Poor agreement between commercial ELISAs for plasma fetuin-A: An effect of protein glycosylation?
Introduction
Human fetuin-A (also known as α2-Heremans–Schmidt glycoprotein) is a 60 kDa glycoprotein which is secreted by the liver into blood plasma [1]. Fetuin-A is a member of the cystatin superfamily of cysteine protease inhibitors and like other Type 3 cystatins has extensive post-translational modifications [2]. Fetuin-A has very complex biology and is purported to function in a diverse array of physiological process. Recent work has focussed on its role as a systemic inhibitor of ectopic mineral deposition, particularly in the setting of chronic kidney disease (CKD) where patients suffer from a heavy burden of early and progressive arterial calcification. Widespread soft-tissue calcification is seen in fetuin-A knockout mice with renal impairment or fed a mineral rich feed [3] and in fetuin-A/apoplipoprotein E-deficient mice, fetuin-A has also been found to protect against atherosclerotic calcification [4].
We recently reported that lower plasma fetuin-A concentrations were a strong and independent predictor of progressive aortic stiffness in cohort with stage 3 and 4 chronic kidney disease (CKD) [5]. However, reports in similar cohorts are not congruent with our results [6], [7]. We sought to explain these discordant data, with the hypothesis that gycosylation of the native protein may be important to the biological action of the protein and that available ELISA assays may differ in their specificity for different glycosylated forms. In the setting of stage 3 and 4 CKD, reported concentrations differ by over 0.5 g/L [5], [6], [8]. In-house methods, based on indirect ELISA and nephelometry have been used extensively by some groups [9], [10], but the majority of publications report the use of one of two commercial ELISA kits manufactured by Biovendor (Modrice, Czech Republic) and Epitope Diagnostics Inc (San Diego, USA). We compared the analytical performance of these two assays, looking at patient and normal plasma and examined the effects of glycosylation of the protein, with reference to potential biological effects.
Section snippets
Study participants
We used 178 lithium heparin plasma samples collected at the baseline visit of patients enrolled in a prospective cohort study of cardiovascular risk in patients with CKD stages 3 and 4. The details of this study have been previously described [5]. In order to evaluate the effect of CKD status, samples were also obtained from a cohort of 78 otherwise unselected patients (50 male; mean age 58 ± 10.4 yr) in whom there was no history of type 2 diabetes mellitus nor biochemical evidence of renal
Results
The analytical characteristics of each ELISA kit are summarised in Table 1. We evaluated the agreement between results in two cohorts: firstly in 178 patients with stage 3 and 4 CKD (Fig. 1A and C) and secondly in a group of 78 patients without biochemical evidence of renal dysfunction or history of type 2 diabetes mellitus but whom were otherwise unselected (Fig. 1B and D). The mean plasma fetuin-A concentration in CKD cohort (Biovendor, 0.236 ± 0.076 g/L; Epitope Diagnostics, 0.526 ± 0.147 g/L) was
Discussion
There is poor agreement between plasma fetuin-A measurements made using different ELISA methods. Overall, the Biovendor assay demonstrated better analytical performance; lower limit of detection, lower imprecision and better recovery of native plasma fetuin-A at the levels tested. The apparent over-recovery of both native and recombinant plasma fetuin-A using the Epitope Diagnostics assays suggests a degree of inaccuracy.
Measurements of plasma fetuin-A concentration in a population of patients
Acknowledgements
We thank Mrs Elizabeth Stainer and Mrs Joanna Lavender for technical assistance with the Grifols Triturus ELISA analyser.
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Serum fetuin-A levels and abdominal aortic calcification in healthy men - The STRAMBO study
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