Vincent M. Brandenburg, MD; Benjamin D. Parker, MD; Joachim H. Ix, MD, MAS
Potential Conflicts of Interest: None disclosed.
Brandenburg V., Parker B., Ix J.; Important Differences in Measurement of Fetuin-A. Ann Intern Med. 2010;153:419-420. doi: 10.7326/0003-4819-153-6-201009210-00017
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Published: Ann Intern Med. 2010;153(6):419-420.
We appreciate Drs. Smith and Holt's insightful comments about the importance of assay characteristics and agree that associations between particular measurements and outcome might be influenced by the assay. Moreover, test specificity and reliability might depend on factors present in the participants and specimens being evaluated.
We used a fetuin-A assay developed by our group in Aachen, Germany, which is not commercially available. This assay has been described in greater detail in our prior articles (1) and was used initially to show the associations of fetuin-A with mortality in patients with end-stage renal disease (2). We evaluated the nephelometric method for low fetuin-A serum measurement in a side-by-side comparison with immunoblot analysis to exclude cross-reactivity of the antibodies with other serum proteins and proteolytic fragments of fetuin-A. We calculated final serum fetuin-A concentrations by regression analysis of a serial dilution curve obtained from standard serum. For comparison and reliability testing, we prepared a control solution of purified serum fetuin-A powder (Boehringer Mannheim, Mannheim, Germany, and Dade-Behring, Marburg, Germany). For both methods, we used the polyclonal rabbit antihuman fetuin-A antibody that does not cross-react with fetuin-B.
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