Removal of interference by erythromycin, phenazopyridine, and methenamine mandelate in the Porter-Silber reaction.

Erythromycin, phenazopyridine, methenamine mandelate, acetazolamide, and ascorbic acid reportedly interfere with the Porter-Silber reaction in the determination of urinary 17-hydroxycorticosteroids. Sodium bisulfite, added to the urine after hydrolysis with glucuronidase, removed almost all nonsteroidal interferences in urine from healthy adults medicated with the first three drugs, but such studies proved that acetazolamide and ascorbic acid do not interfere with the Porter--Silber reaction, whether or not sodium bisulfite is added.