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Standardization of C-Peptide Measurements in Urine by Method Comparison with Isotope-Dilution Mass Spectrometry

¹ Laboratorium voor Analytische Chemie and
³ Laboratoria voor Medische Biochemie en Klinische Analyse, Faculteit Farmaceutische Wetenschappen, Universiteit Gent, Harelbekestraat 72, B-9000 Gent, Belgium

² Klinisch Laboratorium, AZ Middelheim, Lindendreef 1, 2020 Antwerpen, Belgium

^aauthor for correspondence: fax 32-9-264-8198, e-mail linda.thienpont@rug.ac.be

The first 300 words of the full text of this article appear below.

In the past, standardization of measurements of diagnostically important polypeptides and proteins was hampered by the noncommutability of primary standards (1). One way to overcome this problem is to establish a method comparison with a reference measurement procedure. Until now, reference measurement procedures, such as isotope-dilution mass spectrometry (ID-MS), have been scarce. Recent developments in the MS field, however, have made the technique easily applicable to the analysis of polypeptides and proteins (kinetic studies, sequence analysis, and determination of molecular mass and posttranslational modifications). To the best of our knowledge, only two groups have used ID-MS for the quantitative determination of a specific polypeptide/protein. One of these groups described the measurement of apolipoprotein A-I after enzymatic digestion (2), the other described offline ID-liquid chromatography (LC)-MS assays for serum proinsulin, insulin, and C-peptide (3)(4). Neither group, however, examined the potential of ID-MS for standardization of the respective routine test systems (usually, immunoassays).

Here we report on the use of an ID-MS measurement procedure for standardization/recalibration of C-peptide measurements in urine by use of a method-comparison study with split-sample measurements. In view of the model character of the study for future applications, we chose urinary C-peptide over the clinically more important serum C-peptide because of the ease of sample collection and MS measurement. The measurement procedure applies online ID-LC-electrospray tandem MS (ID-LC-MS/MS) and is described in detail elsewhere (5). For calibration, it makes use of a commercial C-peptide preparation with a peptide content of 89% and a purity by HPLC of >99% (according to the manufacturer’s information). This purity was taken into account for calculation of the C-peptide content in the calibrators. The C-peptide preparation was delivered in a vial containing 250 µg of freeze-dried material; a calibration solution was prepared by . . . [Full Text of this Article]

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