In the near-term, the wide-spread use of hybrid assays appears to be 'the ne-t big thing' in bioanalysis as evidenced by its inclusion in recent professional publications (Table 1) and discussions at scientific meetings. Consequently, the US FDA has been prompted to issue guidance/specifications/e-pectations for hybrid assays; however, regulatory e-perience with these modalities has been e-tremely scarce to date. Much of the interest in FDA guidance has been for acceptance criteria around accuracy and precision for hybrid assays. Several authors have privately and publicly promulgated the use of the ligand-binding assay (LBA) criteria (20/25%) because of the use of LBA to enrich the samples [1]. The FDA has not stated a position on this issue yet [2], due to the lack of data in regulatory submissions, but it has indicated that those criteria would be based on the data generated over time. However, review of the recent literature (Table 1) tends to indicate overwhelmingly that chromatographic acceptance criteria are easily met. So, with data like these, chromatographic acceptance criteria would seem to be the more reasonable choice. The typical validation parameters in and of themselves are not the issues of greatest interest. This combinatorial approach of a hybrid assay, and the inclusion of a digestion step is much more comple-than the average assay for either a small-molecule drug, measured by LC-MS or therapeutic protein assayed by LBA. Consequently, the FDA has questions about these 'new' steps in the process and how they affect reliability of the assay.
