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.
It's also the most commonly abused benzo, ahead of similar drugs like Valium and lorazepam. Combined with a growing awareness of prescription-drug abuse, it's getting more and more likely that Xanax can cause you to fail a drug test. Failing a drug test can have serious consequences for your career.