Introducing the "urine biochemical approach": an alternative tool for improving acute kidney injury monitoring in critically ill patients

Abstract

Urine electrolytes and indices assessment as a tool for acute kidney injury (AKI) pathophysiological understanding and management is, until these days, a matter of debate. The classic division of AKI in "pre-renal" (functional/transient) and "renal" (structural/persistent) based on the urinary concentration of sodium and the fractional excretions of sodium and urea has gained popularity for decades and is still present in medical textbooks. Nevertheless, the conclusions of the studies that have used these parameters are very heterogenous and controversial. In the last decade, the pre-renal paradigm has been questioned since urine biochemistry (UB) compatible with "pre-renal AKI" was retrieved from experimental animals with increased renal blood flow, leading some authors to conclude that this approach is not useful for AKI monitoring. Our group has also studied the use of UB in AKI and we think that the key point for adequate use of this tool in clinical practice is a complete mindset change in the way we look and interpret data. In this article, we present the "urine biochemical approach" as an alternative way for UB assessment, which we believe that makes more sense and seems to be more useful for AKI monitoring than the traditional approach. Although the real utility of this alternative approach needs to be confirmed in large, prospective studies, the aim of the present article is to open the mind of critical care practitioners for a potential reappraisal of ancient concepts and ideas regarding the use of urine electrolytes in AKI monitoring.

Keywords: acute kidney injury; fractional excretion of potassium; monitoring; urinary sodium concentration; urine biochemical approach.

Figure 1

Temporal dissociation between serum creatinine (sCr) and urine biochemical parameters (FeK and NaU) regarding the moment of renal dysfunction diagnosis and recovery. Simultaneous evaluation may lead to random combinations and erroneous interpretations such as “renal (structural) AKI” when renal function is actually recovering and “pre-renal AKI” with no obvious evidence of low renal blood flow in many cases. Renal microcirculatory stress (RMS) is a wider diagnosis which includes any condition that triggers avid tubular sodium reabsorption. Gray numbers represent the “missing story” when a single assessment of sCr and urine biochemistry is made (black numbers), as is common in most studies. AKI, acute kidney injury; FeK, fractional excretion of potassium; NaU, urinary sodium concentration.