Suitability Testing is integral to any Microbiological test system. Suitability testing demonstrates that the test system and methodology are suitable for the intended purpose and compatible with the product or material of interest. Because of the many variables in technique, personnel, equipment, media, reagent, environment, procedures, and supplies, suitability is expected to be performed in every laboratory. It is analogous to analytical method verification, as the general method (when following a compendial or other reference method) is validated, but certain parameters may be changed or optimized to a specific test article. The analytical validation parameters incorporated in microbiological suitability testing include accuracy/precision and specificity. Accuracy (and precision) is covered by inoculating (or spiking) the sample solution with known quantities of microorganisms of interest, and comparing recovery in the sample solution to the inoculum in the absence of the product. Typically this control will also incorporate the diluent used, to ensure no interference of the media or additives, although this is often taken care of by a growth promotion test carried out prior to use.

To spike a sample solution for enumeration tests, there should be controls in place to ensure the appropriate final concentration of microorganisms. This is most commonly achieved by a turbidimetric or spectrophotometric reading of a concentrated harvest solution, when culture control cell banking and propagation techniques are employed. This may also be achieved by ready-to-use suspensions, which typically employ a validated system wherein a lyophilized pellet of known purity and concentration is reconstituted in a neutral diluent at the time of use. In either case, the actual concentration is verified It is important to challenge the method during suitability testing by inoculating with a low level of microorganisms (10-100 CFU/mL sample solution for enumeration tests, and <100 CFU for enrichment tests) to ensure sufficient recovery is achievable in the worst case scenario, very low levels. This is the worst case situation because very low levels are closer to the testing sensitivity and are more susceptible to demonstrable loss of viability in biocidal and biostatic solutions. Inhibitory solutions such as these will be covered in more detail in Part 3 of this series.

Inoculation of the solution should also not significantly change the concentration of sample and nutrients in solution. USP requires that inoculation volume not exceed 1% of the volume of sample solution.

Once the sample solution is inoculated as specified by the method, the remainder of the test is carried out per the procedure, exactly as the actual test will take place, with one notable exception. Where most microbiological culture-based methods provide a time window for incubation, suitability must be performed at or below the minimum time allowed for routine analysis. (For those of you unfamiliar with this language, routine analysis is industry speak for regular testing. Synonyms include release testing and release analysis, antonyms include suitability testing, validation testing, verification testing, etc.) Using the shortest possible incubation time ensures adequate recovery will be achieved and observable during the testing window (it is assumed that microbial growth will remain static or continue to proliferate with time, when these timeframes are used).

Successful repetition of the suitability test for 3 representative lots of a product is often considered the industry standard for validation. Suitability and/or validation should be repeated when formulation, manufacturing, supplier, test lab, or other significant process parameters change for a material or product.