Insulin resistance describes the body's lack of sensitivity to the hormone insulin, meaning body cells such as the muscle, fat and liver cells are not adequately stimulated to take up glucose from the blood, even when insulin levels are high. This under-utilization of blood glucose results in hyperglycemia or a raised blood sugar level. Tests for diagnosing insulin resistance include:
Fasting blood sugar and postprandial blood sugar - Blood sugar is almost always raised in people with insulin resistance.
Fasting insulin assessment - In a healthy person who has fasted for 6 to 8 hours (usually overnight), the insulin level is approximately 60 pmol/L. A level higher than this is considered indicative of insulin resistance.
Glucose tolerance testing (GTT) - For a glucose tolerance test, a person fasts for 8 to 12 hours (usually overnight) and is then given a 75 gram oral dose of glucose. After two hours, the blood levels of glucose are measured.
In a healthy person, the blood sugar level after two hours is usually less than 7.8 mmol/L (140 mg/dl). A blood sugar level between 7.8 and 11.0 mmol/dl (140 to 197 mg/dl), however, indicates impaired glucose tolerance. If the level is over 11.1 mmol/dl (200 mg/dl), diabetes mellitus is diagnosed.
Modified insulin suppression test - For this test, patients are given 25 mcg of octreotide (an inhibitor of insulin and glucagon) over 3 to 5 minutes and are then infused with somatostatin (0.27 μgm/m2/min) to suppress the release of insulin and glucose into the blood.
Next, the patient is administered insulin and 20% glucose and a series of blood sugar tests are then performed and the values averaged out to give the steady-state plasma glucose (SSPG) level. An SSPG greater than 150 mg/dl indicates insulin resistance.
Other measures of insulin resistance include the homeostatic model assessment (HOMA) and quantitative insulin sensitivity check index (QUICKI).
Reviewed by Sally Robertson, BSc