Methods and assays for oversulfated glycosaminoglycans are provided. In an embodiment, the present disclosure provides a method for detecting oversulfated glycosaminoglycan (OS-GAG) in a heparin sample. The method comprises placing the heparin sample onto a support comprising immobilized heparin and contacting the heparin sample on the support with a binding compound that attaches to the heparin and forms a heparin-binding compound complex. The binding compound also has a greater affinity for attaching to the OS-GAG than to the heparin in the heparin sample and forms an OS-GAG-binding compound complex. The method can further comprise detecting an amount of the heparin-binding compound complex on the support, and determining an amount of OS-GAG in the heparin sample based on the amount of the heparin-binding compound complex on the support.
