A prompt, simple method for determining the protein-unbound and total salicylic acid concentrations in human, beagle and mouse blood samples by means of the measurement of the fluorescence from salicylic acid was studied. Both protein-unbound and total salicylic acid concentrations were accurately determined with a ca. 50μl of sample, serum or plasma : Protein-unbound salicylic acid was separated from protein-bound salicylic acid by the use of a ultrafiltration membrane (Centriflow[○!R] CF25) ; 15μl of the filtrate or untreated blood sample was added to Tris-HCl buffer ; and the fluorescence intensity of salicylic acid measured at 300 nm (excitation) and 400 nm (emission). It was found that the metabolites of salicylic acid affect hardly the measurement of salicylic acid with this method.
