Evaluation of alternative approaches to the reference method for the urine osmolality test

Introduction: urine osmolality, a crucial test for differential diagnosis of hydro-electrolytic disorders, is mainly assessed by measuring the colligative properties of solutes. Many approaches were attempted to assess urine osmolality without this reference technique, mainly based on the use of formulas. In this study the performance of the Sysmex UF-4000/5000 system, which provides an alternative approach for estimating urine osmolality by means of conductivity, has been evaluated. Methods: the Passing-Bablok regression and Bland-Altman plots have been used to compare the results obtained by the reference method and calculated with seven different forlmulas from 122 routine urine samples. Results: none of the formulas demonstrated acceptable performances. Sysmex-calculated osmolality showed no systematic error and a bias to the reference method lower than the minimum acceptable as calculated according to biological variation data. To evaluate a possible effect of glucose on colligative properties, we also used a subset of samples with glucose ≤5.6 mmol/L (100 mg/dL); in these samples, the Sysmex-calculated osmolality and one of the formulas – 1.09 [1.86 Na (mmol/L) + glucose (mmol/L) + urea (mmol/L)] – showed a variation lower than desirable bias. Discussion: the Sysmex UF-4000 analyzer demonstrated a good agreement with the reference technique, but a significant bias was still present. The seven different formulas tested in our investigation showed even lower analytical performances, so that the use of Sysmex analyzer may be preferable in facilities where osmometers are unavailable