how useful are they in acute kidney injury?

Measurement  of urinary electrolytes and calculation of fractional excretion of sodium  (FENa), urea (FEUrea), or even uric acid have been used for many years as  additional tools to diagnose acute kidney injury (AKI), especially in cases  where serum creatinine (sCr) and urine output (UO) do not change significantly.  However, if these tests are not interpreted correctly they can be misleading;  also, reports of their correlation with clinical and histopathological findings  have been inconsistent. In situations associated with transient hypovolemia or  hypoperfusion, healthy kidneys respond by increasing urine osmolarity and  reducing sodium and/or urea or uric acid excretion. However, this physiological  response may be variable and confounded by chronic kidney disease (CKD) and  interventions such as diuretics, aminoglycosides, and cardiopulmonary bypass.  This is particularly relevant for ICU patients in whom critical illness is usually accompanied by  fluid overload, muscle wasting, sepsis, and reduced effective circulating  volume, all of which may mask the diagnosis of AKI.(1, 2)

Fractional  Excretion of Sodium (FENa)

The  FENa is a measure of the extraction of sodium and water from the glomerular  filtrate. It is the ratio of the sodium filtration rate to the overall  glomerular filtration rate (GFR). An euvolemic person with normal renal function  and moderate salt intake in a steady state will have FENa approximately 1%.  Patients with pre-existing CKD might exhibit FENa >1% in the absence of AKI  depending on their GFR and sodium intake. Traditionally FENa has been used to  discriminate between pre-renal and intrinsic AKI (in which there is tubular  damage leading to an inability to properly reabsorb electrolytes, including  sodium).

In  pre-renal azotemia, the proximal tubules reabsorb filtered sodium resulting in a  very low urine sodium concentration (<20 mmol/L) and FENa <1%, whereas in  intrinsic AKI the urine sodium concentration is >40 mmol/L and the resulting  FENa is >1%. A  low FENa or low urine sodium (together with normal urinary sediment) reflects  poor renal perfusion of any cause, not exclusively volume depletion. However  there are many reasons why FENa might be low in the setting of intrinsic AKI, or  might be high in the setting of pre-renal AKI, including diuretic use, sepsis,  myoglobinuria, acute glomerulonephritis, cirrhosis, congestive heart failure,  and contrast induced nephropathy. A detailed list of the limitations of FENa is  presented by Perazella and Coca.(3, 4)

Fractional  Excretion of Urea (FEUrea)

Calculation  of FEUrea is based on the same principle as FENa. However, urea reabsorption  occurs mainly at the proximal tubule, which should theoretically make FEUrea  more reliable than FENa during use of diuretic agents, which act distally to the  proximal tubule. However studies evaluating the performance of FEUrea in various  clinical settings (including ICU patients) have produced discordant results. A  low FEUrea is usually indicative of pre-renal AKI, however recent studies  indicate that aging and sepsis may alter FEUrea.(5, 6)

How  Can the Clinical Lab Be of Help?

It  is easy to implement these calculated tests in a clinical lab’s routine. A fresh random urine  collection (not catheterized) is required, together with a blood sample, and  calculations can be made within the laboratory information system. However, the  utility of standardized interpretive comments for these tests are a matter of  debate. Personalized interpretive comments can be made by the Clinical Chemist  only if he/she has access to patients history and putative diagnosis, and may be  limited by regulatory restrictions.

In  Conclusion

The  interpretation of urinary electrolytes is challenging, with many limitations  affecting urine concentrations and fractional excretion indices. Serial  monitoring of urinary electrolytes may be more useful than individual  measurements, as sequential alterations in urine composition have been shown to  parallel the development and severity of AKI. However, whether serial  measurement of urine electrolytes can also help diagnosing the etiology of AKI  remains unclear.    

The  original article please refer to AACC CLN News:https://www.aacc.org/community/aacc-academy/publications/scientific-shorts/2018/urinary-sodium-urea-and-fractional-excretion-calculations