DTPA is a chelating agent that chelates Tc-99mO4. Tc-99m-DTPA does not bind to plasma protein, is essentially biologically inert and is the nuclear medicine equivalent of radiological contrast medium. As a small hydrophilic molecule, it diffuses across capillary endothelium and enters the interstitial fluid space of the whole body, except the brain. Being hydrophilic, however, it does not enter the whole-body intracellular space. Its distribution volume is therefore the extracellular fluid volume. Like contrast, it is unable to cross the blood-brain barrier unless the latter is damaged. It is freely filtered at the glomerulus and passes into urine without tubular secretion or reabsorption. It therefore belongs to a family of substances called filtration markers that have identical plasma, renal and urinary clearances and are used for measuring GFR. It is also used for imaging the kidney in renography, although for this application it has been largely replaced by Tc-99m-MAG3.