Evans Blue, often incorrectly rendered as Evan's Blue, is an azo dye which has a very high affinity for serum albumin. Because of this, it can be useful in physiology in estimating the proportion of body water contained in blood plasma.
E 1705 (OTTO) Evans blue Cas 314-13-6 - used to assess the permeability of the blood–brain barrier to macromolecules. Because serum albumin cannot cross the barrier, and virtually all Evans Blue is bound to albumin, normally the neural tissue remains unstained.