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It depends on when someone is suspected of having DI, how they are diagnosed. There are a couple of tests available that I know of:
1. Water deprivation test-they withold water and fluids and then test the concentration of your urine and measure your urine output. This helps them determine how your body processes anti diuretic hormone (ADH).
2. MRI with dye injection-they use the images from the MRI to determine how much ADH you are making or lacking. I THINK this test also helps determine whether or not the patient suffers from Central Diabetes Insipidus (CDI) or Nephrogenic Diabetes Insipidus (NDI).