Urine Sodium
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Urine sodium (Na) analysis is usually ordered when it is necessary to distinguish between various forms of renal failure and to classify hyponatremia. [1]
The reference range for urine Na is 40-220 mEq/L/24 hours. [2]
In evaluating the possible causes of hyponatremia, first establish the patient’s volume status. The following algorithm is useful in diagnosing cause as it relates to volume:
The urine Na concentration tends to be low in prerenal disease, being less than 20 mEq/L (in an attempt to conserve Na), while the concentration is high in acute tubular necrosis (>40-50 mEq/L). [3] However, calculation of the fractional excretion of Na (FeNa), using the following formula, is a more reliable means of differentiating prerenal disease from renal failure [4] :
FeNa = [(urine Na/plasma Na)/(urine creatinine/plasma creatinine)] x 100
Specifics for indirect integrated chip technology (ICT) urine Na measurement (ARCHITECT c system, Abbott Laboratories) are as follows [5] :
Related tests include the following:
Measurement of the urine Na concentration is vital in determining the integrity of tubular reabsorptive function. Low urine Na concentration indicates not only intact reabsorptive function but also the presence of a stimulus to conserve Na, whereas a high urine Na concentration may signify salt-wasting etiologies.
Indications for testing of urine Na include the following:
In all clinical assessments mentioned above, one must understand the effect of various drugs, particularly diuretics and intravenous (IV) fluids, that impact urine Na concentrations. Conclusions cannot be derived when treatments that impact the excretion of Na and water are being used.
Wolfson AB. Renal failure. Walls R, Hockberger R, Gausche-Hill M, eds. Rosen’s Emergency Medicine: Concepts and Clinical Practice. 9th ed. New York, NY: Elsevier; 2018.
Rifai N, Horvath AR, Wittwer CT, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. New York, NY: Elsevier; 2018.
Miller TR, Anderson RJ, Linas SL, et al. Urinary diagnostic indices in acute renal failure: a prospective study. Ann Intern Med. 1978 Jul. 89 (1):47-50. [Medline].
Kaplan AA, Kohn OF. Fractional excretion of urea as a guide to renal dysfunction. Am J Nephrol. 1992. 12 (1-2):49-54. [Medline].
Abbott ARCHITECT c system manual [package insert]. Abbott Park, Ill: Abbott Laboratories. 2009.
Fazia Mir, MD Fellow, Department of Gastroenterology, University of Missouri-Columbia School of Medicine
Fazia Mir, MD is a member of the following medical societies: American College of Physicians
Disclosure: Nothing to disclose.
Ejaz Mahmood, MBBS, MRCGP Resident Physician, Department of Internal Medicine, Einstein Medical Center
Ejaz Mahmood, MBBS, MRCGP is a member of the following medical societies: Philadelphia Endocrine Society
Disclosure: Nothing to disclose.
Catherine Anastasopoulou, MD, PhD, FACE Associate Professor of Medicine, Sidney Kimmel Medical College of Thomas Jefferson University; Attending Endocrinologist, Department of Medicine, Albert Einstein Medical Center
Catherine Anastasopoulou, MD, PhD, FACE is a member of the following medical societies: American Association of Clinical Endocrinologists, American Society for Bone and Mineral Research, Endocrine Society, Philadelphia Endocrine Society
Disclosure: Nothing to disclose.
Eric B Staros, MD Associate Professor of Pathology, St Louis University School of Medicine; Director of Clinical Laboratories, Director of Cytopathology, Department of Pathology, St Louis University Hospital
Eric B Staros, MD is a member of the following medical societies: American Medical Association, American Society for Clinical Pathology, College of American Pathologists, Association for Molecular Pathology
Disclosure: Nothing to disclose.
Judy Lin, MD
Disclosure: Nothing to disclose.
Urine Sodium
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