Carbamylation of albumin is a cause for discrepancies between albumin assays Maarten B. Kok a, ⁎, Frans P.W. Tegelaers a , Bastiaan van Dam b , Jan L.M.L. van Rijn c , Johannes van Pelt a a Medical Center Alkmaar, Laboratory of Clinical Chemistry, Hematology and Immunology, Alkmaar, The Netherlands b Department of Internal Medicine, Alkmaar, The Netherlands c Westfriesgasthuis, Laboratory of Clinical Chemistry, Hoorn, The Netherlands abstract article info Article history: Received 28 January 2014 Received in revised form 31 March 2014 Accepted 31 March 2014 Available online 5 April 2014 Keywords: Bromocresol BCP BCG Hemodialysis Albumin Carbamylation Background: Several investigators have reported discrepancies between the bromocresol-purple (BCP), bromocresol-green (BCG) and immunonephelometric (INP) assays in dialysis patients. This study compared the abovementioned assays and investigated whether hemodialysis itself or carbamylation of albumin is the cause for this discrepancy. Methods: Samples obtained from hemodialysis patients were analyzed by BCP, BCG and INP. Furthermore, albu- min was carbamylated in vitro using isocyanate. Isocyanate converts lysine to homocitrulline. Results: No differences were observed between samples of the pre- and post-hemodialysis groups for BCP. In the control group, BCG averaged 6 g/L higher than INP. BCP did not statistically deviate from INP. In the dialysis group BCG averaged 5 g/L higher when compared to INP, whereas BCP averaged 2 g/L lower. BCP was affected by carbamylation of albumin. BCG and INP measurements were affected to a much lesser extent. Homocitrulline content of hydrolysates was increased in both the carbamylated albumin as well as in the dialysis population. Conclusion: In a hemodialysis population albumin concentrations are not consistently estimated by both BCG and BCP methods. Relative to INP measurements BCG overestimates the albumin concentration (4–10 g/L), whereas BCP leads to an underestimation (0–4 g/L). Carbamylation of albumin is the main attributor to the discrepancy found with BCP. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Analysis of plasma albumin is generally performed using either a bromocresol-green (BCG) or a bromocresol-purple (BCP) based colori- metric method. BCG and BCP are the most frequently utilized methods, since they are cost effective, quick and readily available on a variety of chemistry platforms. Other techniques, such as immunonephelometry (INP) or immunoturbidimetry are used less frequently, since these are more time-consuming and expensive. Main advantage of the use of immunonephelometry is the absence of cross-reactivity with non- albumin proteins [1]. In patients with end-stage kidney disease, albumin is an important predictor of mortality and morbidity. Furthermore, it is required for the interpretation of total calcium levels in patients requiring dialysis in case of hypoalbuminemia [2,3]. Adjustment of total calcium for low albumin levels is advised by the Nephrology Guidelines (KDIGO; [4]) and several correction formulas have been suggested [5,6]. Several publications have reported discrepancies in albumin con- centration between the BCP, BCG and INP assays [1,5,7–13]. In healthy subjects, BCP has been reported to closely agree with INP, whereas BCG reveals a positive bias when compared to BCP or INP, especially at low albumin levels [8]. Carfray et al. demonstrate that BCG has a positive bias up to 10 g/L when compared to either BCP or INP and also show that this discrepancy increases with hypoalbuminemia [14]. Ueno et al. de- scribe that acute phase proteins such as haptoglobin may interfere in the BCG assay [13]. Xu et al. described that serum globulins are the main contributor to the discrepancy between BCG and BCP [7]. Pinell et al. report that immunoglobulins do not affect the BCP assay [15]. Interestingly, several investigators have also reported that differ- ences found between BCP, BCG and INP are more pronounced in a pop- ulation of patients with chronic renal failure requiring hemodialysis [9, 10,16]. A discrepancy of up to 9 g/L has been observed between BCP and BCG [9,10]. Previous studies have hypothesized that the reported in- crease of albumin in plasma of uremic patients could be attributed to ei- ther uremic toxins [17] or possibly to an unidentified chemical modification of albumin, that inhibits the binding of BCP to albumin, but not that of BCG or INP [8,10]. Jaisson et al. and Kraus et al. have Clinica Chimica Acta 434 (2014) 6–10 Abbreviations: BCP, bromocresol purple; BCG, bromocresol green; INP, immuno- nephelometry; CKD, chronic kidney disease; KDIGO, the Kidney Disease Improving Global Outcomes guidelines. ⁎ Corresponding author. E-mail address: maarten.b.kok@gmail.com (M.B. Kok). http://dx.doi.org/10.1016/j.cca.2014.03.035 0009-8981/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Clinica Chimica Acta journal homepage: www.elsevier.com/locate/clinchim