Blood Gases Level 1 Tutorial: Blood Gases
Please wait.. Please wait...
Tutorial: Blood Gases
Learn an organized approach to arterial blood gas analysis, incorporating serum and urine electrolyte values into your more advanced levels of analysis.
How to level up?
Develop your skills by completing our Practice Cases!
Tutorial: Blood Gases Anion Gap Mechanism
Times Practiced
Cases Completed
1h 24m
Total Time spent
1m 24s
Average Time
Anion Gap Mechanism

The anion gap is a very important calculation because the anion gap is an important branch point for your differential diagnosis of a metabolic acidosis. A bunch of specific things will cause a NON anion gap metabolic acidosis, and a completely different bunch of things will cause an anion gap metabolic acidosis.

First things first. We only use anion gap with metabolic acidosis. There is no such thing as an anion gap metabolic alkalosis. There is also no such thing as an anion gap respiratory acidosis (or alkalosis). So anion gap is connected only to processes that cause a metabolic acidosis.

What does anion gap mean? Well, we are all electrically neutral. We have the same number of positively charged ions as we have negatively charged ions. Therefore, if we counted up all the anions (negatively charged ions) and cations (positiviely charged ions), theoretically the positive charges will equal the negative charges.

So let's do that now. Because we are efficient (lazy), we are not going to count every single ion type in the body. We will take a shortcut and only count the most common ions.

The positive ions are:
  • sodium
  • potassium
  • (will ignore magnesium and calcium because their values are low)
The negative ions are:
  • chloride
  • bicarbonate
  • (will ignore phosphate because value is low)
Any other ions I have not listed are in very low levels and adding them to the equation would not change the value of the anion gap significantly.

So the equation for anion gap is all the positives minus all the negatives:

(Na + K) - (Cl + HCO3)

Typical normal numbers are:

(140 + 4) - (110 + 24) = 10

Whoa. Stop the press. Does this mean that we are positively charged? No, it does not because we left something out. Albumin has a negative charge and contributes to the negative charge in your body and makes up most of the difference. However, it is easier (lazier) to leave it out and just acknowledge that without albumin in the equation, the normal gap is around 10.

If the calculated anion gap is around 10-12, then we say there is no gap.

If the value is higher, like around 15 or more, then we say there is a gap.

Significant of Gap vs. Non-gap
Why do some forms of metabolic acidosis cause a gap and others don't? Well, that is a good question and I'm glad you asked. The answer is because an acid takes the form of HA, where H is the hydrogen ion and A is the "anion" of the acid. When you put HA into water, it breaks apart into H+ and A-.

As a general rule, adding this negatively charged anion A- to your blood will make your HCO3- lower. Park this concept into your memory.

If you add acid to your body by adding an acid HA, then your bicarb will drop and take a look at what will happen to your anion gap:
(Na + K) - (Cl   +        HCO3-      )    becomes
(Na + K) - (Cl   +  smaller value ) = larger value here

So when you add some foreign type of acid, the gap between the positive ions and negative ions gets larger because your bicarb value is lower and the this creates a positive ion gap. We don't include these foreign acids in our anion gap equation, so they are missing and this is what causes a larger difference between the positives and negatives.

So how does the body generate a non anion gap metabolic acidosis? There are 2 mechanisms. The first, is administration of chloride. For mechanisms that are not entirely well understood, but related to the body's buffering system as well as to the fact that "normal saline" has a pH of 5.5, giving a chloride load causes your bicarb to drop but not surprisingly, makes your chloride go up.

So if we look at the anion gap equation after administration of chloride:
(Na + K) - (       Cl           +       HCO3-      )    becomes
(Na + K) - (larger value   +  smaller value ) = still normal

In addition to giving someone chloride, there is another way for a patient to develop a non anion gap acidosis. If someone loses HCO3- directly in the urine or through the GI tract (diarrhea), then the direct loss of HCO3- (with positive ions like Na+) results in changes to the anion gap equation that all balance each other out and maintain a normal anion gap.

So in summary, the mechanism that creates:
  • an anion gap metabolic acidosis is the addition of foreign acids that are not measured in the anion gap equation
  • a NON anion gap metablic acidosis is the addition of chloride OR the direct loss of bicarb from the body

In the next lesson, we will summarize specific causes of all acid base disturbances, including the causes of gap and non gap metabolic acidosis.