Effects of insulin-free plasma on the charcoal-separation method for radioimmunoassay of insulin.

Radioimmunoassay of insulin in rat plasma using a popular method (Albano, Ekins, Maritz and Turner 1972) involving charcoal-separation of free and antibody-bound insulin was found to be unsatisfactory despite inclusion in standard tubes of insulin-free plasma prepared in either of two ways. Insulin-free plasma and untreated plasma had different effects on adsorption of free insulin to the charcoal. It was concluded that separation with charcoal is very sensitive to any prior treatment of the plasma. Particular care must be taken to ensure that hormone-free plasma is identical in all other respects to untreated plasma.