The only way to make these significant clinical decisions with confidence is to be sure that before assuming that you are castrate resistant your testosterone levels must be confirmed by either mass spectrometry or an immunoassay method that has been validated at low testosterone levels and interpreted with caution before any changes are made to treatment management.
Germline Mutations As Prognostic Indicators of Castrate Resistance
AR-V7 Liquid Biopsy Coming of Age
Once a man is castrate resistant and moves on a second line hormone therapy drug like Zytiga or Xtandi (aka AR inhibitors) it is inevitable that the Zytiga or Xtandi will also become ineffective.
When this happens, the question that comes is what should be the next treatment? Generally, the options currently available are either to move to the drug not initially used ( Zytiga if Xtandi was first used or Xtandi if Zytiga was used) or instead to use taxane chemotherapy (Taxotere aka docetaxel).