Blood creatinine level. The blood creatinine level shows how well your kidneys are working. A high level may mean your kidneys are not working as they should. The amount of creatinine in the blood depends partly on the amount of muscle tissue you have.
Xanax is stronger. 0.5 mg of Xanax equals 10 mg of Valium. But Xanax is mostly effective for about 2 hours, then you may have to repeat the dose (and regular doses may cause addiction). Valium peak-level effects wear off after 1 hour, but it stays in blood longer, and continues working for about 12 hours.
I agree with psychmajor. Xanax has a short half life and the best benzo would be valium as it has the longest half life. It would be better for you to try another non-addictive med for your anxiety that is not addicting, cause benzo's are hell to get off of.