Anion Gap & Osmolar Gap

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.