The Introduction of the IDS-iSYS Total IGF-1 Assay May Have Far-Reaching Consequences for Diagnosis and Treatment of GH Deficiency

Context: IGF-1 measurements are used for screening and monitoring GH deficiency (GHD) and acromegaly. Objective: Our objective was to study whether the introduction of the IDS-iSYS IGF-1 assay would lead to different clinical interpretations in GHD and acromegaly. Design: In 106 GHD subjects and in 15 acromegalic subjects visiting our university hospital, total IGF-1 levels were measured before and during therapy by using the Immulite (IM) assay and IDS-iSYS (ID) assay. Z-scores were calculated by using assay-specific age-specific normative range values. All treatment decisions were based upon results obtained by the IM assay. Results: In GHD subjects, absolute IGF-1 concentrations differed significantly between both IGF-1 assays before treatment (P < .001) but not during GH treatment (P = .32), and mean Z-scores for IGF-1 differed significantly before starting (IM, −2.23, vs ID, −1.43; P < .001) and during GH treatment (IM, −0.60, vs ID, +0.21; P < .001). In acromegalic subjects, absolute IGF-1 concentra...