Radioimmunoassay of insulin in urine.

Abstract

1.Details are given of a double antibody immunoassay method suitable for measuring insulin in urine. — 2. The method proved specific and reproducible. Human insulin added to urine was quantitatively recovered and dilution and concentration resulted in proportional changes in measured insulin. — 3.Urines kept at 4°C for 24 hours showed no loss of assayable insulin, while decreases of 4–23 % were observed after standing at room temperature for 24 hours; samples of urine adjusted to pH 7.4–8.4 and centrifuged were kept at −20°C for 9 months without loss. Addition of albumin to urine to prevent adsorption of insulin on the container was found to be unnecessary. — 4. Tests of the first antibody reaction showed that non-specific co-precipitation of radioactive insulin was less than 3.5 % and that addition of sodium chloride did not alter the standard insulin curve. Monitor tubes containing131I-guinea-pig gamma-globulin showed optimal second antibody precipitation in all the urine assays surveyed. — 5. In normal subjects, the renal clearance of insulin was relatively constant over a wide range of serum insulin levels, but varied with body size. However, preliminary studies suggested that the clearance was reduced in pregnancy, hypertension and peripheral vascular disease, all of which are characterized by high serum insulin values. In contrast, the clearance in acromegaly was normal as both serum and urine insulin levels were raised. In chronic renal disease the urinary excretion of insulin was very high. — 6. Direct measurement of urinary insulin was feasible in insulin-treated diabetics provided that renal function was normal; insulin-binding antibodies were found in the urine of these diabetics only in association with proteinuria.