Good manufacturing practice would warrant testing bottles from the beginning, middle and end to assure that segregation has not occurred. Such samples should not be composited. In-process testing for suspensions might also include an assay of a sample from the bulk tank. More important, however, may be testing for viscosity. Additional important specifications for suspensions include particle size of the suspended drug, viscosity, pH, and in some cases dissolution.
Viscosity can be important from a processing aspect to minimize segregation.
Additionally, viscosity has also been shown to be associated with bioequivalency. With regard to dissolution, there are at least three products which have dissolution specifications. These products include phenytoin suspension, carbamazepine suspension, and sulfamethoxazole and trimethoprim suspension. Particle size is also important and at this point it would seem that any suspension should have some type of particle size specification. As with other dosage forms, the underlying data to support specifications should be reviewed.