ABG Interpretation: The Anion Gap (Lesson 5)

A discussion of the anion gap, including its calculation and its use in categorizing metabolic acidoses, including multiple examples of acid-base interpretation.

[drharveyshahnam]

Amazing lecture Dr Strong, I'm studying for my final medical school exams and have struggled with acidosis since first year of medical school. You lecture series has addressed my confusion with this topic. Thank you

[StrongMed]

Hersch, that's a great question. What the adjusted anion gap does is to provide an estimate of what the AG would be if the pt's low albumin could be corrected. In other words, if a pt has an alb of 2mg/dL & AG of 12, it superficially looks normal. However, if u could correct the low alb to 4, you'd find the AG incr. to 17. It's why I favor the term "adjusted anion gap" over "corrected anion gap", because the later implies the measured AG is wrong, which isn't the case.

[StrongMed]

An anion gap of 4 is generally abnormal, but you would want to check with the lab that ran the sample to see what its normal range is. The normal range of any lab test varies slightly depending upon the specific method the lab uses. If your anion gap is truly low, by far the most common explanation for this is low albumin, which is often from poor diet but can also be seen with a large variety of chronic diseases.

[sunving]

Thank you Doctor Eric Strong . although i might need to watch a couple time but it is a lot clearer, Doctors around the world thank you . You use your valuable free time to do this.

[sardarsakhiyar]

Can’t thank you enough for such a brilliant lectures dr strong

[yashdshah]

TOO GOOD... EXCEPTIONAL... WHAT I COULD NOT LEARN IN MY 5 AND HALF YEARS, YOU TAUGHT ME IN 15MINS.... tHANK YOU SO MUCH...KEEP IT UP SIR...:)

[lilycrochet]

this is the first time i ever understood anion gap ... thank you very much

[Sponge]

Awesome video! I think something that would also be helpful, in addition to walking through how to determine the type of acid-base disturbance in the examples, is to provide the diagnosis or cause of the acid-base disturbance (e.g. salicylate toxicity, DKA, or whatever it may be)

[StrongMed]

I'm not aware of an obvious one other than to simply convert mmHg to kPa or vice versa for the PCO2. (1 kPa = 7.5mmHg, 1mmHg=0.133 kPa). Converting mmol/L to meq/L is a little tricky because it depends upon the chemical properties of the ion or molecule, but for sodium, bicarb, and chloride, 1meq/L=1mmol/L. If there is anyone else on here that has a lot of experience interpreting ABGs in a location that uses SI units, please feel free to leave a comment explaining what approach you use.

[StrongMed]

@ramseet chhabra, you're correct to question whether the AG of 7 is truly "normal", when I mentioned that labs commonly report a normal range of AG of 8-12 mEq/L. I suppose it's actually lower than normal, though we don't typically discuss "low anion gap metabolic acidosis" as a specific entity, since it's usually the consequence of a normal anion gap acidosis + a condition that leads to a lowering of the AG that's not directly related to acid-base status (e.g. hypoalbuminemia - at least in the conventional model of acid-base analysis).  In this particular example, the normal gap metabolic acidosis is likely from either CKD or hypoaldosteronism.  While severe and/or end-stage renal failure typically leads to an elevated AG acidosis (caused by accumulation of unmeaured anions such as sulfates and phosphates), mild or early renal failure can lead to a normal AG acidosis through an impairment in acid excretion.

[Martina]

Thank you I made it through the RRT exams but your explanation is the best so far... Thank you

[EgyptianLotus]

The examples at the end are great! Thank you so much!

[campusEMS]

Great set of lectures, clear and informative.

[laurentiu]

Very helpful video in understanding how to operate these concepts .Thank you.

[dorindragos]

BUN refers only to the nitrogen in urea. Nitrogen accounts for a little less than half of the urea's molecular mass is (28 out of 60). In countries where urea (and not only the nitrogen in urea) is determined, a typical normal range for urea is 20 to 40 mg/dL, which is about twice the normal range for BUN (7 to 20 mg/dL).

[mihollow]

just ran across these videos... they all look very informative. Thank you

[scottsantinon]

Hi Dr Strong,

Thank you SO much for your videos. They are incredibly well structured and I can't imagine how long they would have taken you to make. A quick question, in patients with electrolyte disturbances caused by pathophysiology independent on acid:base disturbances, does the anion gap become hard or even impossible to interpret?

[maheshr]

Thank you for ur Videos
Very informative

[StrongMed]

DrMythology, good question that I probably should have either clarified in the narration or just have changed the numbers by one to prevent the appearance of a contradiction. In this case, the patient has mild hyperkalemia, and as potassium is an "unmeasured" cation, and excess of it leads to a lower than anticipated anion gap. (every 1meq/L that the K is above 4.0, the expected anion gap should be 1meq/L lower than "normal").

[semsemtiger]

THANKS SO MUCH, IT IS SEEMED TO BE EASY TO UNDERSTAND ABG