A Practical Derivation of the Uncertainty Factor Applied to Adjust the Extractables/Leachables Analytical Evaluation Threshold (AET) for Response Factor Variation

The analytical evaluation threshold (AET) establishes which chromatographic peaks, produced during organic extractables/leachables (E&L) screening, require toxicological safety risk assessment because the peaks are associated with compounds of potentially unacceptable toxicity. Thus, the AET protects patient safety as its proper application ensures that all potentially unsafe E&L are necessarily assessed. Generally, application of the AET involves the presumption that all organic E&L have the same detector response factor, an assumption that is not valid for any of the detection methods commonly used in E&L screening. Thus, the AET's ability to be protective is compromised for poorly responding compounds, as they will appear to be below the AET when in fact they are not. This unacceptable outcome is addressed by adjusting the AET with an uncertainty factor (UF) whose value is dictated by the magnitude of response factor variation, with a larger variation resulting in a larger UF and a lower adjusted AET. Although the concept of the UF is straightforward, setting a generally accepted, scientifically valid, and practical value for the UF has been challenging. In this article, a database of relative response factors obtained for nearly 1200 E&L via the most commonly applied chromatographic screening methods (gas chromatography/mass spectrometry [GC/MS], liquid chromatography/mass spectrometry with atmospheric pressure chemical ionization [LC/MS-APCI], and LC/MS with electrospray ionization [LC/MS-ESI]) is used to justify UFs for these methods, individually and as a combined practice, based on the practical principle of "the point of diminishing returns". Using this concept results in nearly 92% of the compounds in the database being properly flagged as above an AET adjusted with a UF = 3. Ninety-five percent (95%) coverage of the compounds can be achieved when a UF of 4 is applied to the combination of GC/MS and LC/MS methods or with other combinations of UF values applied to the various methods individually. Coverage is increased to 97% when a UF of 4 is individually applied to the GC/MS method and a UF of 10 is individually applied to the LC/MS methods. Furthermore, the available data suggest that application of both APCI and ESI ionization in LC/MS screening (as opposed to either method separately) provides the greatest coverage of E&L.