Health care providers require patients to fast before blood sugar, glucose tolerance and cholesterol tests, according to Mayo Clinic. In addition, individuals must refrain from eating before iron blood tests, Summit Medical Group states.
Doctors use blood sugar and glucose tolerance tests when they suspect patients have diabetes, Mayo Clinic explains. A blood sugar test indicates the amount of sugar still in a person's blood after several hours without food. This is often followed by a glucose tolerance test, which monitors changes in blood sugar levels for two hours after a patient consumes a sugary drink. Though fasting blood tests are common for checking cholesterol levels, research suggests that abstaining from food is not necessarily required. Health care providers use fasting iron blood tests to diagnose anemia, certain autoimmune diseases, cancer or an abnormal accumulation of iron, Summit Medical Says.