Anion Gap Metabolic Acidosis: MUDPILES Mnemonic
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Example Case
A female patient presents with abdominal pain, nausea, and vomiting. The patient has a history of diabetes. She admits that she has not been compliant with her insulin regimen and has not been monitoring her home glucose.
Her glucose on arrival is > 600. A blood gas is obtained and shows a metabolic acidosis with partial respiratory compensation. You want to determine if the anion gap is elevated.
Introduction
Arterial blood gases (ABGs) are useful in determining a patient’s acid base status, and can also provide some insight regarding the severity of the patient’s illness or the underlying pathophysiology taking place.
To easily learn how to interpret blood gases using the tic-tac-toe method, make sure to check out the previous EZmed blog post about blood gases.
This post will now focus on metabolic acidosis, specifically high anion gap metabolic acidosis, and the main causes of this presentation.
Every EZmed blog post provides some kind of trick or strategy to make the medical topic easy to understand.
Today you will learn a useful MUDPILES mnemonic that will help you remember the main causes of anion gap metabolic acidosis.
What is Normal?
Before we can determine if metabolic acidosis is present, we need to know the normal blood gas values.
The main components of a blood gas used to determine a patient’s acid base status is the pH, the carbon dioxide (CO2) level, and the bicarbonate (HCO3) level.
The normal pH range is 7.35-7.45.
A normal PaCO2 range is 35-45. This is easy to remember - simply take the decimal numbers from the pH (0.35 - 0.45) and now you have your 35-45 CO2 range.
The normal values for HCO3 is 22-26.
Metabolic Acidosis
Now that we know the normal blood gas ranges, we can determine what acid base status is present.
The previous EZmed blog post about blood gases made interpreting ABGs easy using the tic-tac-toe method.
If you do not know the tic-tac-toe method, then I recommend checking it out as it will provide you with an easy strategy on how to analyze blood gases.
We learned that metabolic acidosis is present when the pH and bicarb levels are both below the normal range.
Next, we will need to calculate the anion gap to determine if it is normal or high.
A pH < 7.35 is consistent with acidosis. A metabolic acidosis will have a low pH and low bicarb level, whereas a respiratory acidosis will have a low pH and high CO2 level.
How to Calculate Anion Gaps
Anion gaps are fairly easy to calculate, but can be confusing to learn at first.
The general way to think about this is to subtract the total amount of negative anions from the total amount of positive cations.
The positive cations used are sodium and potassium.
The negative anions used are bicarbonate and chloride.
A normal anion gap is typically < 12.
Anion gap = [Na + K] - [HCO3 + Cl]
Pay attention to the normal anion gap values at your hospital/institution or lab as they can vary slightly.
Some clinicians exclude potassium in calculating anion gaps, as the level of potassium is fairly negligible.
An anion gap is calculated by adding up the total amount of cations (sodium and potassium) and the total amount of anions (bicarbonate and chloride). The anion total is then subtracted from the cation total. A normal anion gap is < 12.
MUDPILES
Now that we know how to calculate an anion gap in the presence of metabolic acidosis, it is important to know what disease states can cause this presentation.
The mnemonic many health professionals use is MUDPILES.
This will help you remember the following.
M = Methanol
U = Uremia
D = Diabetic Ketoacidosis (DKA)/Alcohol Ketoacidosis
P = Paraldehyde
I = Isoniazid
L = Lactic Acidosis
E = Ethanol/Ethylene Glycol
S = Salicylates
You can also add CAT to MUDPILES to help you remember a few more causes of high anion gap metabolic acidosis.
C = Carbon Monoxide/Cyanide
A = Aminoglycosides
T = Theophylline
Therefore, if you have a patient with a high anion gap metabolic acidosis, this mnemonic can help you think through a differential diagnosis.
It may even help you consider what to order such as: glucose (DKA), ethanol level, lactate, chemistry (uremia), volatile alcohol panel (methanol), co-oximetry (carbon monoxide), etc.
Conclusion
Hopefully this post provided you with an easy explanation on how to calculate an anion gap.
Use the CAT MUDPILES mnemonic to help you remember the main causes of high anion gap metabolic acidosis.
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