Urine Anion Gap Calculator

How does this urine anion gap calculator work?

This is a health tool designed to assess kidney function based on the urine anion gap that is also used to differentiate metabolic acidosis cases. The concentrations for the two cations [Na⁺], [K⁺] and one anion [Cl^-] are expressed in units of miliequivalents per liter per day [mEq/L/day].

1. Urine sodium [Na⁺] – measured in either mEq/L/day or mmol/L/day. Sodium is both an electrolyte and mineral and the normal range in urine is 40 to 220 mEq/L/day. Higher levels of urine sodium indicate amongst others an impaired function of the adrenal glands, too much salt in diet or inflammation of kidneys, salt losing nephropathy. Lower than normal levels are indicative for hyperaldosteronism, dehydration, fluid loss, diarrhea, heart failure or kidney failure.

2. Urine potassium [K⁺] – measured in mEq/L/day or mmol/L/day. The normal range is between 25-125 mEq/L/day. Higher results may be due to metabolic acidosis such as diabetic acidosis, eating disorders, hypomagnesemia, acute tubular necrosis. Lower than normal levels are indicative amongst others for hypoaldosteronism and usage of anti inflammatory drugs or beta blockers.

3. Urine chloride [Cl^-] – measured in mEq/L/day or mmol/L/day. The normal daily range is 110- 250 mEq/L/day and higher than this levels may be due to an impairment of the adrenal glands, polyuria, nephropathy or too much salt intake. Lower than normal levels are indicative for sodium retention, decreased salt intake, fluid loss, dehydration, Cushing syndrome.

The formula used by the urine anion gap calculator is computed based on urinary ions:

UAG = [Na⁺] + [K⁺] - [Cl^-]

which is the difference between the most important measured cations: sodium and potassium and urinary anions: chloride. The other unmeasured urinary anion is ammonia. The results can be either positive or negative while the normal values for this type of gap range between -10 to 20 mEq/L.

This urine anion gap calculator can also be used in cases of non gap metabolic acidosis, mixed metabolic acidosis or cases suspected of RTA and can also distinguish between renal or extra renal causes.

Non gap metabolic acidosis conditions are known as the HARDUP group:

- Hyperchloremia
- Acetazolamide, Addison’s disease
- Renal tubular acidosis
- Diarrhea, vomiting, ileostomies, fistulae
- Ureteroenteric fistulae
- Pancreatoduodenal fistulae

Increased UAG

A positive urine anion gap suggests a renal non anion gap metabolic acidosis in which the kidneys do not excrete ammonia NH4⁺ properly, in conditions of type 1 and type 2 renal tubular acidosis.

- Urinary acidosis of different etiology.
- Impaired renal distal acidification
- Impaired/ low NH4⁺ (ammonium) excretion because of loss of base in the kidney e.g. renal tubular acidosis.

Decreased urine anion gap

A negative UAG indicates extra renal non anion gap metabolic acidosis.

- high levels of NH4⁺ (ammonium) excretion or gain of mineral acid e.g. HCl infusion.
- below -10 (in average between -27 and -10) is indicative for loss of bicarbonate, e.g. diarrhea, bowel conditions.

Serum anion gap vs Urine anion gap

The anion gap is basically the difference between the measured cations (positive ions) and the measured anions (negative ions) and there are two mediums that can be analyzed: serum and urine. When used casually anion gap usually implies serum anion gap. They are both used in discerning the presence and causes of metabolic acidosis and monitoring the treatment response. Formulas are similar but in the UAG, bicarbonate is excluded as its level in urine can be neglected:

- Serum AG = [Na⁺] + [K⁺] + [HCO₃^-] - [Cl^-] where normal range is 6 to 11 mEq/L.
- Urine AG = [Na⁺] + [K⁺] - [Cl^-] where normal range varies -10 to 20 mEq/L.

Example calculation:

Taking the following urine determinations: [Na⁺]= 56 mEq/L/day; [K⁺]=  40 mEq/L/day; [Cl^-]=  110 mEq/L/day

UAG = [Na⁺] + [K⁺] - [Cl^-] = -14 Urine anion gap negative

References

1) 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 Oct; 292(4): 198-202.

2) Batlle, Daniel C., et al. "The use of the urinary anion gap in the diagnosis of hyperchloremic metabolic acidosis." New England Journal of Medicine 318.10 (1988): 594-599.