Anion Gap

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Study Tools



  1. Anion Gap
    1. Normal Value Range
    2. Pathophysiology
    3. Special considerations
    4. Elevated anion gap
    5. Low anion gap

Nursing Points


  1. Normal value range
    1. 3-11 mEq/L
  2. Pathophysiology
    1. Difference between cations and anions
      1. Cations
        1. Potassium
        2. Sodium
      2. Anions
        1. Chloride
        2. Bicarbonate
    2. Formula
      1. [K+]+[Na+]-[Cl]+[HCO3]
      2. Difference (or gap) represents the other anions in the serum
        1. Elevations in the amount of anions changes the pH of the blood/serum
        2. Decreases pH
        3. Creates more acidic environment
    3. Certain diseases or illnesses result in the increase in anions, thus increasing the gap
  3. Special considerations
    1. Commonly submitted with other panels and electrolytes
    2. Green or red (serum) top tube
  4. Increased anion gap
    1. Methanol
    2. Uremia
    3. Diabetic ketoacidosis
    4. Propylene glycol toxicity
    5. Infection
    6. Lactic Acidosis
    7. Ethylene glycol toxicity
    8. Salicylate
      – *MUDPILES
  5. Normal anion gap
    1. Loss of bicarbonate (via stool)
    2. Medications (carbonic anhydrase inhibitors)
    3. Renal tubular acidosis
  6. Low anion gap
    1. Loss of albumin

Nursing Concepts

  1. Lab Values
  2. Acid-Base Balance
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In this lesson we’re going to talk about the anion gap at what it means for your patient.

So to get started, the anion gap normal measurement is 3 to 11 ml equivalent per liter and what anion gap is is it’s a measurement of cations, so you positively charged ions in the body and anions or the negatively-charged ions in the body. So let’s look into how we get this measurement.

So in our body we have all of these molecules. And some of them are positively charged and some of them a negatively charged. So you have the positively charged particles or ions which are called cations, and you have the negatively charged ions. examples of positively charged ions are hydrogen, potassium, or sodium, and then negatively charged ones are chloride or bicarb. And all of these ions basically change the pH of the serum or the blood in the body. If you go to our lesson on arterial blood gases, you can get better ideas and understanding some things that cause issues like metabolic acidosis or respiratory alkalosis, but these are conditions where the pH of the blood is changed. And what we need to look at is how we measure does changing the pH looking specifically at these molecules.

The way we do that is by using this equation. We look at the the difference between potassium, sodium, chloride, and bicarb. Because potassium and sodium are positively charged you add those together, and because chloride and bicarbonate are negatively charged, you add those together. And then you look at the difference. So let’s say our potassium is 4 are sodium is 135, when we add those two together it gives us 139. Now let’s say our chloride is 105, and our bicarb is 25. that means the total of are negatively charged ions is 130. Now we’re going to look at the difference between are positive and negatively charged ions. That difference is 9, which is normal.

What eventually happens is that if the gap ends up increasing, and we can’t account for it with these four standard variables. That means that there’s a buildup of some sort of other acid that’s contributing to this Gap. So these are things like lactic acid or keto acid and because you know that the accumulation of acids in the body is going to drop the pH, we can use the anion gap to look at specific causes for systemic changes in PH.

When you submit you labs, you’re going to submit them in a green top tube or red top tube and they’re going to be either submitted with other panels, or they’re going to be commonly submitted with electrolytes. Its usually standard and most of your comprehensive metabolic panel is, or in conditions where you’re concerned about some sort of acidosis, so let’s get into reasons for acidosis or changes in that Gap.

There are three situations where you’re actually going to have changes in your Gap. You’re going to have an increased anion gap, you’re going to have a normal anion gap but the patient’s pH is off, or you’re going to have a decreased anion gap. With your increased anion gap, remember the mnemonic MUDPILES so it literally stands for the things that are going to cause increases in anion gap, or this increase in the amount of a negatively charged ions or anions. So methanol, uremia which is a problem with the kidneys, you’ve got Diabetic ketoacidosis where fats are being broken down for energy and that causes the increase in anions.. Propylene glycol toxicity, infection. You’ve also got lactic acidosis which could be due from different types of trauma or sepsis which is going to cause an increase in acidity, you’ve got ethylene glycol toxicity, and you’ve got salicylate toxicity as well.

When you have decreased anion gaps, so this is going to be less than three, you’re going to look for situations where you have a loss of albumin. So you’ll want to pay attention to your albumin levels.

There is a situation where your patients pH is low, but the anion gap is actually normal or what we call closed. So the main cause of it is a loss of bicarbonate, so you’re going to look to things like renal failure or diarrhea. So correcting those two things will actually correct the patient’s pH even though the Gap is normal.

In this lesson for our nursing Concepts, we look at lab values and really pay attention to that acid-base balance because the anion gap is going to give us so much information about it.
So let’s recap.

The normal values for an anion gap is 3 to 11 ml equivalent per liter.

Remember the formula that were looking for the difference in positively charged ions and negatively charged ions. They’re going to add your sodium & potassium together and take away your chloride and bicarb, and that’s going to give you your Gap.

The Gap is actually caused by the accumulation of other acids in the body

When you’re looking at causes for an increased anion gap remember the mnemonic mudpiles. So methanol, uremia, diabetic ketoacidosis, propylene glycol poisoning, infection, lactic acid build up, ethylene glycol poisoning, and salicylate poisoning.

Also remember that you can have a normal anion gap with a decrease pH, and that’s going to be caused by losses of bicarb through the kidneys or the stool.

That’s it for our lesson on anion gap. Make sure you check out all the resources attached to this lesson. Now, go out and be your best selves today. And, as always, happy nursing!!

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