EVALUATION OF VARIABLES INFLUENCING THE MEASUREMENT OF INSULIN-LIKE GROWTH FACTOR-1

Objective: Our aim was to identify the frequency and possible clinical and analytical factors leading to "falsely" elevated insulin-like growth factor-1 (IGF-1). Methods: We performed a retrospective review of patients with IGF-1 concentrations above the reference interval. They were selected from a database of patients who had consecutive IGF-1 measurements between the years 2007-2010. Results: Out of 2,747 unique patients, 117 (4%) were found to have IGF-1 concentrations above the reference interval that were considered false-positives following a review of the clinical data. There was no relationship between the percentage of increase in IGF-1 and age, sex, body mass index (BMI), comorbidities, medication use, gonadal status, baseline growth hormone (GH), or insulin-like growth factor binding protein-3 (IGFBP-3) concentrations. An abnormal IGF-1 concentration led to performance of an oral glucose tolerance test (OGTT) for GH suppression in 21 of 117 patients (18%) and repeat IGF-1 measurements in 52 patients (44%). In 34 of 52 patients (65%) with a median follow-up of 36 months, the subsequent IGF-1 concentration was within the reference interval. Conclusion: Discovery of "falsely" elevated IGF-1 concentrations may lead to unnecessary testing and physician visits. Although the cause of the "falsely" elevated IGF-1 concentrations during the study period remains to be elucidated, it appears they are caused by a combination of high biological variability and method-specific assay performance issues. Clinicians should consider retesting patients with potentially spurious IGF-1 elevations after some time interval before embarking on more extensive investigations.