Hello, Emrald. In the U.S., the clinical lab statutes are written by each state, and most laws appear to limit the request for lab tests to "medical practitioners." Some of these statutes (such as California) even prevent labs from revealing test results to patients -- they can only be released to a med prac. I hate these laws -- they're idiotic and assume we have to be saved from ourselves by the medical community. God forbid we decide on our own to have (and pay for) a medical test!
If you've been shown to be negative for Candida through a reputable serum test and lab, I'd leave it at that. No real need to have this gamma globulin test done unless you have symptoms which are still in need of diagnosis. (Even then, this particular test may or may not be helpful for you since no yeast has been detected in your bloodstream.) It's always possible that one has yeast overgrowth in the gut, but it hasn't passed through a "leaky gut" lining into the bloodstream, leaving the blood clean. A Comprehensive Stool Analysis (link below) from a reputable lab would detect this gut-only yeast overgrowth. But unless you have the strictly gut-related symptoms such as gas, bloating, mucus, constipation, diarrhea, etc. listed in a Candida questionnaire (link below), I wouldn't bother with a $300 CSA. Do you have specific symptoms you're concerned about?