The urine anion gap is calculated using measured ions found in the urine. It is used to aid in the differential diagnosis of metabolic acidosis

Metabolic acidosis

In medicine, metabolic acidosis is a condition that occurs when the body produces too much acid or when the kidneys are not removing enough acid from the body. If unchecked, metabolic acidosis leads to acidemia, i.e., blood pH is low due to increased production of hydrogen by the body or the...

.

The term "anion gap" without qualification usually implies serum anion gap

Anion gap

The anion gap is the difference in the measured cations and the measured anions in serum, plasma, or urine. The magnitude of this difference in the serum is often calculated in medicine when attempting to identify the cause of metabolic acidosis...

. The "urine anion gap" is a different measure, principally used to determine whether the kidneys are capable of appropriately acidifying urine.

Calculation

Urine anion gap is calculated by subtracting the urine concentration of chloride

Chloride

The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...

 (anions) from the concentrations of sodium

Sodium

Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

 plus potassium

Potassium

Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

 (cations):

= Na⁺ + K⁺ − Cl⁻

where the concentrations are expressed in units of milliequivalents

Equivalent (chemistry)

The equivalent , sometimes termed the molar equivalent, is a unit of amount of substance used in chemistry and the biological sciences.The equivalent is formally defined as the amount of a substance which will either:...

/liter

Litér

- External links :*...

 (mEq/L).

In contrast to the serum anion gap equation, the bicarbonate

Bicarbonate

In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...

 is excluded. This is because urine is acidic, so the bicarbonate level would be negligible.

Uses

Urine anion gap is an 'artificial' and calculated measure that is representative of the unmeasured ions in urine. Usually the most important unmeasured ion in urine is NH₄⁺

Ammonium

The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...

.

• A positive urine anion gap suggests a low urinary NH₄⁺ (e.g. renal tubular acidosis
  Renal tubular acidosis
  Renal tubular acidosis is a medical condition that involves an accumulation of acid in the body due to a failure of the kidneys to appropriately acidify the urine. When blood is filtered by the kidney, the filtrate passes through the tubules of the nephron, allowing for exchange of salts, acid...
  
  ).
• A negative urine anion gap suggests a high urinary NH₄⁺ (e.g. diarrhea
  Diarrhea
  Diarrhea , also spelled diarrhoea, is the condition of having three or more loose or liquid bowel movements per day. It is a common cause of death in developing countries and the second most common cause of infant deaths worldwide. The loss of fluids through diarrhea can cause dehydration and...
  
  ).