# Approach to Acid Base Status: Step 4 – Laboratory Diagnostics

by Carlo Raj, MD

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00:02 Approach to acid base. Well, now we get into specifically metabolic acidosis.

00:07 Keep that in mind.

00:08 So for metabolic acidosis, you will calculate the anion gap.

00:11 This is what you will know clinically.

00:13 And this is what you stick with, and do not, I repeat, DO NOT, for the sake of time, use anion gap for metabolic alkalosis That will drive you crazy.

00:21 Metabolic, and it's really no need.

00:23 A metabolic acidosis. I'm gonna explain what this anion gap means.

00:27 I'll show you graphs and pictures, and then we'll go one step further.

00:30 We already discussed where it's your cation, sodium, do not use potassium, negligible.

00:38 3.5 to 5.0 potassium negligible, And you have your anions, chloride and bicarb.

00:44 Normal anion gap 10 to 14.

00:48 You can use 10 as a point of reference.

00:51 Next, what do you have to do clinically? You know, so many conditions in which your patient could actually present with hypoalbuminemia.

01:01 So many? Yes, you do. You know, nephrotic disorders.

01:05 And you also know about liver damage As a pretty major in terms of society? Sure it is.

01:12 Now, what type of charges and protein have? Negative charge.

01:17 So, therefore, for every 1 g/dl below 4 you will subtract 2,5 mEq/L to "normal anion gap".

01:26 By normal anion gap we mean approximately 10.

01:29 So, you have to clinically then correct for hypoalbuminemia, if need be, if that is a patient who has something like cirrhosis, something like nephrotic disorder, and the question is asking you about metabolic acidosis, and to please figure out the anion gap.

01:48 What else do you want to do? Well, this is the unmeasured anions, and the unmeasured cation, the sodium.

01:55 You rearrange the formula, you end up getting your anion gap.

01:59 What's your normal? Approximate 10.

02:02 What does it all mean? Let me set this up for you.

02:05 On your left, all cation that you will use.

02:10 Should I say, the only cation that you use clinically, is a sodium of 140-145. What have you? Okay, so that's my cation.

02:19 What must always exist in your body as a measure of homeostasis? Electroneutrality.

02:25 So whatever positive charges you have, you have to be able to equate that to negative charges.

02:31 It is that simple? So opposites will attract.

02:35 So let's add this up, shall we? Say of 140 of sodium. What does that? Cation.

02:40 We have chloride between 95 to 105. We have an anion of 105, chloride.

02:46 So now we have a difference of how much? 35.

02:49 So, if you add up all the anions well, now you have 25.

02:53 And what are you anions in the formula? Take a look.

02:56 Chloride plus bicarb.

02:58 Well, that then adds up to, what? Well, we have at 130.

03:02 Which anion gap? 10.

03:05 Is that normal? Absolutely.

03:07 Absolutely. So the unmeasured anions of 10 is perfectly normal.

03:12 So what are they? That's your proteins.

03:14 By proteins, I don't mean albumin.

03:15 And so therefore, they will be like sulfates and phosphates, organic anions.

03:19 So some of these will then contribute to that an anion gap.

03:23 In other words, excess anions.

03:28 What does this mean in terms of moving above 10? We'll take a look.

03:32 Say that you have a patient who's exercising.

03:35 And when they exercise excessively, is it possible that you start feeling cramps and pain in your muscles? Aw.

03:42 Well, what does that mean to you? That means that you are now shifting from aerobic to anaerobic glycolysis, aren't you? As you shift into anaerobic glycolysis, then would you produce more of and less of? A less of ATP and you produce more lactic acid.

03:58 To lactic acid buildup in your muscles then gives you that cramp.

04:02 Doesn't it? Sure it does.

04:03 Another big topic as well is if there's just straight up ischemia.

04:08 Anytime there's ischemia.

04:09 You know that there's an increase in lactic acid.

04:11 To begin with, when would you even consider doing anion gap? Metabolic acidosis.

04:16 What is lactic acidosis? The metabolic acidosis.

04:20 So now you do anion gap.

04:22 You find your cation of 140.

04:26 Lets do your anions while the board's give you something like your bicarb of 10 and a chloride of 105.

04:33 What do you get? You get a measly 115.

04:37 So you get 115.

04:39 So what is the anion gap at this point? 140 minus 115. I do believe that that is your 25, isn't it? Okay. So you get a 25 there has been anion gap.

04:48 What's normal? Approximate 10.

04:50 What accounts for 15 additional anion gap? The lactate.

04:57 An example, here you go, of anion gap metabolic acidosis, in fact, is lactic acidosis.

05:03 Can you replace that lactic with ketones? Sure you can.

05:07 Will you end up getting diabetic ketoacidosis, don't you? Keep this picture in your head, as far as the electrolytes that are responsible for contributing to anion gap, because we will have to go one step further.

05:18 And another step further, and another step further, so that you get every single perspective of anything that's thrown at you absolute correct.

05:27 The more knowledge that you have, the more aware that you know things exist, the less you'll be confused, right? But be careful though, you also have to have a firm understanding.

05:35 Let's move on.

05:36 So elevated anion gap refers to increase in unmeasured anions.

05:41 These include ketones, or lactate. I just sent you a new picture.

05:50 For elevated anion gap and metabolic acidosis.

05:55 AG stands for anion gap.

05:57 MA? When would you only use anion gap, please? Good, metabolic acidosis.

06:02 So when you're using this step, if you need to? Remember, every step of the way, you're asking yourself this question, "Have I answered the question? Have answered the question? Have I come to the proper conclusion about my patient?" If you have, then step away, you've done your job.

06:18 Let's move on to the next one.

06:20 Here says, that you haven't.

06:21 And you do an anion gap metabolic acidosis.

06:24 That's what you have. Give yourself an example such as lactic acid or DKA.

06:28 We'll go through differentials. Big time.

06:30 We've talked about some of that, the mnemonic that you may use such as MUDPILES.

06:35 Right, we talked about Methanol, we talked about Uremia, so on and so forth.

06:40 So those are your differentials.

06:41 Check urine ketones, and if not present, check renal function, lactate, toxin screen, and osmolal gap.

06:50 What does that mean? Please give me two ways in which a patient can develop metabolic acidosis.

06:56 Two ways. Keep it simple.

06:59 Endogenous within the body. Exogenous you're consuming it.

07:04 Endogenous, I just talked to you about exercise.

07:07 What are you producing? Good, lactic acid.

07:10 What is that? An anion gap metabolic acidosis? What about diabetic ketoacidosis? That is also, what? An anion gap metabolic acidosis.

07:20 Where is the origin of that? Good, endogenously.

07:24 What if you were consuming an acid? What do you mean? Such as ethylene glycol, such as methanol? Such as, ah, salicylate.

07:34 So, exogenous sources? And what can you possibly do mathematically, clinically, so that you, if you can figure out how is it that the patient has developed an elevated anion gap metabolic acidosis? Understand something here.

07:50 Whenever you're given a piece of information of anion gap that automatically puts in the realm of above 10, maybe 25, maybe 30.

07:59 What if it says non-anion gap? That means normal, which is not our discussion yet. Not yet.

08:04 Keep in mind the big picture.

08:06 So, what is the osmolal gap? It is the measured osmolality, subtract this from the calculated osmolarity.

08:13 For example, let's say that you calculated osmolarity was approximately 300.

08:17 So, now, let's say that your osmolal gap is greater than 10.

08:21 If your osmolal gap is greater 10, guess where this anion gap is coming from? Possibility of ingestion of cause of anion gap metabolic acidosis.

08:31 So if you find that the osmolal gap is increased between the measured and calculated, guess what, you have most likely confirmed or highly now suspect that there's going to be an ingestion of anion gap metabolic acidosis.

08:47 Interesting, isn't it? So this might then give you a differential of well, huh, exogenous? Now you're looking for sources? What exactly did your patient consume? Was it ethylene glycol? Was it methanol? Was it alcohol? Was it type, some type of drug, so on and so forth? Let's continue.

09:05 Now, I'm going to go through this efficiently.

09:09 It's just enough where you have enough information.

09:12 So at this point, you have absolutely diagnosed elevated anion gap metabolic acidosis.

09:17 You have now figured out that this patient has exogenously taken some kind of acid.

09:23 So therefore, you got an increase in osmolal gap.

09:27 Is there a possibility that your patient has an anion gap metabolic acidosis, along with a mixed non-anion gap? True.

09:37 So therefore, this brings us to an interesting concept called the delta gap.

09:41 All that I wish to point out to you based on feedback that I've gotten, making sure that I give you everything that you need for proper tools. Let's begin.

09:52 Calculate patients anion gap.

09:54 But we already know the patient has an anion gap metabolic acidosis.

09:58 So that will be given. You'll know or you've calculated.

10:01 And then from this, you subtract your normal anion gap, that literally means your delta gap.

10:07 What the heck does that even mean? That means if the change in bicarb, remember that picture that I just showed? I asked you to please conceptualize and keep that bar graph of sodium on one side, of 140? What are the two anions on the other side? You buy carbon chloride, which gave you the original? Well, there, that gave you approximately, 125.

10:30 And so therefore, should I say, 130.

10:34 And we had a normal anion gap of 10, right? So everything equaled out. That was normal.

10:38 And I told you, in our next figure, that if that well, you started adding in lactate, you're taking away, what? Bicarb.

10:47 And you already know, a metabolic acidosis, that your bicarb has decreased.

10:51 My point is this. Is that changing bicarb? Is that changing bicarb enough or sufficient to properly explain the change an anion gap? This will then give you a Delta gap.

11:05 Let's take a look. Compared to change in bicarb.

11:09 What does that mean? Once again, who's your patient so far? Metabolic acidosis anion gap.

11:15 Where that anion gap come from? The osmolal gap would tell you most likely, if it is going to be exogenous, or endogenous and greater than 10, exogenous. Stop there.

11:26 So now we have anion gap metabolic acidosis.

11:30 And now you want to see as to whether or not there's an underlying issue.

11:34 So underlying issue here is going to be, well, is the change in bicarb equal to the change in anion gap.

11:42 If those two equal out, then you know that your patient has normal change.

11:47 and it is strictly a anion gap metabolic acidosis with no mixed underlying issue.

11:54 Let's take a look at a few.

11:55 Example, a bicarb of 16, stop there.

12:00 Normal bicarb. About 24. We'll use 24.

12:04 Dropped down to 16.

12:06 So far, very clear that we have metabolic acidosis.

12:10 Remember, this entire topic is anion gap metabolic acidosis.

12:13 So, all I want to do is make sure that we're on the same page.

12:17 There is an anion gap of 20.

12:19 Either they gave it to you or you figured it out.

12:22 How did you do that? Sodium minus your anions.

12:26 Okay. Normal anion gap is 10.

12:30 But here we have 20.

12:32 So this is definitely an anion gap metabolic acidosis.

12:39 Now, we take the difference, the absolute difference of 20 and 12.

12:46 We get 8, unmeasured anions.

12:48 Okay, let me stop you there.

12:51 If there's a change of 8 unmeasured anions, and you're thinking about that graph, the bar graph that I showed you with cations, obviously, we're on the side of the anions.

13:03 And if there's a change of 8, what can it change should we find in bicarb? Eight.

13:10 If both of these are equal, then it is a pure anion gap metabolic acidosis.

13:16 What is this called? A delta gap.

13:18 Now, before any of this, obviously, you did your degree of compensation.

13:23 And by that we mean, if it was metabolic acidosis, and that respiratory compensation? What was it respiratory compensation, and metabolic acidosis? A decrease in bicarb should equal a decrease in carbon dioxide.

13:35 So that step was already done by you, and that's in the initial stages.

13:39 So, normal bicarb 24.

13:42 The patient's bicarb 16.

13:44 What's the absolute difference? Eight.

13:48 It is the change in bicarb equal to the anion gap change.

13:52 Yes, it is.

13:52 So therefore, what's my diagnosis? A pure anion gap, metabolic acidosis.

13:59 Now, if the change is greater than changing bicarb, it's a concomitant metabolic alkalosis.

14:08 Let me show you something as you move through here as an example.

14:10 If the change in anion gap is less than bicarb, and it's a concomitant non or normal anion gap metabolic acidosis.

14:19 Stop there. Now, I'm going to show you something so this makes better sense.

14:23 For example, let's do this one.

14:26 Bicarb of 10. Normally, by 24.

14:30 So we have metabolic acidosis.

14:33 Okay. Anion gap of 20.

14:36 Either you calculate it or they've given it to you, and so therefore we have, what's my diagnosis? Obviously, anion gap metabolic acidosis.

14:43 That's the change in anion gap. We get 8.

14:48 Stop there. Now, what? Well, we should find a change of bicarb being 8.

14:54 Let's see whether or not that is taking place.

14:57 Normal bicarb of 24 and the patient's bicarb is 10.

15:01 What's the absolute difference? 14.

15:04 Uh-oh.

15:06 The unmeasured anions, anion gap, does not add up to the change in bicarb.

15:13 Do you see this? Wow, so what's happening? There's a difference of how much there? The difference of 6, isn't there? 8 and 14.

15:22 So the six means what? It's accounted for.

15:26 So the fact that you do not find an anion gap, which explains this, there must be an underlying issue.

15:33 What is the underlying issue? Here, you'll notice that go back to the previous discussion, that we just had about anion gap being less than.

15:42 If the anion gap, the 8 is less than the change and the bicarb, than this to you gives you a combined, combined the diagnosis of anion gap and non-anion gap metabolic acidosis.

15:57 This is as tricky as they will get, I wish to introduce the concepts to you.

16:03 If I were you, I would go through the first few steps where you figure out the pH.

16:08 You forgot the primary, you figure out the degree of compensation.

16:13 If need be, for metabolic acidosis, you do an anion gap.

16:18 With anion gap, you then figure out if it's exogenous or endogenous.

16:22 It would behoove you to make sure you pay attention to the osmolal gap.

16:26 Once it get as far as that maybe perhaps you take a look at delta gap, if need be.

16:32 Most of your questions will be answered as far as what I've taken you.

16:37 Now, if it is respiratory, and it's renal compensation, what did you want to know about that? It can have an acute and chronic change.

16:45 Continue.

16:48 So for a normal anion gap is the only thing that we're left with now.

16:51 With the normal anion gap, what do you want to do? This is interesting as well.

16:56 With a normal anion gap metabolic acidosis, you know, that there is no anion gap.

17:03 I mean to say that, or in other words, really, it's within normal limit.

17:07 Okay. So what does that mean to you? Well, check urine anion gap. Why? Because you don't know as to whether or not that bicarb is being lost through the urine, or if the bicarb is lost, where? Through the intestine. For example, think diarrhea.

17:26 When you have diarrhea, what happens? You lose bicarb.

17:30 It has nothing to do with the urine.

17:32 So you're non-anion gap metabolic acidosis, by using clinically, a urine anion gap will then give you how your patients lost their bicarb. Is that beautiful? Well, not so much for the patient, but for you.

17:46 Urine sodium + urine potassium.

17:50 From this, you subtract your urine chloride. Stop there.

17:53 The first time that we're seeing potassium in the urine, it is significant.

17:56 And when I bicarb, why isn't bicarb here? Because that's what you're trying to figure out.

18:00 If the urine anion gap is positive, think of it as being positively getting rid of bicarb.

18:08 You want to think of potential causes such as renal tubular acidosis, type 2.

18:13 And type two wasn't a problem. What was it? Two lines to make an X.

18:19 Proximal convoluted tubule is type two RTA.

18:22 What could be a potential problem? Oh, carbonic anhydrase inhibition.

18:27 So therefore, your anion gap will be positive for bicarb.

18:31 What if you do the formula and that urine anion gap is negative? And how would you get? Well this means that the bicarb is not being lost through the urine.

18:41 In fact, it's being lost, where? Through the GI tract.

18:45 Welcome to diarrhea.

18:46 This is for non-anion gap metabolic acidosis.

18:50 This is all for what? A metabolic acidosis.

18:54 You see as to how important it is for you to absolutely do the anion gap.

18:57 So put you into two major families of differentials.

The lecture Approach to Acid Base Status: Step 4 – Laboratory Diagnostics by Carlo Raj, MD is from the course Pulmonary Diagnostics.

### Included Quiz Questions

1. 10-14
2. 12-18
3. 11-24
4. 60
5. 40
1. For every 1 mg/dL below 4, subtract 3.
2. For every 2 mg/dL below 4, subtract 3.
3. For every 1 mg/dL below 4, add 3.
4. For every 1 mg/dL below 4, divide by 3.
5. For every 1 mg/dL below 3, subtract 3.
1. Plasma magnesium is included in the formula.
2. Anion gap = unmeasured cations - unmeasured anions
3. Anion gap = Na - (Cl + HCO3)
4. May include organic anions and proteins.
5. It is influenced by albumin levels.
1. 50
2. 35
3. 15
4. 10
5. 24
1. Ketones
2. Lactic acid
3. Pyruvate
4. Salicylic acid
5. Hydrochloric acid
1. Measured Osmolality - Calculated Osmolality
2. Urine Osmolality - Serum Osmolality
3. Calculated Osmolality - Measured Osmolality
4. Serum sodium - (Serum bicarbonate + Serum chloride)
5. 2 x (Na) + (glucose) + (urea)
1. Osmolar gap > 10
2. Osmolar gap < 10
3. Elevated ketones in the urine.
4. Increased blood urea nitrogen.
5. Anion gap > 20
1. Change in HCO3 = Change in the anion gap.
2. Change in HCO3 > Change in the anion gap.
3. Change in HCO3 < Change in the anion gap.
4. Change in the delta gap = Change in the anion gap.
5. Change in the delta gap > Change in the anion gap.
1. 2 (Na+) + Glucose/18 + BUN/2.8
2. 2 (Na+) + Glucose/18 + BUN/2
3. 2 (Na+) + Glucose/8 + BUN/2.8
4. 2 (Na+) + Glucose/18 + Uric acid/2.8
5. 2 (HCO3) + Glucose/18 + BUN/2.8
1. Patient's anion gap - Normal anion gap
2. Patient's cation gap - Normal anion gap
3. Patient's anion gap - Normal cation gap
4. Patient's anion gap - Urine anion gap

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