Urine biochemistry in acute kidney injury: are we moving in the right direction?
© BioMed Central Ltd. 2013
Published: 15 November 2013
We read with interest the article by Pons and colleagues  in a recent issue of Critical Care and we would like to ask them a few questions. They excluded patients ‘for whom urine could not be collected according to the study protocol’ . What were the reasons for this - anuria, renal replacement therapy (RRT), ICU discharge or death? The authors said that there was no patient receiving RRT at the time of the study, but then they mentioned 14 patients requiring RRT in the first 24 hours. How many patients were excluded due to less than 72 hours of follow-up? Since acute kidney injury (AKI) diagnosis was mandatorily done on admission in order to define AKI reversal in the first 3 ICU days, what about patients who were ‘no-AKI’ on admission but developed AKI in these 3 days?
The authors evaluated urinary indices which are actually calculated variables, dependent on multiple measured parameters. In a pilot study , we demonstrated that urinary sodium (NaU) was lower on admission in patients who developed AKI in the first 4 ICU days. We believe that sequential NaU measurement is useful, especially in the absence of diuretics, and that early AKI development is characterized by decreases in NaU which may precede increases in creatinine, in both transient and persistent AKI (unpublished data). These findings suggest that transient and persistent AKI are different magnitudes of the same pathophysiological process and not synonyms of functional/structural AKI . This may partially explain the absence of discernment ability of the urinary indices. What was the time course of NaU in your three groups in the absence of diuretics? We believe that there is a role for urine biochemistry in AKI assessment and to exclude it from daily practice is the wrong direction.
Bertrand Pons and Michael Darmon
We thank Maciel and Vitorio for their comments and appreciate their interest in our work. We enrolled 202 patients, and 58 were excluded as consequences of missing urine samples or an early ICU discharge precluding patient classification.
Patients without AKI at the time of urine collection were considered ‘no AKI’ even if they developed AKI after day 1. We fully agree that the rise in creatinine is delayed following renal injury and that some patients therefore might have been misclassified . However, AKI was defined according to the Acute Kidney Injury Network criteria of both urine output and serum creatinine, and this may have limited the risk of patient misclassification [5, 6].
Overall, 20 patients (21%) without AKI at inclusion developed AKI between days 1 and 3. These patients had similar NaU concentration at inclusion to patients without AKI from inclusion to day 3: 77 mmol/L (40 to 120) versus 79 mmol/L (34 to 110) (P = 0.79). Similarly, fractional excretion of urea and fractional excretion of sodium or other urinary indices in this subgroup of patients were similar to those of patients without AKI from inclusion to day 3.
Lastly, NaU concentration at inclusion was similar across patient groups - 72 mmol/L (38 to 113) in patients without AKI, 55 mmol/L (24 to 84) in patients with transient AKI, and 64 mmol/L (35 to 99) in patients with persistent AKI at inclusion (P = 0.07) - or at later time points.
Acute kidney injury
Renal replacement therapy.
- Pons B, Lautrette A, Oziel J, Dellamonica J, Vermesch R, Ezingeard E, Mariat C, Bernardin G, Zeni F, Cohen Y, Tardy B, Souweine B, Vincent F, Darmon M: Diagnostic accuracy of early urinary index changes in differentiating transient from persistent acute kidney injury in critically ill patients: multicenter cohort study. Crit Care 2013, 17: R56. 10.1186/cc12582PubMed CentralView ArticlePubMed
- Maciel AT, Park M, Macedo E: Urinary electrolyte monitoring in critically ill patients: a preliminary, observational study. Rev Bras Ter Intensiva 2012, 24: 236-245. 10.1590/S0103-507X2012000300006View ArticlePubMed
- Schneider AG, Bellomo R: Urinalysis and pre-renal acute kidney injury: time to move on. Crit Care 2013, 17: 141. 10.1186/cc12676PubMed CentralView ArticlePubMed
- Waikar SS, Bonventre JV: Creatinine kinetics and the definition of acute kidney injury. J Am Soc Nephrol 2009, 20: 672-679. 10.1681/ASN.2008070669PubMed CentralView ArticlePubMed
- Mehta RL, Kellum JA, Shah SV, Molitoris BA, Ronco C, Warnock DG, Levin A: Acute Kidney Injury Network: report of an initiative to improve outcomes in acute kidney injury. Crit Care 2007, 11: R31. 10.1186/cc5713PubMed CentralView ArticlePubMed
- Ralib AM, Pickering JW, Shaw GM, Endre ZH: The urine output definition of acute kidney injury is too liberal. Crit Care 2013, 17: R112. 10.1186/cc12784View Article
- Darmon M, Vincent F, Dellamonica J, Schortgen F, Gonzalez F, Das V, Zeni F, Brochard L, Bernardin G, Cohen Y, Schlemmer B: Diagnostic performance of fractional excretion of urea in the evaluation of critically ill patients with acute kidney injury: a multicenter cohort study. Crit Care 2011, 15: R178. 10.1186/cc10327PubMed CentralView ArticlePubMed