Test Explanation
Insulin regulates blood glucose levels by facilitating the movement of glucose out of the bloodstream and into the cells. Insulin secretion is primarily reactive to the blood glucose level. Normally, as the blood glucose level increases, the insulin level also increases; as the glucose level decreases, insulin release stops.
Some investigators believe that measuring the ratio of the blood sugar and insulin on the same specimen obtained during the oral glucose tolerance (GT) test is more reliable than measuring insulin levels alone. Combined with the oral GT test, the insulin assay can show characteristic curves. For example, patients with juvenile diabetes have low fasting insulin levels and display flat GT insulin curves, because there is little or no increase in insulin levels. Patients who are mildly diabetic have normal fasting insulin levels and display GT curves with a delayed rise.
Type 2 diabetes (adult onset) is characterized by an excess of insulin production in response to GT testing. This hyperresponse of insulin may precede hyperglycemia by many years, allowing the patient time and opportunity to take action to reduce the incidence of outright diabetes through diet management and lifestyle changes.
When combined with a fasting blood sugar, insulin assay is very accurate in detecting insulinoma. After the patient fasts for 12 to 14 hours, the insulin/glucose ratio should be less than 0.3. Patients with insulinoma have ratios greater than this. To increase the sensitivity and specificity of these combined tests for insulinoma, Turner and others have proposed the “amended” insulin/glucose ratios using variable mathematic “fudge” factors:
A Turner amended ratio greater than 50 suggests insulinoma.