Intact Insulin and C-Peptide Levels Measured By Multiplexed Mass Spectrometry: Correlation with Fasting Glucose in Apparently Healthy Volunteers

Program: Abstracts - Orals, Poster Previews, and Posters
Session: SUN 699-739-Insulin Signaling/Insulin Action and Pathopathology of Diabetes (posters)
Basic/Clinical
Sunday, April 3, 2016: 1:15 PM-3:15 PM
Exhibit/Poster Hall (BCEC)

Poster Board SUN 706
Steven W. Taylor1, Dov Shiffman2, James J. Devlin2, Richard E. Reitz1, Nigel J. Clark3 and Michael J. McPhaul*1
1Quest Diagnostics Nichols Institute, San Juan Capistrano, CA, 2Quest Diagnostics, Alameda, CA, 3Quest Diagnostics, San Juan Capistrano, CA
Introduction

Elevated insulin levels have been shown to be associated with increased risk for the development of diabetes mellitus. Although clinical tests for insulin and C-peptide have been available for decades, insulin measurements have not been broadly used in clinical practice, at least in part due to the wide range of available immunoassays and the difficulties in relating results obtained from one assay platform to those obtained from others. This shortcoming has been noted in the literature for both insulin and C-peptide. Therefore, we have developed a multiplexed mass spectrometry–based assay that measures both intact insulin and C-peptide. We investigated the relationship between insulin, C-peptide, and glucose levels in fasting serum samples from a cohort of apparently healthy volunteers.

Methods

Apparently healthy subjects provided informed consent (WIRB # 20121940) and fasting venous blood samples were obtained. Glucose levels were determined using an Olympus AU2700TM chemistry-immuno analyzer (Melville, NY); insulin and C-peptide levels were determined by multiplexed mass-spectrometry assay. Anthropomorphic measurements were obtained at the time of blood collection.

Results

This study included 103 apparently healthy volunteers (46.7% males, median age = 35, median BMI = 26.1). Median insulin level in this population was 8.07 μIU/ml (IQR 5.38 to 12.55); insulin was elevated (≥15 μIU/ml) in 19.4% of the subjects. Insulin levels were elevated in 50% and 40%, respectively, of those with either impaired fasting glucose or fasting glucose between 90 and <100 mg/dL; in contrast, only 9.6% of those with fasting glucose <90 mg/dL had elevated insulin levels. Median insulin levels were 14.78 μIU/ml (IQR 6.44 to 42.29) in the 10 subjects with impaired fasting glucose, 9.79 μIU/ml (8.43 to 17.70) in the 20 subjects with fasting glucose between 90 and <100 mg/dL, and 7.26 μIU/ml (4.49 to 9.47) in the 73 subjects with fasting glucose <90 mg/dL (P=0.0004 for difference between medians). Insulin levels in those with BMI >26 (median = 9.17 μIU/ml, IQR 6.96 to 17.33) were higher than in those with BMI ≤26 (median = 6.92 μIU/ml, IQR 4.08 to 9.09; P=0.0003). Insulin and C-peptide levels were highly correlated (r=0.88).

Discussion

This study demonstrates the application of a multiplexed mass spectrometry–based assay to measure intact insulin and C-peptide, an assay standardized by calibration to the WHO insulin reference material 83/500, and SI traceable. Measurements of C-peptide were carefully quantified with calibrators assigned by quantitative amino acid analysis. Our findings indicate that (1) elevations of insulin are observed in a substantial number of individuals who demonstrate fasting blood glucose that are within the normal range, and (2) the proportion demonstrating elevation of insulin increases progressively at higher glucose levels within the normal range.

Disclosure: SWT: Employee, Quest Diagnostics. DS: Employee, Quest Diagnostics. JJD: Employee, Quest Diagnostics. RER: Employee, Quest Diagnostics. NJC: Employee, Quest Diagnostics. MJM: Medical Director, Quest Diagnostics Inc..

*Please take note of The Endocrine Society's News Embargo Policy at https://www.endocrine.org/news-room/endo-annual-meeting/pr-resources-for-endo