Assay and content uniformity (CU) are two tests generally required for establishing quality of a pharmaceutical product. In one case (assay), the testing is done by pooling the content of multiple units (tablets/capsules) together while in the second case, CU, evaluation of the units is done individually. The evaluation procedure usually remains the same, or almost the same, that is grinding the product, dissolving/extracting the drug using an appropriate solvent and then assaying it using an analytical technique e.g. chromatographic/spectroscopic.
The drug dissolution testing is based on the extraction (procedure) step, perhaps simpler as it does not require grinding of the product. Another advantage of using dissolution tests for extraction purposes is that the choice of a solvent is simple (water, with or without a solubilizer) which is physiologically relevant as well. Thus, one can evaluate CU and assay while conducting a dissolution test.
The question is why has not this simple approach been adopted or commonly used currently. The reason is that currently used dissolution methods, based on Paddle and Basket apparatuses, do not provide accurate and reproducible results. A more appropriate and reproducible apparatus can certainly provide such simplicity and efficiency. For further detail on this topic please see, The Open Drug Delivery Journal, 2008, 2, 33-37, (link) and AAPS Poster presentation (link).
