Urine Anion Gap Calculator
Read more about the urine electrolytes measured and the formula used, in the text below the tool.
The urine anion gap uses the values of the urine electrolytes, the two cations [Na+], [K+] and one anion [Cl-], to differentiate between renal non-anion gap, extra renal non-anion gap and mixed metabolic acidosis or cases suspected of renal tubular acidosis.
The formula employed is based on the values of urinary ions:
UAG = [Na+] + [K+] - [Cl-]
This is the difference between the most important measured cations: sodium and potassium and a urinary anion: chloride. The other unmeasured urinary anion is ammonia.
UAG normal values are between -10 and 20 mEq/L, therefore UAG can be either positive, negative or 0, suggesting a different metabolic acidosis causality.
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Variables and formula
The urine anion gap assesses kidney function and distinguishes between the renal or extrarenal cause of metabolic acidosis based on three electrolytes:
|UAG variables||Normal values in mEq/L/day||Elevated levels indicate||Lowered levels indicate|
|Urine sodium [Na+]||40 - 220||Impaired function of the adrenal glands, too much salt in diet, inflammation of kidneys or salt losing nephropathy.||Hyperaldosteronism, dehydration, fluid loss, diarrhea, heart failure or kidney failure.|
|Urine Potassium [K+]||25 - 125||Metabolic acidosis such as diabetic acidosis, eating disorders, hypomagnesemia or acute tubular necrosis.||Hypoaldosteronism and usage of anti inflammatory drugs or beta blockers.|
|Urine Chloride [Cl-]||110 - 250||Impairment of the adrenal glands, polyuria, nephropathy or too much salt intake.||Sodium retention, decreased salt intake, fluid loss, dehydration or Cushing syndrome.|
Formula: UAG = [Na+] + [K+] - [Cl-]
The UAG normal values are between -10 and 20 mEq/L. It is used in determining the cause of an unexplained non-gap metabolic acidosis, in diagnosing mixed metabolic acidosis and in the evaluation of patients with suspected renal tubular acidosis.
Measurement and interpretation of electrolytes from a simple urine test helps find the cause of several non-gap metabolic acidosis, also known as the HARDUP group of conditions:
■ Acetazolamide, Addison’s disease;
■ Renal tubular acidosis;
■ Diarrhea, vomiting, ileostomies, fistulae;
■ Ureteroenteric fistulae;
■ Pancreatoduodenal fistulae.
A positive urine anion gap value greater than 20 mEq/L indicates a renal cause to the metabolic acidosis.
This occurs in type 1 and type 2 renal tubular acidosis where the kidneys do not excrete ammonia [NH4+] properly. Other causes may be:
■ Urinary acidosis of different etiology;
■ Impaired renal distal acidification.
Lower than normal UAG
A negative urine anion gap value lower than -10 mEq/L suggests an extrarenal cause to the metabolic acidosis. This may be caused by:
■ High levels of NH4+ (ammonium) excretion or gain of mineral acid e.g. HCl infusion;
■ Loss of bicarbonate, e.g. in diarrhea or bowel conditions.
Serum vs Urine anion gap
The anion gap is defined as the difference between cations (positive ions) and anions (negative ions) in either serum or urine. When used casually, the term usually refers to the serum anion gap.
Both determinations are used to confirm the presence and retrieve the cause of metabolic acidosis. They are often used in monitoring treatment response as well.
Formulas are similar but in the UAG, bicarbonate is excluded as its urinary level can be neglected:
■ Serum AG = [Na+] + [K+] + [HCO3-] - [Cl-] | where normal values are between 6 and 11 mEq/L.
■ Urine AG = [Na+] + [K+] - [Cl-] | with normal values between -10 and 20 mEq/L.
Goldstein MB, Bear R, Richardson RM, Marsden PA, Halperin ML. The urine anion gap: a clinically useful index of ammonium excretion. Am J Med Sci. 1986; 292(4):198-202.
Kim GH, Han JS, Kim YS, Joo KW, Kim S, Lee JS. Evaluation of urine acidification by urine anion gap and urine osmolal gap in chronic metabolic acidosis. Am J Kidney Dis. 1996; 27(1):42-7.
1. Kraut JA, Madias NE. Differential Diagnosis of Nongap Metabolic Acidosis: Value of a Systematic Approach. Clin J Am Soc Nephrol. 2012; 7(4): 671–679.
2. Hertford JA, McKenna JP, Chamovitz BN. Metabolic acidosis with an elevated anion gap. Am Fam Physician. 1989; 39(4):159-68.
No. Of Variables: 3
Year Of Study: 1986
Published On: June 2, 2017 · 01:06 PM
Last Checked: June 2, 2017
Next Review: June 2, 2023