It has been reported that ultracentrifugation of plasma will allow direct measurement of free insulin in the serum of diabetic subjects with insulin antibodies. To validate this method, we determined recovery of immunoreactive insulin and immunoglobulin G from the plasma of normal individuals after ultracentrifugation. The upper and middle fractions of plasma after ultracentrifugation were evaluated at several combinations of time and temperature (4°C, 25°C, or 37°C for 3, 4 or 5 hours). None of these conditions effectively removed all immunoglobulin G without causing concomitant loss of insulin. We conclude that ultracentrifugation of plasma prior to radioimmunoassay cannot be used to reliably determine free insulin concentration in the plasma from subjects with circulating insulin antibodies.
- Free insulin
- Insulin antibodies