the dosage may reduce the risk of excessive bleeding.