White box testing of PLC ladder logic source code

Is it compulsary to carry out white box testing of GAMP catagory 4 and 5 PLC source code. The code is mostly written in Ladder Logic and ST. (This is automation within the pharmaceutical industry) Any advice would be very helpful.
Yes ! It is perhaps assumed you would do nothing less.

I must admit I did not appreciate the gigantic difference between GAMP category 4 & 5 until now,
however White Box testing is a must.

It was in the food industry not pharma we got as far as client test when we had multiple ethernet crashes.
Despite the comms. software originating from the Senior Controls Engineer it was fundamentally flawed.
Never make assumptions where it matters.
Great, thank you so much for your reply. Yes GAMP catagory 4 does not require a full source code review but catagory 5 does. We bought Siemens PLC's with configurable software which are catagory 4, so I was hoping we didn't need to do white box testing for this code but only for catagory 5 code written from scratch by the programmers on site. I take it the white box testing is for both catagories? Thanks again