Additional considerations
Urine studies can also be useful for assessment of proteinuria and albuminuria in a patient with CKD or diabetes, diagnosis of plasma cell dyscrasias, the diagnosis and prevention of nephrolithiasis, and a wide variety of other conditions.
Back to the case
Our patient was admitted with an elevated creatinine of unclear chronicity, and subacute symptoms of uremia. Because she was oliguric, urine and serum sodium and creatinine were measured before intravenous fluids were administered. Her FENa was 2%, which was not consistent with prerenal azotemia or ATN. She was found to have CKD secondary to previously undiagnosed diabetes. Upon further questioning, she had been taking high-dose NSAIDs for her chronic knee pain. Her renal function improved mildly by withholding NSAIDs, and she was discharged with appropriate nephrology follow-up.
Bottom line
Urine electrolytes have specific indications and utilities for different clinical scenarios, and should be ordered in a targeted manner that can aide in diagnosing AKI, hyponatremia, hypokalemia, and normal anion gap metabolic acidosis.
Dr. Tummalapalli, Dr. Krouss, and Dr. Goetz are hospitalists in the department of medicine at the Icahn School of Medicine at Mount Sinai in New York City.
References
1. Brosius FC, Lau K. Low fractional excretion of sodium in acute renal failure: role of timing of the test and ischemia. Am J Nephrol. 1986;6(6):450-7.
2. Gottfried J, Weisen J, Raina R, Nally J. Finding the cause of acute kidney injury: Which index of fractional excretion is better? Cleve Clin J Med. 2012;79(2):121-6.
3. Steiner, RW. Interpreting the fractional excretion of sodium. Am J Med. 1984;77(4):699-702.
4. DeVita MV, Gardenswartz MH, Konecky A, Zabetakis PM. Incidence and etiology of hyponatremia in an intensive care unit. Clin Nephrol. 1990;34(4):163-6.
5. Shepshelovich D, Leibovitch C, Klein A, et al. The syndrome of inappropriate antidiuretic hormone secretion: distribution and characterization according to etiologies. Eur J Int Med. 2015;26(10):819-24.
6. Mount DB. Fluid and Electrolyte Disturbances. In: Kasper D, Fauci A, Hauser S, Longo D, Jameson J, Loscalzo J. eds. Harrison’s Principles of Internal Medicine, 19e. New York, NY: McGraw-Hill; 2015.
7. Kamel KS. Intrarenal urea recycling leads to a higher rate of renal excretion of potassium: an hypothesis with clinical implications. Curr Opin Nephrol Hypertens. 2011 Sep;20(5):547-54.
8. Kamel KS, Davids MR, Lin S-H, Halperin ML. Interpretation of Electrolyte and Acid-Base Parameters in Blood and Urine. In: Brenner and Rector’s The Kidney, 27, 804-45.e2. Philadelphia, PA: Elsevier; 2016.
9. Goldstein MB, Bear R, Richardson RMA, Marsden PA, Halperin ML. The Urine Anion Gap: a clinically useful index of ammonium excretion. Am J Med Sci. 1986;198-202.
Key Points:
• In acute kidney injury, the FENa and FEUrea may be calculated to distinguish prerenal azotemia from ATN; however, FENa and FEUrea may be low in a wide variety of conditions other than prerenal azotemia.
• Urine sodium and osmolality values are helpful in diagnosing the cause of hyponatremia, but have a number of limitations in nonoliguric patients and those with CKD.
• An elevated transtubular potassium gradient (TTKG) may indicate renal loss of potassium in patients with hypokalemia.
• A positive urine anion gap (UAG) in the setting of a normal anion gap metabolic acidosis points to renal causes of the metabolic acidosis, whereas a negative UAG points to extrarenal causes such as bicarbonate losses in the GI tract.Ad
Additional Reading:
Goldstein MB, Bear R, Richardson RMA, Marsden PA, Halperin ML. The Urine Anion Gap: A Clinically Useful Index of Ammonium Excretion. Am J Med Sci. 1986;198-202.
Gotfried J, Wiesen J, Raina R, Nally Jr JV. Finding the cause of acute kidney injury: which index of fractional excretion is better. Cleve Clin J Med. 2012;79(2):121-126.
Kamel KS, Davids MR, Lin S-H, Halperin ML. Interpretation of Electrolyte and Acid-Base Parameters in Blood and Urine. In: Brenner and Rector’s The Kidney, 27, 804-845.e2. Philadelphia, PA: Elsevier; 2016.