My U.S. manufacturing location purchases a USP pharmaceutical raw material (which has already been certified as meeting all USP requirements by its manufacturer's QC) and uses that to make a finished product for which that raw material is the active material. The QC assay for the level of that active in the finished product is a fully-validated HPLC test methodology. My question is: for the incoming USP raw material, don't I just have to do QC testing to show it meets my company specifications (including my company's test for % assay) or do I have to purchase instrumentation to set up and perform the % assay test by the USP procedure?

I think the answer to your question depends on your vendor qualification program. I know that for some of our raw mat vendors, we accept their material with only a vendor Certificate of Analysis (C of A). These vendors are of "certified" status, based on a long and successful track record of in-house, full monograph USP testing. Other vendors are "qualified" and we accept their material based on C of A and in-house ID and assay. Eventually, a qualified vendor may attain "certfied" status. Still other vendors are "approved". Their material may only be released after full USP testing. I hope this addresses your question. So based on your question as I understand it, you need to look up the appropriate monograph and perform the full USP testing. If you don't have a vendor qualification program, start one. It will go a long way towards ensuring that quality is manufactured into your product.

Basically the trust that you have on your vendor and based on your past material history and with a good vendor qualification policy and guideline, a reduced testing can be designed with a proper specification

Is there a limit for Y-intercept when conducting linearity from 70% to 140%?