question on erbitux and KRAS gene test?

my mom started on erbitux 2 weeks ago. she was on avastin/oxaliplatin/leucoverin, but her CEA level started to rise so her doc changed it to erbitux since she says the body eventually gets used to the chemo and stops responding. my only question is: should she have had a KRAS gene test BEFOREHAND to see if she will respond to erbitux?  she didnt. she did start to get acne on her nose (which is a normal...and good?...reaction i think) so can i assume that the chemo is working because she had a reaction? thanks, ive learned so much from this board!!!! best of luck and God Bless you all