Stability of insulin-like growth factor (IGF)-I and IGF binding protein (IGFBP)-3 measured by the IMMULITE automated chemiluminescence assay system in different blood specimens.

Insulin-like growth factor I (IGF-I) and IGF binding protein 3 (IGFBP-3) are measured to diagnose disorders of the somatotropic axis in children and adults. In clinical studies samples for IGF-I and IGFBP-3 measurement must often be stored for months and sent to specialized laboratories. Therefore, we tested the stability of IGF-I and IGFBP-3 in whole blood, serum and plasma from 12 volunteers at 4 degrees, 22 degrees, and 37 degrees C for several hours and at -25 degrees C for several months. The effect of only one protease inhibitor (Aprotinin = Trasylol, Bayer, Germany ) on IGF-I and IGFBP-3 measured in the automated IMMULITE assay system (DPC, Los Angeles) was tested. IGF-I and IGFBP-3 were stable in heparinized whole blood, plasma and serum at 22 degrees C up to 24 hours. IGF-I was stabilized by aprotinin for up to 72 hours at 37 degrees C. Factor concentrations were not altered after storage at -25 degrees C for at least 12 months. Recognition of IGFBP-3 fragments by the antibody used in the automated IGFBP-3 IMMULITE was excluded by measurement in 26 sera from pregnant women which usually contain IGFBP-3 fragments. In conclusion, samples for measurement of IGF-I and IGFBP-3 should be kept on ice and cooled if shipment takes more than 48 hours or alternatively 5000 IU/ml aprotinin should be added. IMMULITE assays are also valid to measure IGF-I and IGFBP-3 after at least 12 months storage at -25 degrees C.