To understand what anion gap is, it is first
necessary to understand a few other terms. If
you already know the following terms, feel
free to skip to the next section.
ION: An ion is an atom or a group of atoms
that have an electric charge by gaining or
losing one or more electrons. An atom is the
smallest part of a substance that can exist
alone or in combination with something else.
An electron is a negatively charged particle
that is smaller than an atom.
Since an electron has a negative charge, if an atom gains too many electrons it is
considered negative (because there will be more negative charges than positive
charges). If an atom has too few electrons it is considered positive (because there
will be more positive charges than negative charges).
CATION: Cations are ions with a positive electric
ANION: Anions are ions with a negative electric
WHAT IS ANION GAP?
Anion gap (pronounced an-eye-on gap) is a
measurement of the anions in the arterial blood
(besides chloride and bicarbonate anions). Arterial
blood is blood in the arteries.
Arteries are blood vessels that carry blood away from the heart. Chloride is a combination of the
element, chlorine, with another element. Bicarbonate is a substance in the blood that prevents it from
becoming too acidic or too alkaline (non-acidic).
The anion gap measurement is taken from the blood serum or blood plasma. Blood serum is the clear,
thin, and sticky fluid part of the blood that remains after the blood has changed from a liquid into a solid
form. Serum does not contain blood cells, platelets (cells that help change the blood from a liquid to solid
form), or fibrinogen (a protein in the blood). The part of blood that contains these and many other types
of cells is known as plasma.
HOW IS ANION GAP MEASURED?
Anion gap is measured after taking a blood sample. One way to calculate the anion gap is to add the
number of chloride and bicarbonate anions together and subtract them from the number of sodium and
potassium cations. Sodium and potassium are two of the most important elements in the body. By
subtracting the chloride and bicarbonate anions from the sodium and potassium cations, the resulting
measurement provides an estimate of the total amount of anions in the blood (besides chloride and
bicarbonate anions). See the previous section for a discussion of chloride and bicarbonate.
The main anions in the blood (besides chloride and bicarbonate) are phosphate (a type of salt), organic
acids (types of acids), sulfate (usually a combination of a metal with a type of acid called sulfuric acid),
and proteins in the blood plasma. See above for a description of blood plasma.
A less accurate way to measure anion gap is to take the number of chloride and bicarbonate anions and
subtract them from the number of sodium cations. This method is less accurate than the previously
mentioned method because it does not take the potassium cations into account. The reason for this is
because the level of potassium in the blood is usually very small. Even though it is less accurate, most
hospitals use this equation.
WHAT IS THE NORMAL LEVEL OF ANION GAP?
The normal level of anion gap is generally considered to be between 8 and 16 mEq (milliequivalents) per
liter of blood. A milliequivalent is a very small unit of weight that is about one thousandth of a gram. To
understand how small a gram is, you need about 453 grams just to get one pound. So just imagine how
small a millimole is. A liter is a measurement of the amount of space that a liquid takes up in a container.
As an example of how much a liter is, four liters of milk equals a little bit more than one gallon of milk.
Sometimes the words "mmol (millimoles) per liter" are used instead of "milliequivalents per liter." In most
cases, a millimole is the same thing a milliequivalent. Usually, the number of anions in the blood plasma
(besides chloride and bicarbonate) equal 12 milliequivalents per liter.
It is worth noting that different hospitals use different ranges for determining normal blood count levels.
The difference in ranges is often due to the different levels of technology used to measure the ions. So
whereas one hospital may use the range of 8 to 14 milliequivalents per liter as the normal range for
anion gap, a hospital using a different machine may use a range of 8 to 16 milliequivalents. The normal
lab values from the instrument being used to measure the ions should always be used when interpreting
the anion gap level.
WHAT IS AN EXAMPLE OF AN ANION GAP CALCULATION?
Here is an example of an anion gap calculation, based on the most common method (subtracting the
chloride and bicarbonate anions from the sodium cations). For this example, the total number of sodium
cations is 140 milliequivalents (mEq) and the total number of chloride and bicarbonate ions (added
together) is 128 mEq. Thus, 140 mEq minus 128 mEq = 12mEq. Thus, the anion gap level is 12 mEq.
This means that we are estimating that there are 12 mEq of anions per liter, other than the chloride and
bicarbonate anions. See the previous section for a description of milliequivalents.
WHY IS IT HELPFUL TO MEASURE ANION GAP?
Measuring anion gap helps to diagnose and treat a condition known as acidosis. Acidosis is when there
is too much acid in the blood. There are many different types of acidosis, and each type is named for its
cause. Some examples are listed below.
WHAT DOES IT MEAN IF THE ANION GAP LEVEL IS TOO HIGH?
It is generally accepted that if the anion gap level is 20 or above, this signals a problem. Too high of an
anion gap level can mean that there is acidosis (too much acid in the blood) due to diabetes mellitus.
The high anion gap level can also be due to lactic acidosis, in which the high level of acid is due a
buildup of a substance called lactic acid. Lactic acid buildup is what causes the burning feeling in your
muscles when lifting weights for many repetitions. A high anion gap can also be due to drug poisoning or
kidney failure. The kidneys are two organs located on each side of the spine, behind the stomach. The
kidneys filter (remove) wastes from the blood. When the anion gap is high, further tests are usually
needed to diagnose the cause of the problem.
WHAT DOES IT MEAN IF THE ANION GAP LEVEL IS TOO LOW?
If the anion gap level is too low, this can mean that the person has hyponatremia (a decreased level of
sodium in the blood). It can also mean that the person has multiple myeloma, which is cancer of the
bone marrow. Bone marrow is a tissue that fills the center of long bones. Additional testing needs to be
done to find out the reason for the anion gap level being too low.
IF THE ANION GAP IS NORMAL, CAN ACIDOSIS BE PRESENT?
Yes, but it is unlikely. Thus, an increase or decrease in anion gap can mean that acidosis is present,
but the absence of an increase or decrease does not always mean that acidosis is not present.
WHAT ELSE IS ANION GAP KNOWN AS?
Anion gap is also known as the cation-anion difference.
WHAT IS THE ORIGIN OF THE TERM "ANION GAP?"
Anion gap comes from the Greek word "anion" meaning "backward going," and the Old English word
"gapa" meaning "hole." Put the words together and you have "backward going hole."