If the single entity combination product is not intended to mechanically infuse the drug into the patient but instead the drug is intended to passively diffuse into the patient, then the rate of drug release or elution needs to be determined. To perform this experiment, the single entirty combination product is placed in an appropriate media designed to model the target tissue. The analytical method is then used to measure the increase in drug concentration in the media over time.  The instrument conditions used are usually similar to those used for the assay; however, more sensitive methods may need to be developed if the release of drug is expected to be slow.  The method will need to be validated similar to an assay method with special consideration to sensitivity and additional ruggedness testing for the sample preparation factors that might impact the release rate.