A chemical circulating in the plasma may be responsible for focal segmental glomerulosclerosis. Focal segmental glomerulosclerosis is a kidney disease characterized by scarring of the glomeruli, which are the part of the kidney that filters blood. Researchers took serum samples from 100 patients with focal segmental glomerulosclerosis and tested them in an assay using rat glomeruli. The permeability of rat glomeruli to the protein albumin was 0.18 when exposed to the patients' serum, compared to 0.06 when exposed to the serum of healthy volunteers. Some serum samples from patients with focal segmental glomerulosclerosis caused the permeability of rat glomeruli to jump to 0.50 or higher. Several patients were treated with plasmapheresis, a procedure in which plasma is replaced. Following plasmapheresis, their plasma lost the ability to increase the permeability of rat glomeruli. This indicates that a chemical in plasma may contribute to focal segmental glomerulosclerosis.