Immunoradiometric assay measurements of insulin-like growth factor-I (IGF-I): Comparison with radioimmunoassays using native or des (1–3) IGF-I as radioligands

Insulin-like growth-factor-binding proteins (BPs) in serum interfere with the measurement of insulin-like growth factor-I (IGF-I). Various assays have been developed to overcome this interference. We evaluated an immunoradiometric (IRMA) assay and compared it with the radioimmunoassay (RIA) using both native IGF-I and a truncated form of IGF-I [des (1-3) IGF-I] as radioligands. The IRMA was simpler (one step assay) and faster (3 hr incubation) than RIA(s) (overnight incubation). Sera were extracted with acid ethanol (AE) before all three assays. Analysis of serum samples (n = 78) performed by use of the two different radioligands in the RIA assays were highly correlated (r = 0.967, P < 0.0001). Measurements of serum IGF-I by IRMA in same samples were also highly correlated with those of the RIA assays (r = 0.952 for RIA and 0.947 for triGF-I RIA, P < 0.0001 for both). To assess the effect of binding protein-3 (BP-3) levels (after AE extraction) on these assays, BP-3 levels were measured in sera from 36 healthy women. The mean BP-3 level was 3.6 +/- 0.79 (S.D.) mg/L (range 1.3-5.0), and there was no significant difference in IGF-I levels measured by the three assays. Also, BP-3 levels were inversely correlated with IGF-I levels as measured by all three methods (r = 0.73 for IRMA, 0.71 for triGF-I, and 0.75 for IGF-I RIA). To assess the effect of binding protein-1 (BP-1) levels on these assays, IGF-I was also measured by IRMA and trlGF-I RIA in 19 women with advanced breast cancer. Women with breast cancer had significantly higher (P < 0.001) BP-1 levels than age matched healthy women. IGF-I levels measured by IGF-I IRMA were slightly lower than those measured by trlGF-I RIA in breast cancer patients. However, this difference was not statistically significant (P = 0.56). These findings suggest that variations in BP-3 or BP-1 levels after AE extraction have no significant effect in any of these assays. "We conclude that trlGF-I as a radioligand provides no added advantage over the standard IGF-I RIA. We also conclude that the IRMA assay is valid for measuring IGF-I and is faster and more convenient than RIA.