The urine anion gap (UAG) calculator works by finding the difference between positive (cations) and negative (anions) ions in your urine. In the post below you can find information about the urine anion gap, its use, its normal range, and the risks of high values.

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What is the urine anion gap?

The urine anion gap (UAG) is the difference, or the gap, between the cations, which consist of potassium (K+) and Sodium (Na+) ions, and the anions, which consist of chloride (Cl) ions, in someone’s urinary tract. In medicine, we use it to diagnose certain metabolic diseases.

Cations in urine

In terms of cations, next to the ones we have already mentioned, there are NH4+ (ammonium), Mg+2, and Ca+2.

NH4+ is the ammonium ion. It is generated when ammonia, which is a weak base, reacts with proton donors such as Brønsted acids. Ammonium is mildly acidic, so when it comes into contact with Brønsted bases, it goes back to the neutral ammonia molecule.

Mg+2 is the cation of magnesium. It is a divalent metal cation, meaning it is actively involved in the physiology and biochemistry of cells.

Ca+2 is the strongly basic oxide of calcium. It is metabolically active and not bound to proteins. Just like magnesium, it is also a divalent metal cation.

Anions in urine

Next to the chloride ions, there are also HCO-3, SO42-, PO43- and some other organic anions.

HCO-3 is bicarbonate, a polyatomic anion (constructed of multiple atoms). It is very important for the pH buffering system in human organisms (the balancing of acids and bases). In the body, it is converted into carbonic acid – H2CO3.

SO42- is sulphate or sulfate, another polyatomic anion. In urine, it is the end product of hydrogen sulfide – H2S.

PO43- is the phosphate anion. It is derived from phosphoric acid by removing three protons, or three H+ ions.

Urine anion gap formula

As we said, the urine anion gap is the difference between the cations and anions that are present in a person’s urine. So, naturally, the formula for calculating it will be:


We can be more precise with this. Since not all cations and anions are used in this calculation, we can rewrite the formula as:

UAG = UNa + UK - UCl

How to calculate the urine anion gap?

Calculating anything is simple when you have the required formula. Since we have the formula, we can use it to easily calculate the urine anion gap. Of course, to calculate your urine anion gap, you need to know the values of cations and anions in your urine. To find out this information, you need to get a urinalysis.

A urinalysis is, as the name implies, an analysis of your urine. It is not a new procedure. Urinalysis involves checking the appearance, concentration, and overall contents of a person’s urine. Usually, your health care provider will give you a small container in which you will collect your urine. To do this, you will collect a sample from the middle of the stream. You do this because, at the beginning of the excretion, your stream will, in a way, wash out your urinary tract, and you will get more accurate results. They might also tell you to take it in the morning when your urine is more concentrated.

Once your urinalysis is done, you should know the values of the required cations and anions in your urine. Now, you can use our calculator to calculate your urine anion gap, and if the values are too high, consult your health care provider for further instructions.

Normal urine anion gap value

According to an article by Medscape, the range for the urine anion gap is between 12-20 mEq/L, with the normal value being right in the center, at 16 mEq/L. Unit mEq/L stands for milliequivalents per liter. An equivalent is the amount of a substance that reacts with a certain amount of another substance in a chemical reaction. In this case, the other substance is water, and the amount is 1 mole. A milliequivalent is 1000 times smaller than an equivalent.

So, what happens when your urine anion gap value is out of the normal range. Let’s talk about that next.

High urine anion gap values

High UAG values are related to hyperchloremic metabolic acidosis.

Medicine has taught us that the human body needs to be very precise with balancing acids and alkalines, as the only acceptable pH level is between 7.35 and 7.45, which is slightly basic. If it goes above that, an alkalosis occurs, and if it goes below that, an acidosis occurs.

pH is the scale used to determine the acidity or basicity of a compound or environment. It represents the concentration of H+ ions. A value of 7 indicates that the concentration of H+ ions is equal to the concentration of OH ions, meaning the environment is neutral. If the value is below 7, there are more H+ ions than OH ions, so the environment is acidic. Similarly, if the value is above 7, there are more OH ions, and so the environment is alkaline or basic. For more information about pH in medicine, you can check out our Venous Blood pH Calculator.

Hyperchloremic metabolic acidosis is a state in which the body enters an acidosis because of the high concentration of Chlorine ions (Cl). The easiest way to diagnose this state is with a UAG test. It comes as a consequence of some other diseases, such as renal tubular acidosis.

Renal tubular acidosis

Renal tubular acidosis is a disease of the kidneys. The kidneys are dual organs, that serve a cleansing function in the body. They produce urine, which contains all the unnecessary substances that are removed from your blood when it enters a kidney. Among other things, they remove acids from your blood. This is why urine is acidic. If the kidneys fail and don’t remove the acids from your blood, you can’t excrete the excess acid, the blood becomes too acidic, and your body enters acidosis.

According to an article from KidsHealth, proximal renal tubular acidosis mostly happens in newborns, and is caused by a disorder called Fanconi’s syndrome.

For patients with renal tubular acidosis, the excretion of urine happens less often because the kidneys don’t produce as much of it as they should. Other symptoms include muscle weakness, cramps, and sometimes rickets, which is a disease where the bones are soft and weak. Some patients also develop kidney stones (colicae renales).

So, why does this disease occur? Your kidneys filter blood through tubules, which are small tubes that serve as filters. New blood comes into the kidneys and is taken through a filter where the waste is removed. The waste is, among other things, a part of urine. The excretion of urine is how you ultimately get rid of the waste. If these tubules don’t do a good job at filtering out the waste, some of the waste will remain in your blood, including the acids.

Low urine anion gap values

On the other side of the spectrum is a state where the organism is in alkalosis.

Alkalosis is a state where the pH level of the organism is above 7.45. It occurs when there is too much bicarbonate in bodily fluids, as bicarbonate is a regulator of the pH level in your body. Alkalosis can be metabolic and respiratory, depending on the cause.

Metabolic alkalosis usually happens because of vomiting or diarrhea. Both of these disrupt the balance of acids and alkalines in your blood. It can also happen as a consequence of renal and cardiovascular diseases.

Respiratory alkalosis is related to a person’s breathing pattern. When someone takes very deep and/or fast breaths for a long time, their lungs get rid of too much carbon dioxide, which turns into bicarbonate in your organism, causing the blood to become alkaline.