New concepts for bringing urine biochemistry back to clinical practice in the intensive care unit

In the last few years, great attention has been given to the composition of the fluids that are administered to critically ill patients.(1,2) In particular, the amount of Na+ and Clis of paramount relevance because these two electrolytes are major determinants of the strong ion difference (SID) in the blood, which is the most important variable in the regulation of metabolic acid-base homeostasis, according to the quantitative physicochemical approach.(3,4) The concern about fluid electrolyte composition is justifiable because iatrogenic, low-SID acidosis may be the result of administration of high volumes of unbalanced solutions, such as normal saline.(2) In an attempt to minimize this problem, certain balanced solutions were developed, with the advantage of being more neutral in terms of acid-base equilibrium.(1) It is known, however, that many other variables have a direct influence on the final concentrations of these electrolytes in the blood. One of these variables is certainly the concentrations of these same electrolytes in the urine, the main fluid responsible for the excretion of Na+ and Cl-.(5) Unfortunately, the attention given to urinary electrolyte composition in daily practice is far from ideal. The aim of this commentary is to present why we think that urine biochemistry evaluation must be part of daily practice in the intensive care unit (ICU).