CPG Sec. 420.500 Interference with Compendial Tests
The recurring question is: What is the legal status of a compendial drug in which an added substance interferes with the compendial assay of the product, even though the product may be fully potent as shown by other methods of analysis?
Section 501(b) of the Federal Food, Drug, and Cosmetic Act states that a drug is deemed to be adulterated if it is recognized in an official compendium and its strength differs from, or its quality or purity falls below the standards set forth in the compendium. Determination as to strength, quality, or purity shall be made in accordance with tests or methods of assay set forth in such compendium.
The USP XX in the section on Added Substances (p. 4) states that suitable substances such as bases, carriers, coatings, colors, flavors, preservatives, stabilizers, vehicles may be added to a pharmacopeial dosage form to enhance its stability, usefulness, or elegance, or to facilitate its preparation. The USP restrictions on the use of such added substances include "if they do not interfere with the assays and tests prescribed for determining compliance with the pharmacopeial standards."
A compendial drug product containing an added substance which interferes with the compendial assay of the product would be adulterated under 501(b) of the Act.