Paraprotein Interference in Automated Chemistry Analyzers

Abstract

Paraproteins can interfere in chemical measurements when they form precipitates during the testing procedure (1)(2)(3)(4)(5)(6)(7)(8)(9). Here we describe ways to identify paraproteins that interfere in methods for bilirubin and HDL on an automated analyzer. The approaches appeared to be effective in identifying these rare cases of interference and did not hinder the autovalidation of appropriately high or low values for either assay. Software analysis and reporting programs currently in use that rely on rigid two-point calculations derived from reaction monitors may be susceptible to errors in analysis. We encountered an artifactually increased total bilirubin concentration and an artifactually low HDL in a patient with a monoclonal IgM paraprotein. The tests were initially performed on plasma samples with the Hitachi 917 automated analyzer (Roche Diagnostics) using the Roche total bilirubin assay and the Roche HDL-C Plus assay. For this patient, the reported total bilirubin was 318 mg/L, direct bilirubin was 2 mg/L, total protein was 127 g/L, and HDL was undetectable. Serum protein electrophoresis revealed a monoclonal IgMκ M component at a concentration of 66 g/L. Similar results were seen on subsequent samples (both in plasma and serum), with a high total bilirubin and undetectable HDL. On a previous admission 2 years before, her total bilirubin concentration had been within the reference interval at 4 mg/L, and her HDL was measured as 390 mg/L by the same assay systems. The Roche total bilirubin assay is a liquid, end-point, chromogenic assay based on a modification of the diazo method (10). The assay uses detergent as the accelerator and to help avoid protein precipitation. After solubilization with this reagent (reagent 1), the diazo reagent (reagent 2) is added to the cuvette to produce a pink color change …