How useful is riboflavin as a tracer of medication compliance?

A quantitative spectrofluorometer assay of urinary riboflavin is described. Using the method, levels produced by prescribed riboflavin are reliably distinguished from normal background levels without requiring dietary restrictions. The method is successfully used to verify the intake of a single 50-mg dose of riboflavin. However, when more frequent dosage is used, verification becomes unreliable because of a "spillover" effect produced by the slow rate of elimination of the substance. This seriously limits the utility of riboflavin as a tracer, as does its tendency to change the appearance of therapeutic compounds into which it is incorporated. The need for further work on validating combined measures of compliance is stressed.