Anion Gap & Osmolar Gap

by Thad Wilson, PhD

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    00:00 We have a number of different ways to get a metabolic acidosis.

    00:07 So, what we have to do is have to diagnose this to understand which one they might have.

    00:14 To do that, we use an Anion gap. And what is an Anion gap? And anion gap allows us to look at the total number of positive charges in the blood.

    00:27 And compare that to the total number of negative charges in the blood.

    00:31 So, it’s cations versus anions.

    00:35 They have to equal each other.

    00:39 The positive charges will always equal the negative charges.

    00:43 And so, I know you’re wondering now, if they are equaling each other, why they have a gap? Well, there’s some amount of an anion that you’re not taking into a count.

    00:54 So, let me explain this.

    00:57 Metabolic acidosis with a normal anion gap, you have a certain amount of sodium.

    01:05 Sodium is your only cation that you’re worried about.

    01:10 Potassium you’re not worried about it, only sodium.

    01:13 Then you compare it to your anions, chloride, bicarb and then something else.

    01:21 That something else is your gap.

    01:24 You don’t know what it is. It’s something out there that it’s taking a part of that total cation to anion comparison.

    01:37 What should your anion gap be? Normally, somewhere between about 8 to 16.

    01:43 So, in our example here having an anion gap of 10, perfectly normal.

    01:48 Let’s compare it to a problem that has an abnormal anion gap.

    01:53 You have a sodium value, you get this from your blood chemistry.

    01:57 You put in your chloride, you put in your bicarb, and then something else is taking up a larger piece of those cations.

    02:08 Cos again, they have to equal each other.

    02:11 They call this a gap because it’s not something that you measure.

    02:16 You’re only measuring the ions in the blood.

    02:19 And so, you don’t know what was there that take up all the extra cations.

    02:25 If it’s above 16, now that you know you have an elevated plasma anion gap.

    02:32 So, you’re looking for a value between 8 and 16.

    02:38 Once you have an increase plasma ion gap you know that you have a certain list of disorders.

    02:46 You either have ketoacidosis, lactic acidosis.

    02:50 You might have renal failure, aspirin overdose, methanol poisoning, propylene and glycol poisoning or maybe you’re not eating enough.

    03:00 All those would have an increased plasma ion gap.

    03:05 The ones you can eliminate if you have an elevated plasma ion gap are the ones that are associated with diarrhea, a renal tubule acidosis, you fear on certain drugs like carbonic anhydrase inhibitors or if you have a disease such as Addison’s Disease.

    03:22 So, you can automatically, tease out which one of this list you were on if you have a metabolic acidosis.

    03:30 After you’ve calculated the anion gap, there’s another differential diagnosis you can go through, and that is calculating on an Osmolar Gap.

    03:40 So, if you were looking at a difference between two numbers, just like you were between cations and anions.

    03:47 What you’re going to look at now is the difference between your measured osmolality to your estimated osmolality.

    03:54 How do you do that? Estimate your osmolality you can take two times your sodium value.

    04:01 You take glucose divided by 18 and then, you take your blood urea nitrogen divided by 2,8.

    04:07 You sum those together and that should be within 10 of your measured osmolality.

    04:15 If it is greater than that, something is taking up that osmolality. And it’s usually a poison.

    04:23 Something like propylene glycol, there’s some other item that is causing an osmotic pull that normally would not be there.

    04:33 And that is another way you can further differentiate if you have a metabolic acidosis with elevated plasma anion gap to tease out between those problems that were on that list.

    04:49 You can use the osmolar gap to figure out if the person has ingest the poison.

    About the Lecture

    The lecture Anion Gap & Osmolar Gap by Thad Wilson, PhD is from the course Acid-Base Balance.

    Included Quiz Questions

    1. Renal tubular acidosis
    2. Ketoacidosis
    3. Lactic acidosis
    4. Ethylene glycol poisoning
    1. 8 to 16 mM/L
    2. 4 to 8 mM/L
    3. 16 to 24 mM/L
    4. 24 to 30 mM/L
    5. 30 to 36 mM/L
    1. <10
    2. <20
    3. <15
    4. <30
    5. <25
    1. The units of an osmolar gap is mM/L.
    2. Osmolar gap is the difference between the measured osmolar gap and estimated osmolar gap.
    3. In a healthy person, the cations must be equal to anions.
    4. Usually, the osmolar gap increases in the consumption of poisons.
    5. Ethylene glycol poisoning can have an increased anion gap.

    Author of lecture Anion Gap & Osmolar Gap

     Thad Wilson, PhD

    Thad Wilson, PhD

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    An anion gap is a tool to differentiate different types of metabolic acidosis.
    By Julián David F. on 19. August 2021 for Anion Gap & Osmolar Gap

    This review, expose the useful tools Anion GAP and Osmolar GAP, it is easy and complete.