Insulin was the first hormone measured successfully by radioisotope immunoassay, and insulin assay is now available in most sizable reference laboratories. Insulin is excreted primarily through the kidneys. In general, juvenile diabetics have low fasting insulin levels, and an OGTT using insulin determinations usually produces a flat curve. Mild diabetics have normal fasting insulin levels and display an insulin GTT curve that has a delayed rise, either to normal height or to a point moderately above normal; in either case the curve thereafter falls in a normal fashion. Decreased tolerance due to many other causes produces similar curves; an insulin OGTT has not been more efficient in uncovering subclinical diabetes than blood glucose OGTT. Some maintain that the ratio of insulin values to glucose values obtained on the same specimen during the OGTT is more reliable than insulin values alone. At any rate, most investigators believe that, at present, plasma insulin levels should not be used for diagnosis of diabetes mellitus.

Plasma anticoagulated with ethylenediamine tetraacetic acid (EDTA) is reported to produce plasma insulin values equal to serum, but heparin is said to be associated with plasma insulin values greater than serum.

Patients being treated with insulin frequently develop antiinsulin antibodies after approximately 6 weeks. These antibodies interfere with insulin RIA measurement by competing with insulin antibodies used in the test. Whether values will be falsely increased or decreased depends on the method used. Endogenous antibodies do not interfere with tolerance tests, since the quantity of endogenous antibody remains unchanged throughout the test; only the baseline value is affected.