The effects of a freeze-thaw cycle and pre-analytical storage temperature on the stability of insulin-like growth factor-I and pro-collagen type III N-terminal propeptide concentrations: Implications for the detection of growth hormone misuse in athletes

Abstract

A method based on two serum biomarkers - insulin-like growth factor-I (IGF-I) and pro-collagen type III N-terminal propeptide (P-III-NP) - has been devised to detect growth hormone (GH) misuse. The aims of this study were to determine the stability of IGF-I and P-III-NP concentrations in serum stored at -20°C and to establish the effects of one freeze-thaw cycle. Blood was collected from 20 healthy volunteers. Serum aliquots were analyzed after storage for one day at 4°C and one day, one week, five weeks, and three months at -20°C. IGF-I and P-III-NP results were combined to calculate a GH-2000 discriminant function score for each volunteer. Inter-assay precision was determined by analysing one quality control sample at each time-point. A single freeze-thaw cycle, storage of serum at 4°C for one day and at -20°C for up to three months had no significant effect on IGF-I or P-III-NP concentration. Intra-sample variability for IGF-I was 6.8% (Immunotech assay) and 12.9% (DSL assay). Intra-sample variability for P-III-NP was 10.9% (Cisbio assay) and 13.7% (Orion assay). When IGF-I and P-III-NP results were combined, intra-sample variability of the GH-2000 score expressed as a standard deviation varied between 0.31 and 0.50 depending on the assay combination used. Variability in IGF-I and P--III-NP results of stored samples is largely determined by the characteristics of the assays. A single freeze-thaw cycle, storage of serum at 4°C for one day or at -20°C for up to 3 months does not result in a significant change in GH-2000 score.