Nonspecific interferences in the fluorometric determination of plasma cortisol.

The popular Mattingly [J. Gun. Pathol. 15, 374 (1962)] fluorometric assay for plasma cortisol is shown to give rise to falsely high values when performed with the recommended optical arrangement. By exciting at 470 nm, however, results are obtained that correlate well (r = 0.972) with those of competitive protein binding. The source of the interference is not identified, but it is related to both the dichloromethane used for extraction and the nature of the individual plasma sample.