BOTH Calculating Anion Gap for Acidosis

[pone]

The Anion Gap is a clinical calculation made in order to break down causes for metabolic acidosis:

http://en.wikipedia.org/wiki/Anion_gap

Roughly, you take Sodium + Potassium - Chloride - Bicarbonate.

I have questions on the formulas used to calculate Anion Gap:

1) In practice, potassium is left out of the calculation. A normal potassium serum reading is in the area of 4 mmol/L. Given that the a "normal range" for the Anion Gap is calculated at around 8 to 16 mmol/L, removing a 4 mmol/L component of the calculation does not seem like a trivial thing. Why is potassium ignored here?

2) In terms of bicarbonate, most labs show "Carbon Dioxide, Total." 80 to 90% of the Carbon Dioxide is the Bicarbonate. Should we just take 85% (mid-point) of the Total CO2 value and use that as the estimate for Bicarbonate?

Any help in understanding these concepts is appreciated.

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[pone]

A related question: would a high anion gap > 20 always indicate acidosis, even in absence of an arterial gas reading of pH?

 

[Ismet]

1) If you use K in the equation, the normal range for anion gap is different, I believe 10-20 mmol/L. In order to avoid confusion, it's generally accepted to leave out K so that the 8-16 normal range is more universal.

2) If they want you to calculate the anion gap, they'll give you the lab values for serum bicarb.

3) The anion gap is what you use to distinguish the etiology of metabolic acidosis. So yes, even if they don't give you the pH from the ABG, if you determine the patient is acidotic and has a high anion gap, it's metabolic acidosis and consider the MUDPILES list of causes.

 

[Ismet]

Are you a pre-med? This forum is for med students taking Step 1, not homework help.

 

[Handinhand]

A related question: would a high anion gap > 20 always indicate acidosis, even in absence of an arterial gas reading of pH?

There is a difference between acidosis and acidemia. If your pH is normal you by definition do not have an acidemia, but that does NOT mean you're not acidotic. For example, in the case of a diabetic ketoacidosis, I think you can fairly frequently see someone with a normal pH on ABG who is still certainly acidotic (increased ketones in blood and urine) and for two reasons:

- people with DKA often present with vomiting, which will results in a metabolic alkalosis as you lose H+ through vomitus
- Respiratory compensation through tachypnea causing respiratory alkalosis

These two things together can often bring a pH back into a normal range. You will still obviously have an anion gap in this case however, and that information paired with increased ketones and glucose will lead you to a diagnosis of DKA.

 

[pone]

Are you a pre-med? This forum is for med students taking Step 1, not homework help.

Anion gap is a legitimate step 1 question, isn't it? SDN is loaded with questions in the Step 1 forum about anion gap, such as:

http://forums.studentdoctor.net/threads/anion-gap-acidosis.1072511/
http://forums.studentdoctor.net/thr...-in-metabolic-acidosis.1005623/#post-14044127

So it's not clear why you would single out my question as homework?

This was NOT a homework assignment of any kind. These are questions that came to my own mind when I read about anion gap. I find the clinical application of anion gap a little confusing and I thought a good starting place would be to firm up my understanding of how it is calculated.

 

[pone]

There is a difference between acidosis and acidemia. If your pH is normal you by definition do not have an acidemia, but that does NOT mean you're not acidotic. For example, in the case of a diabetic ketoacidosis, I think you can fairly frequently see someone with a normal pH on ABG who is still certainly acidotic (increased ketones in blood and urine) and for two reasons:

- people with DKA often present with vomiting, which will results in a metabolic alkalosis as you lose H+ through vomitus
- Respiratory compensation through tachypnea causing respiratory alkalosis

These two things together can often bring a pH back into a normal range. You will still obviously have an anion gap in this case however, and that information paired with increased ketones and glucose will lead you to a diagnosis of DKA.

Your answer helped my understanding of these concepts a lot, particularly understanding the compensation of acidosis and alkalosis producing a neutral pH.

What I would like to understand is does anion gap > 20 mmol/L by itself suggest acidosis that requires further investigation? Or is anion gap > 20 mmol/L without other markers of acidosis considered a normal case? In the real world, if a patient is not acutely ill, there is a low chance that they will get an arterial gas. So the person with a low level of chronic acidosis might slip by. I'm trying to understand if there is a clinical marker to flag such people.

 

[Handinhand]

You should not have an increased anion gap in a normal individual. If someone had an anion gap greater than 20 it would warrant further investigation.