Colloid fluids are crystalloid electrolyte solutions with a macromolecule added that binds water by its colloid osmotic pressure. As macromolecules escape the plasma only with difficulty, the resulting plasma volume expansion is strong and lasts many hours. The clinically used colloid fluids include albumin, hydroxyethyl starch, gelatin, and dextran.

The plasma volume expansion shows one-compartment kinetics. Marketed iso-oncotic fluids are usually composed so that the infused volume expands the plasma volume by the infused amount. Exceptions include hyperoncotic variants such as 20% albumin.

The main indication for colloid fluid is as second-line treatment of hemorrhage. Because of inherent allergic properties, crystalloid electrolyte fluids should be used when the hemorrhage is small. A changeover to a colloid should be performed only when the crystalloid volume is so large that adverse effects may ensue. The only other clinical indication is that dextran can be prescribed to improve microcirculatory flow.