Method validation

I am analyzng a drug of a brand with a method of an article and try to validate it. The method is using Uv-Vis analysis and the assay of the drug is reported as 109.5%, is this assay ok? how can I find the range of assay change of the drug? The drug is produced 20 months ago.

This indicates, there is some interference of excipient in the drug formulation. Check interference of excipient in method validation. Other reason could be method is not specific to the drug under analysis. Please verify specificity. Other reason could be there is no linearity for the range you have selected to perform assay. Please check linearity.

As I think you definitely performed specificity and this may be due to diluent interference