The association between IGF-I/IGFBP-3 and subclinical end points: epidemiology faces the limits.

BACKGROUND The variation in laboratory measurements represents a challenge in clinical practice and epidemiological research. The use of different analytical platforms might have led to different results, which were often discussed in the interpretation of conflicting results. We aim to study the impact of two different IGF-I and IGF binding protein 3 assays on published epidemiological studies. METHODS We compared epidemiological results based on the previous gold standard Nichols Advantage, which is no longer available, with these based on the IDS-iSYS assay. The latter follows the recently proposed Keswick criteria. We reinvestigated published association studies between IGF-I or IGF binding protein 3 and anthropometry, subclinical cardiovascular diseases including intima-media thickness or left ventricular mass index, and hard end points like mortality and single-nucleotide polymorphisms of our genome-wide association study in the Study of Health in Pomerania. RESULTS We demonstrated that there are significant differences in the associations of IGF-I measured by the Nichols or IDS-iSYS assay and subclinical outcomes including intima-media thickness and left ventricular mass index. However, concerning hard outcomes like mortality or single-nucleotide polymorphisms, our analyses revealed similar results with comparable regression estimates. CONCLUSION With our study we queried not only the accuracy of measurement but also the effect of different methods on study results. The establishment of laboratory standards like the Keswick criteria should be enforced to allow reliable comparisons of different methods and thus clinical and epidemiological studies. Single-center studies have to be interpreted carefully. Moreover, to assure the reliability of studies, their results should be replicated in a meta-analysis, and a generated hypothesis by epidemiology should be proven by intervention studies.