Radioimmunoassay of Insulin in Serum and Plasma

Paired serum and plasma samples from subjects were analyzed to determine if there was a difference in the insulin content of serum and plasma. Although there was no difference in the serum and plasma insulin concentrations in the samples as a group, there were differences in the serum and plasma insulin concentrations in individual subjects. Recovery of radioimmunoassayable insulin from clotted or heparinized blood is less than the recovery from serum or plasma, apparently because of association of insulin with the blood cells. Serum and plasma insulin concentrations are not altered when blood stands at room temperature for as long as 4 h. Storage at -20 °C for 28 months results in similar decreases in the insulin content of serum (74%) and plasma (76%). We conclude that one should consistently use either serum or plasma for insulin measurements in individual patients.