It is important to have means of studying the key characteristics of quality of formulations during development and for quality control purposes during production and testing. Obviously performance in patients is a product’s ultimate test. However, laboratory tests to measure physical properties in vitro can give insights into the quality of the product to be used by patients. Formulations are, after all, physical constructs and various aspects of their physical chemistry can inform us about differences between batches of products, and can sometimes explain their behaviour, aberrant or otherwise.

This chapter presents the basics of some in vitro tests that can be applied to pharmaceutical products, using the background physical chemistry discussed in earlier chapters. In particular, we will examine how we can measure the influence on drug release rates or product performance using some of the key parameters of pharmaceutical systems, such as aerosol particle size, viscosity of topical products, adhesion of films and patches and tablet hardness, inter alia. It is necessary to appreciate the importance of in vitro testing in formulation development and in batch-to-batch control of uniformity but also in assessing product defects. In vitro tests might be preferred to in vivo measures when there is a good correlation between the two, but it should be appreciated that such correlations and physiological verisimilitude are not essential for validity in quality control, where reproducibility of product characteristics is itself a goal.

It is hoped that this brief account of quantitative tests – some simple, some complex – will stimulate thought on how in practice we can assess the products we handle.