Impact of albumin carbamylation on immunoturbidimetric measurement of urinary albumin

Absolute Quantification of Human Serum Albumin Isoforms by Internal Calibration Based on a Top-Down LC-MS Approach

Well-characterized biomarkers using reliable quantitative methods are essential for the management of various pathologies such as diabetes, kidney, and liver diseases. Human serum albumin (HSA) isoforms are gaining interest as biomarkers of advanced liver pathologies. In view of the structural alterations observed for HSA, insights into its isoforms are required to establish them as reliable biomarkers. Therefore, a robust absolute quantification method seems necessary. In this study, we developed and validated a far more advanced top-down liquid chromatography-mass spectrometry (LC-MS) method for the absolute quantification of HSA isoforms, using myoglobin (Mb) as an internal standard for quantification and for mass recalibration. Two different quantification approaches were investigated based on peak integration from the deconvoluted spectrum and extracted ion chromatogram (XIC). The protein mixture human serum albumin/myoglobin eluted in well-shaped separated peaks. Mb allowed a systematic mass recalibration for every sample, resulting in extremely low mass deviations compared to conventional deconvolution-based methods. In total, eight HSA isoforms of interest were quantified. Specific-isoform calibration curves showing good linearity were obtained by using the deconvoluted peaks. Noticeably, the HSA ionization behavior appeared to be isoform-dependent, suggesting that the use of an enriched isoform solution as a calibration standard for absolute quantification studies of HSA isoforms is necessary. Good repeatability, reproducibility, and accuracy were observed, with better sensitivity for samples with low albumin concentrations compared to routine biochemical assays. With a relatively simple workflow, the application of this method for absolute quantification shows great potential, especially for HSA isoform studies in a clinical context, where a high-throughput method and sensitivity are needed.

Semi-synthetic human albumin isoforms: Production, structure, binding capacities and influence on a routine laboratory test

Human Serum Albumin (HSA) undergoes Post-Translational-Modifications (PTMs) leading to isoforms affecting its oncotic and non-oncotic properties. HSA is comprised of several isoforms whose abundance may vary with pathologies such as diabetes, kidney and liver diseases. Studying their impact separately may help to understand their sources and potential pathogenicity and further their evaluation as biomarkers. The present study examined semi-synthetic HSA isoforms to investigate independently their structure by means of advanced mass spectrometry techniques (LC-TOF-MS and ICP-MS), influence on the HSA binding/antioxidant activities using a binding capacity test, and potential impact on albumin quantification by a routine immunoturbidimetric assay. Applying different chemical reactions to a commercial HSA solution, we obtained different solutions enriched up to 53 % of native HSA, 78 % of acetylated HSA, 71 % of cysteinylated HSA, 94 % of oxidized HSA, 58 % of nitrosylated HSA and 96 % of glycated HSA, respectively. Moreover, the semi-synthetic isoforms showed differently altered binding capacities for a panel of ligands (Cu, Cd, Au, Ds and L-T4). Furthermore, immunoturbidimetry was found to be insensitive to the presence and abundance of the different isoforms. The fully characterized semi synthetic HSA isoforms obtained should be useful to further investigate their pathogenicity and potential roles as biomarkers.

Impact of Albumin Assays in the Diagnosis of Malnutrition in Hemodialysis Patients: A Cohort Study

OBJECTIVE

In hemodialysis (HD) patients, malnutrition should be diagnosed by several assessment tools including a plasma albumin concentration of less than 3.8 g/dL or 3.5 g/dL using bromocresol green or immunonephelometry (IN), respectively. However, albumin measurement is not yet standardized and two alternative methods are also commonly used in laboratories: bromocresol purple (BCP) and immunoturbidimetry (IT). This study aimed to revisit the hypoalbuminemia thresholds for BCP and IT, in HD patients.

METHODS

Plasma albumin was measured by the four analytical methods during the monthly HD nutritional assessment of 103 prospectively included patients.

RESULTS

Significant differences in albumin levels were observed in HD patients depending on the method used. Using BCP or IT with the cut-off at 3.5 g/dL (determined for the general population) we obtained 33% and 9.7% of false hypoalbuminemia in comparison to IN (mean bias of -0.4 g/dL and -0.065 g/dL, respectively). The best hypoalbuminemia threshold for BCP was 3.05 g/dL and 3.4 g/dL for IT. Twenty percent of HD patients were classified as malnourished when albumin was determined by IN. Similar rates were obtained using the new hypoalbuminemia cut-offs for BCP (18.5%) and IT (19.5%).

CONCLUSION

To avoid nutritional misclassification of HD patients, we should adjust hypoalbuminemia thresholds when BCP or IT methods are used in laboratories.