CVM establishes assay limits for drugs in medicated feeds (i.e., Type B and Type C medicated feeds) as part of the approval process for Type A medicated articles. The term “assay limits” refers to how much the measured amount of drug can deviate from the labeled amount of drug. The assay limits for Type B and Type C medicated feeds are specified as a range and published in 21 CFR §558.4(d). When a medicated feed assay value falls within this range using a validated method accepted by CVM, it indicates that the measured drug level in medicated feed is consistent with the labeled drug level. The assay limits are not intended to permit a feed manufacturer to add more or less than the labeled amount of drug to the medicated feed. Assay limits are used pre-approval to ensure that medicated feeds in Target Animal Safety (TAS), Effectiveness (EFF), Chemistry, Manufacturing, and Controls (CMC), Bioequivalence (BE), and Human Food Safety (HFS) residue chemistry studies contain the appropriate amount of drug, and post-approval for compliance and customer service purposes.

This guidance recommends a standardized set of assay limits for medicated feeds. Standardized medicated feed assay limits allow predictability in the review process as sponsors can determine early in the drug development process what assay limits they should expect to meet for medicated feeds used in TAS, EFF, CMC, and HFS residue chemistry studies. 

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