The fasting insulin test measures the amount of insulin in your blood after a period of not eating. Insulin is a hormone made by the pancreas that helps move sugar (glucose) from the blood into cells for energy. Measuring insulin while fasting gives information about how the body produces and responds to this hormone. This test is used to help evaluate how the body is producing insulin and how well cells respond to it. It is often ordered along with a fasting glucose test to help assess conditions such as insulin resistance, prediabetes, and certain causes of low or high blood sugar. It may also be used to help investigate metabolic concerns.