A turbidimetric alternative for analysis of total light chains in urine

Commercial assays quantifying total immunoglobulin light chains in urine are generally intended for nephelometry but are used on turbidimetric analyzers. We evaluated Siemens N Antiserum (N AS) for total kappa chains and total lambda chains on a cobas 502 turbidimeter (Roche Diagnostics). Imprecision was calculated at several levels based on repetitive measurements of controls, patient samples, and calibrator solutions. To approximate functional sensitivity, a precision profile was generated. Bias was estimated using 150 patient urine samples, both on the cobas and a BNII nephelometer (Siemens Healthineers). The comparisons were performed using Passing-Bablok regressions and Bland-Altman plots. Our results showed a CV <10% at concentrations >10 mg/L for both the kappa and lambda assays. A CV of approximately 20% (functional sensitivity) was seen at approximately 5 mg/L for both methods. The cobas and BNII analyzers showed good agreement, although kappa measurements on the cobas gave slightly higher values at low concentrations. For the lambda assay, regression analysis showed no clinically relevant difference. In summary, we found no major difference between Cobas c502 and BNII. Analysis with N AS Kappa and Lambda on Cobas c502 gives clinically acceptable results at concentrations ≥8 mg/L and ≥5 mg/L, respectively.