If your blood magnesium is abnormal, this test is the essential next step to find the why. It measures the amount of magnesium your kidneys are excreting. This helps your doctor determine if the imbalance is caused by your kidneys "wasting" too much magnesium or by an issue with your body's absorption. This test, often a 24-hour collection, is critical for pinpointing the root cause. This test provides the deeper, diagnostic data needed to fix the underlying problem, not just the symptom.