Sample exchange to compare insulin measurements between the San Antonio Heart Study and the Framingham Offspring Study

Lack of assay standardization has precluded cross-study comparison of insul in levels. We exchanged blood samples between the San Antonio Heart and Fra mingham Offspring Studies to compare insulin measurements. Two randomly sel ected specimens were chosen for each non-diabetic man and woman in each of the bottom four quintiles and top two deciles of the originally assayed fas ting and 2-hour post-challenge insulin distributions: 48 plasma samples fro m Framingham. and after further stratification by ethnicity, 96 serum sampl es from San Antonio. Total immunoreactive insulin was originally measured i n both studies; we repeated the identical assay on exchanged samples. Repea t assays were performed a mean (SD) of 7.0 (0.8) years after collection in the Framingham study and 4.6 (1.1) years in the San Antonio study. Repeat i nsulin levels were highly correlated with original levels for both San Anto nio samples repeated in Framingham (Pearson r=0.923) and for Framingham sam ples repeated in San Antonio (r=0.959). Original and repeat San Antonio ser um insulin levels were similar (mean fasting and 2-hour combined original l evel 154 pmol/l vs. 142 pmol/l on repeat in Framingham). Framingham plasma insulin levels repeated in San Antonio were substantially lower than origin al levels (120 pmol/l vs. 336 pmol/l), as were an additional 12 samples rep eat assayed in Framingham (93 pmol/l vs. 320 pmol/l). Repeat rank ordering in both studies was excellent: over 90% of subjects originally classified a s hyperinsulinemic (top tertile of the combined distribution) were again cl assified as hyperinsulinemic upon repeat assay. We conclude that sample exc hange for insulin measurement is simple and feasible. Original and repeat i nsulin levels are highly correlated; subjects originally classified as hype rinsulinemic remain so classified upon repeat assay. Associated regression curves can be used to calibrate insulin levels to a common reference standa rd, allowing epidemiology studies to compare levels of insulin and associat ed risk factors. (C) 2001 Elsevier Science Inc. All rights reserved.