Clinically relevant urine creatinine underestimation in the low concentration range on the Siemens Dimension Vista®

While considerable efforts have been accomplished to standardize the measurement of plasma creatinine (PCr), urine creatinine (UCr) has not been subject to the same scrutiny. UCr is importantly used when measuring biomarkers in spot urines, to assess urine output and variable dilution of urine samples. Here, we report underestimation of Jaffe UCr measurements on the Siemens Dimension Vista® analyzer, critically affecting samples with UCr ≤2 mmol/L. We demonstrate that this error is caused by automatic urine pre-dilution by the Vista's «urine mode», and that UCr measured in «plasma mode» without pre-dilution does not present this error. In the absence of a comprehensive solution proposed by Siemens, we propose simple formulae that can be easily implemented in a laboratory to correct these low UCr measurements. Importantly, the observed UCr underestimation can significantly influence reported results for biomarkers/UCr ratios measured in spot urine. Indeed, these results can be overestimated up to +84.4 % before correction using our formulae. This can sometimes lead to misclassification according to clinical thresholds, e.g. Kidney disease: improving global outcomes (KDIGO) guidelines for urine albumin/creatinine. This highlights the need for every clinical laboratory to assess the detection limits of their assays, including for lesser-discussed parameters such as UCr. Indeed, the error we reported here may affect other urine assays performing systematic urine pre-dilution and could have significant repercussions on the clinical management of patients.