Estimated urinary osmolality based on combined urinalysis parameters: a critical evaluation

What is it about?

Urinary conductivity can be used as a marker for measuring urinary dilution, but is insensitive to uncharged particles and presence of roentgen contrast media. Urinary osmolality can be estimated on urine analyzers using conductivity. We evaluated the performance of conductivity - based osmolality estimations. We also aimed to improve the manufacturer’s algorithm for estimating urinary osmolality, based on standard urinalysis parameters (creatinine, glucose, density). A prediction model to estimate urinary osmolality based on urinalysis parameters was developed. In addition, the influence of roentgen contrast media on osmolality was studied. The within-run and total imprecision for osmolality and conductivity measured ranged from 1.1% to 4.9% and 0.7% to 4.8%, respectively. Multiple regression analysis revealed urinary creatinine, conductivity and relative density to be the strongest predictors to estimate urinary osmolality. A mean difference of 1.3 mOsm/kg between measured and predicted osmolality demonstrated that the predictive performance of our model was favourable. An excellent correlation between density and % contrast media was demonstrated. Urinary osmolality is an important parameter for assessing specimen dilution.

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