Is detected igg with lamba in bloodwork of breast cancer patient serious

May be: That means you likely have blood disorder- that is likely completely separate- to the breast cancer diagnosis. You would need to have further evaluation to figure out what kind of blood disorder that is related to the igg lambda. Differential diagnosis will include plasma cell disorder vs other lymphoproliferative disorder.Go to see your hematologist/oncologist and discuss further with your doctor.

MGUS: If your blood count, kidney function and calcium are normal, it probably is a monoclonal gammopathy of undetermined significance. I agree your oncologist should be able to evaluate this.