Analytical method specificity is assessed using ICH International Conference on Harmonisation Q2. Although, certain methods aren’t specific enough for their intended purposes, they might have other benefits. The two titrimetric and UV Ultra-violet spectroscopy assays are non-invasive, but have superior accuracy ca. 0.1-0.5percent RSD Residual Standard Deviation compared with the corresponding specific HPLC High Performance Liquid Chromatography assay methods >0.5% RSD. A method’s ability to quickly Identify substantial changes in the real mean of an API Active Pharmaceutical Ingredient is vital for any quality critical evaluation. Hofer et al modelled a situation where for the first 50 batches of an API, the true mean was 99.5% and the standard deviation σ was 0.5%; subsequently the true mean dropped to 99.0%, with unchanged σ. The modelling showed an inability to recognize this substantial change by trending the HPLC assay data. The authors proposed using a mass balance approach 100% –% total impurities demonstrating unequivocally it is relatively simple to detect changes using this approach.
The Analytical methodology variability is often larger than the production process variability, especially for hplc testing manufacture. Generally, the analytical method σ should be ≤one-sixth of this proposed specification scope; i.e., 6σ capability. Ermer4 evaluated the maximum allowable σ for an API assay method. He revealed for an API assay method using replicate determinations, where the lower specification limit is 98.0% and with 0.5% total impurities, the analytical method σ should be ≤0.17%. Remer et al5 utilized 44 unique APIs, and 156 distinct balance studies to ascertain a generic precision value for HPLC assays, i.e., 1.1%6. Likewise, Hofer et al3 reported that the average intermediate precision values were between 0.6-1.1percent and Görög7 indicated variability was about 1 percent.
Based on This normal analytical variability of ca. 1%, and assuming that an API specification ranges of ±2.0% i.e., 98.0-102.0percent, many commentators3,4,5,7 have voiced significant concerns about the usefulness of HPLC assay methods to track API caliber trending API potency, trending API stability, releasing batches whose accurate potency is 98.0-102.0% or meaningfully investigating OOS Out of Specification results. Sarda et al8 suggested that ‘assay results are just not stability-indicating’ due to the inherent assay variability. The effects of method variability on OOS results can also be constrained by the FDA’s 2006 guidance, as individual repeats, in addition to the mean value, should lie within the approval criteria9. Hofer et al3 modelled the probability of finding a ‘false OOS’ and demonstrated that this is highly determined by method variability, in addition to the API accurate mean.