Serum IGF-1 levels measured by different ELISA assays and IRMA – a comparability study

Background: While techniques for measuring IGF-1 have evolved over the decades (Blum 1998), immunoassays are still the primary tool used in routine laboratories. ELISA, IRMA and Chemiluminiscence IGF-1 kits are today on the market and their calibrators, specifity, sensitivity, cross-reactivity and prices differ substantially. Comparability studies are therefore valuable, especially if IGF-1 results obtained by one method should be interpreted retrospectively after transfer of patients from the pediatric to the adult endocrinologists. Aim : To determine which of the used ELISA assays is closest to the IRMA IGF-1 assay and responds best to the Consensus Statement IGF-1 requirements from 2011. Material and methods: 24 patients aged 37.9±14.4 years, median 35; 12 females (42.25± 16.6, median 38.5); 12 males (33.58±10.9, median 34.5) years. The IGF-1 serum levels were measured by three different assays: Immunoradiometric (IRMA), IGFBP-blocked Immunoenzymetric (ELISA1) and Immunoenzymetric with acid-ethanol extraction (ELISA2). The results were interpreted according to the kit referent values adjusted for age and gender. Results: A very high positive correlation between the three assays (r=0.986 to 0.990) (p<0.01) was found. A significant difference was established only regarding the mean IGF-1 values measured by IRMA vs ELISA2 (р<0.05). The lowest IGF-1 mean values were measured by IRMA. Conclusions: Closest to the IRMA method were IGF1 levels measured by the indirect “sandwich” ELISA1 (IGFBP blocked) assay. Due to the significant difference between IRMA and the direct ELISA2 method, diagnostic pitfalls and therapeutic misinterpretations are possible. Priority has to be given to IGF-1 assays which best correspond to the Consensus Statement requirements from 2011. In this study this is fulfilled for the ELISA1, IGF blocked immunoassay. Future developments lie in fully automated assay systems, adapted to pediatric endocrine laboratories.