In research reports, they’re always talking about glucose clamps. Two types of clamps are quite commonly used, but they have nothing to do with the common definition of the word clamp. Instead, they are used to measure either how well you metabolize glucose or how sensitive you are to insulin.
They’re called clamps because your glucose is clamped, or held, at a certain concentration. The hyperglycemic (high blood sugar) clamp is a way to quantify beta-cell response to glucose. The hyperinsulinemic (high insulin) euglycemic (normal blood sugar) clamp is a way to quantify your sensitivity to insulin.
In a hyperglycemic clamp, researchers raise your glucose concentration to 125 mg/dl above your optimal glucose level by a giving you an infusion of glucose. The desired high blood sugar level is maintained by adjusting the glucose infusion that’s going in: They give you just enough to keep you at that high blood glucose level. How much glucose they have to infuse into you to keep your blood sugar at that constant high level is a measure of how fast you’re metabolizing the glucose.
The hyperinsulinemic euglycemic clamp takes about two hours. Your plasma insulin concentration is raised to approximately 100muU/ml by infusing insulin into you through a peripheral vein, and it’s kept at that level by continually infusing insulin. Then you’re given a variable glucose infusion of a twenty percent glucose solution, just enough to keep plasma glucose concentration constant at a normal level.
Blood sugar levels are measured every five minutes, so that enough glucose can be infused to keep blood sugar just right. Under these steady-state conditions of euglycemia, the glucose infusion rate equals the glucose uptake by all the tissues in the body. Basically, the test measures the amount of glucose needed to compensate for the increased insulin levels.
The rate of glucose infusion (called the GIR) during the last thirty minutes of the test determines insulin sensitivity. If high levels of glucose infusion (7.5 mg/min or higher) are required, you are insulin-sensitive. Very low levels (4.0 mg/min or lower) indicate that your body is resistant to insulin. Levels between 4.0 and 7.5 mg/min are not definitive and suggest “impaired glucose tolerance,” an early sign of insulin resistance.