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Abstract Background Icterus is a common endogenous interference that can cause falsely low results for serum creatinine and total protein. It can be defined by elevated unconjugated (indirect) and conjugated (direct) bilirubin levels and is most commonly associated with liver dysfunction. Recent literature suggests that dilutions can be a reliable method to reduce bilirubin interference. The purpose of this study was to determine whether the vendor’s on-board sample dilution can eliminate icteric interference with creatinine and total protein. Methods Creatinine (enzymatic) and total protein interference studies were performed on an automated chemistry analyzer (Roche Diagnostics, cobas c702). Low, mid, and high sample pools for each analyte were divided into four equal volumes. To achieve an icterus index (II) of approximately 40 (approximately 40 mg/dL of bilirubin), one sample pool was spiked with unconjugated bilirubin, and one pool was spiked with conjugated bilirubin. An equal amount of diluent was added into each control pool and remained at an II of approximately 0. The icteric and control samples were measured for conjugated and unconjugated bilirubin, serum indices, and the target analyte. Using the on-board dilution function, the icteric creatinine and total protein pools were diluted using a 1:4 and 1:3 dilution, respectively. All samples were analyzed in triplicate. Percent variance was found by calculating the difference between the average dilution result and control result and then dividing by the average control result. Results Conclusions Only modest alleviation of icteric interference was noted with dilution. The average bias was 10% for creatinine and 24% with total protein, after dilution. The on-board sample dilution is not effective in eliminating icteric interference when measuring both creatinine and total protein.
Zakrajsek et al. (Tue,) studied this question.