Insulin resistance is recognized as a major risk factor for cardiovascular disease. Fasting glucose concentration can be determined easily, but glucose response to an oral glucose challenge appears to be a more sensitive predictor of cardiovascular disease. Because compensatory hyperinsulinemia correlates strongly with insulin resistance, insulin measurements may better identify insulin-resistant patients. Unfortunately, standardization of insulin quantitations among laboratories is not yet reliable, making identification of normal and elevated levels difficult. Tuan and associates used plasma insulin concentrations and estimates of insulin-mediated glucose disposal in 490 patients without diabetes to establish insulin levels that would identify insulin-resistant patients.
Healthy patients 48 years of age, on average, who had normal glucose tolerance and normal insulin-mediated glucose disposal underwent serial glucose and insulin concentration testing after a 75-g oral glucose challenge. These same patients also completed an insulin suppression test with somatostatin, a test used extensively to document insulin resistance. This insulin suppression testing was intended to identify the one third of the population who were most insulin resistant and most likely to develop glucose intolerance and cardiovascular disease.
After correlation of all test results with those patients who were most insulin resistant, the plasma insulin concentration, either fasting or 120 minutes after oral glucose challenge, compared well with impaired fasting glucose (IFG) or impaired glucose tolerance in the diagnosis of participants with insulin resistance. Insulin quantitations had almost the same specificity but a significantly higher sensitivity with plasma insulin concentration. This finding is not surprising because insulin-resistant persons without diabetes maintain normal glucose tolerance by increasing insulin secretion.
The authors conclude that, although measurement of IFG and plasma glucose concentration 120 minutes after glucose challenge can identify patients who are insulin resistant and remains the standard for diagnosis of diabetes, measurement of plasma insulin concentrations is a much more sensitive test of insulin resistance. Because of interlaboratory variations, reporting of insulin concentrations would be more useful in identifying the patients with the most insulin resistance if accompanied by a facility-specific frequency distribution of insulin levels among persons without diabetes.