An improved spectrophotometric procedure for the determination of urinary metanephrines.

Spectrophotometric assays for urinary metanephrines are still widely used because of the simple equipment needed. These methods, however, have the major drawback of being very susceptible to interference. Using the original Pisano technique (Pisano, J. J. (1960) Clin. Chim. Acta 5, 406-414) we have often observed overestimation of the metanephrine concentration. By measuring absorbances at three different wavelengths we were able to considerably reduce this positive bias. With many patients this resulted in a significant downward adjustment of the values found for the total metanephrine excretion.