Diabetes mellitus is caused by the lack of (or insufficient) insulin. This is when the body is unable to handle the standard amounts of sugar in a normal diet. Obesity, Heredity, stress and drugs are also common factors that can lead to diabetes mellitus. The required diagnostic tests are a Fast blood sugar-24hr test, a post-prandial test and a Glucose tolerance test. The treatment requires the correct sugar/insulin intake and medication to prevent its progression. It also includes a change of diet.
On the other hand, Diabetes insipidus is indicated by the lack of the hormone vasopressin or the inability of the kidneys to respond to it. It is not connected with either sugar or the lack of it. The diagnostic tests as well as their treatments too are entirely different. The tests needed to diagnose Diabetes insipidus are a Water deprivation test and a vasopressin test. The most important part of the treatment is to replace vasopressin (usually with its synthetic counterpart desmopressin) also fluid intake and urine must be balanced.