Certain perfusates have been shown to have toxic effects on kidneys as a result of the inadvertent inclusion of particular chemicals in their formulation. Collins showed that the procaine included in the formulation of his flush fluids could be toxic, and Pegg has commented how toxic materials, such as PVC plasticizers, may be washed out of perfusion circuit tubing. Dvorak showed that the methyl-prednisolone addition to the perfusate that was thought to be essential by Woods might in some circumstances be harmful. He showed that with over  g of methyl-prednisolone in 650 mL of perfusate (compared with 250 mg in 1 litre used by Woods) irreversible haemodynamic and structural changes were produced in the kidney after 20 hours of perfusion. There was necrosis of capillary loops, occlusion of Bowman's spaces, basement membrane thickening and endothelial cell damage.