The anion gap is the difference in the measured cations and the measured anions in serum

Blood serum

In blood, the serum is the component that is neither a blood cell nor a clotting factor; it is the blood plasma with the fibrinogens removed...

, plasma

Blood plasma

Blood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...

, or urine

Urine

Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...

. The magnitude of this difference (i.e. "gap") in the serum is often calculated in medicine when attempting to identify the cause 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...

. If the gap is greater than normal, then high anion gap metabolic acidosis

High anion gap metabolic acidosis

High anion gap metabolic acidosis is a form of metabolic acidosis characterized by a high anion gap.The list of agents that cause high anion gap metabolic acidosis is similar to but broader than the list of agents that cause a serum osmolal gap.-"Mudpiles":...

 is diagnosed.

The term "anion gap" usually implies "serum

Blood serum

In blood, the serum is the component that is neither a blood cell nor a clotting factor; it is the blood plasma with the fibrinogens removed...

 anion gap", but the urine anion gap

Urine anion gap

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

 is also a clinically useful measure.
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) or in millimoles/litre (mmol/L).
It is calculated by subtracting the serum concentrations 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...

 and bicarbonate

Bicarbonate

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

 (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...

 and 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⁻] + [HCO₃⁻])

However, the potassium is frequently ignored because potassium concentrations, being very low, usually have little effect on the calculated gap.