If the test is simply a fasting insulin test, it is primarily to determine insulin levels in the fasting state as well as c-peptide levels. This allows a more accurate determination of whether type 1 diabetes is insulin depleted, as well as the degree of insulin resistance based on the ratio of blood glucose to insulin.
Fast insulin, also called basal insulin, is an indicator of insulin function in the fasting state, where the pancreas also secretes insulin slowly and continuously to maintain blood glucose within a relatively safe range. If fasting insulin levels fall, it means there is an absolute insulin deficiency, and such patients may need to use injectable insulin to control their own blood sugar.