Table 3. Parameters Applicable to Different Analytical Procedures

Physical and Chemical Analyses

The nature of physical and chemical analyses is such that they may be relied upon to be more accurate and reproducible, and therefore easier to validate, than biological analytical methods. Many tests required for evaluation of drug substances and drug products are defined in various pharmacopoeias and national formularies. In addition, ICH has published Q6A and Q6B, which give test procedures and acceptable criteria for new drug substances and products and for biotechnological/biological products, respectively.^11,12 These tests are defined by the FDA as "regulatory analytical procedures" and are generally preferred by the agency. Alternative test methods may be used, but they must be demonstrated to be equivalent to or better than the compendial test in every respect. Thus, non-compendial tests present greater validation problems, as the adoption of an analytical method as a regulatory analytical procedure indicates it can be validated by any competent laboratory.

Validation of physical and chemical analytical methods involves a well-characterized procedure, according to an established SOP, using properly qualified and calibrated instruments. The test should be run a sufficient number of times, using a clearly defined and acceptable reference standard and, if necessary, using different analysts, so that a proper statistical analysis can be performed to determine such things as accuracy and precision, as defined for each type of test. For quantitative measurements, statistics should be able to determine the linearity of the observed response, or to indicate the optimum manipulation to achieve linearity, e.g., log/log plotting. Equally important is the determination of the parallelism between the standard curve and those created by test samples. The statistical methods for this test have been described by Kleinbaum, Kupper, and Muller.¹³

BIOLOGICAL ASSAYS

The form of biological test that most often requires validation is that which provides a quantitative estimate of the biological activity or "potency" of a biological or biopharmaceutical product — an assay. The FDA's "Analytical Procedures Validation" does not provide help in this area, as it merely states, "Biological and/or immunochemical assays should be assessed using the same analytical procedures used to determine product potency. These can include animal-based, cell culture-based, biochemical, or ligand/receptor-binding assays. While these tests may be needed for complete characterization of certain reference standards, specific recommendations for validation of biological and immunochemical tests are not contained in this guidance document."⁸

FDA's later document, "Bioanalytical Method Validation," goes some way toward resolving this problem, although it is specifically designed to assist in the validation of tests used in clinical pharmacology, bioavailability, and bioequivalence studies. The document describes how to validate analyses for quantitative measurement of an analyte in a biological matrix such as blood, plasma, serum, or urine. The critical parameters for this type of validation are, however, similar to those already discussed, namely accuracy, precision, selectivity, sensitivity, reproducibility, and stability. Therefore, biological assays can be approached from the same standpoint as physical or chemical assays, as long as the inherent variability of biological systems is taken into account.⁹