Determination of free-insulin in antibody containing sera: comparison of polyethylene glycol and Staphylococcus aureus cells.

Polyethylene glycol (PEG) and killed Staphylococcus aureus cells (S. aureus) were used as agents to separate free insulin from antibody-bound insulin in diabetic sera. Insulin was determined by conventional double antibody radioimmunoassay. The free insulin values after PEG treatment were almost half of those after S. aureus treatment. The free insulin levels in high-antibody containing sera preincubated at 37 degrees C, 2 h were double the value of fresh sera. PEG treatment caused about 40% loss of total serum protein. The addition of bovine serum albumin (BSA) to the PEG-treated serum greatly increased the immuno-reactive insulin values. This may suggest that protein concentration plays a role in insulin radioimmunoassay. PEG treatment may also enhance the interaction between free insulin and free antibodies resulting in underestimation of free insulin level.