Understanding Anion gap in OSCE questions

everything about anion gap


I feel the concept of nion gap is always overshadowed in the process of understanding metabolic acidosis. This is an effort to give a deserving credit to Anion gap by dedicating the entire space to this topic.


Anion gap calculation and its interpretation

Anion gap is an important part of questions related to ABG scenarios in pediatrics OSCE examinations, however, this post is not just limited to OSCE but rather on how to use Anion gap at the bedside.


Let us start with few OSCE questions to warm up and then try to understand Anion gap in the easiest possible way, step by step!


STATION 1

A patient is admitted to the ICU with the following lab values: BLOOD GAS in room air shows pH: 7.199, PCO2: 32.2,  HCO3: 12,  PO2: 86.6. Electrolytes are: Na: 136 K: 4 Cl: 103


Question: Calculate Anion gap



Ans 3 – Anion gap 21, can the answer be 25 also, if we include K in the calculation?


STATION 2 

Your patient is a 8-year-old boy admitted with DKA with acute renal injury, bloods investigation are as PH 7.12, Paco2 12 mm Hg, HCO3 is 5 meq/l. Na - 140 K - 4 Cl- 115


Question: What is the anion gap in this patient, choose correct options

  1.  Anion gap is 14 and indicates he has metabolic alkalosis. 
  2. Anion gap is 20 and he has respiratory acidosis. 
  3. Anion gap is 21 and he has high anion gap metabolic acidosis. 
  4. Anion gap is 25 and he has normal acid-base status.


Ans 3: Anion gap is 21 and he has high anion gap metabolic acidosis. But whether there is sufficient data to say that? can there be a mixed non anion gap metabolic acidosis?


STATION 3

Anion gap is calculated as follows

  1. [ K+ ] - [ HCO3- + CL- ]
  2. [ Na+ ] + [ HCO3- + CL- ] 
  3. [ Na+ ] - [ HCO3- + CL- ] 
  4. [Na+ + K+ ] - [ HCO3- + CL- ]


Ans: Both 3 and 4 are correct, but which one to choose in exams?



What is Anion gap?


Anion gap is the difference between Positively Charged Ions (Cations) and Negatively Charged Ions (Anions). well, that is very simple!

But ...


The law of electro-neutrality states that each atom in a stable substance has a charge close to zero therefore the negative charges (Anions) must be equal to positive charges (Cations) [ Linus Pauling 1948 ]


The body must balance the difference between major cations Na+ and major anions Cl- on either side by using Compensatory buffers like negatively charged bicarbonate and positively charged protons. 


Imagine the body as a large pool of water with an abundance of H+ and OH- ions. When the body tries to maintain this electro-neutrality during various pathologies, it has to do so by making alteration in the H+ and OH- ions concentration, thereby resulting in ALTERED PH. 


So abnormal PH is a result, not the effect. In reality, the body is just trying to maintain stable chemistry by trying to equalize the charges on both sides by changing readily available ions like HCO3.


This clearly establishes two facts:

1. Abnormal PH is not a cause but a result. So treating the PH cannot be the solution to the problem

2. Bicarbonate should not be in the center of understanding and treating acid-base disorders. (Unless the pathology is a loss of bicarbonate)

( I will strongly recommend reading Stewart's approach to acid-base disorder as against Handerson Hasselback's, after all its just an equation which calculate PH based on H+ and OH-  )


Why a normal anion gap is not zero if the body Follows the law of electro-neutrality


Because The Anion Gap is the Gap between the MEASURED and not the total anions and cations. 

Since our ability to test ions in ABG is limited to Na+, K+ Cl-, HCO3-  (at least in past). This leaves a gap representing the number of unmeasured anions and cations. However, most of the new ABG's have electrolytes, albumin, and ketones as well.


Where from the concept of anion gap originates?


This is very important to understand as it opens more dimensions in utilizing anion gap other than just using it to differentiate between types of metabolic acidosis.


Acid-base pioneer James L. Gamble, created Gamblegrams (see the image below) which helps to understand the broader utility of the anion gap.


Gamblegram to understand anion gap concept


Looking at the Gamblegram, try to imagine that whenever one ion will increase  ( eg chloride), the other ion ( eg HCO3-) will decrease to squeeze-in the additional chloride causing what we call hyperchloremic acidosis or whenever HCO3 is lost via urine, Cl- has to increase to maintain the neutrality of total charges.


In the same way, when new acids will start accumulating say lactate or ketones, the bicarbonate has to decrease to accommodate causing lactic acidosis in sepsis or ketoacidosis in DKA.


Gambelgram shows a significant presence of albumin on the negative charge side and therefore albumin is very important while interpreting anion gap metabolic acidosis, this is explained below.


The anion gap is a small application of Gamblegram and in my view is unnecessarily kept limited to metabolic acidosis as Gamblegram also can be applied to explain conditions such as metabolic alkalosis in the same way.


How Anion gap is calculated?

There are two ways of calculating anion gap

[Na+] - [Cl−] + [HCO3] Or [Na+] [K+] - [Cl−] + [HCO3]

Hence in OSCE station 3, both the answers are correct.

Because potassium concentrations in serum is relatively low, it usually have little effect on the calculated gap in blood. Therefore, not using potassium is widely accepted, however its not wrong.


In U.S., anion gap is calculated as [Na] – ([Cl] + [HCO3]) and while in UK and also Australia the anion gap is calculated as ([Na] + [K]) – ([Cl]+[HCO3])


The important point is if you using the K, the normal range of anion gap should be 12-20 mEq/L and if you are not considering K then it should be 8 to 16.


Which formula we should use in OSCE?

I will go ahead and calculate the anion gap without using K and maybe note that down in the answers sheet in a bracket.


When to use Anion gap ?

In metabolic acidosis, to decide whether the metabolic acidosis is due to increased acid production: eg sepsis or intake: eg. salicylate) against decreased acid excretion: eg RTA or loss of HCO3– : eg renal or GI losses.


Let us simplify

Acidosis is a result of these three factors 

  1. Increased production or intake 
  2. Decreased excretion of acid
  3. Loss of bicarbonate

Increase acid will give rise to a high anion gap 

Decrease acid excretion or loss of bicarb will produce normal anion gap

( Further reading:

Urinary anion gap is very important and helps to differentiate between a GI and renal cause of a normal anion gap acidosis  )

Are there disorders with low anion gap?

Rare but yes, Read here


How to use anion gap in the differential diagnosis of metabolic acidosis?



High Anion gap metabolic acidosis (HAGMA)

Add CAT to MUDPILES in 2020

Normal Anion gap metabolic acidosis (NAGMA)

CO & CN
Alcoholic ketoacidosis
starvation ketoacidosis
Toluene
Metformin, Methanol
Uremia
DKA
Pyroglutamic acidosis, paracetamol, phenformin, propylene glycol, paraldehyde
Iron, Isoniazid
Lactic acidosis
Ethylene glycol
Salicylates
(CAGE)

Chloride excess
Acetazolamide/Addisons
GI causes – diarrhea/vomiting, fistulae (pancreatic, ureters, Billary, small bowel, ileostomy)
Extra – RTA

(ABCD)

Addison's 
Bicarbonate loss (GI or Renal)
Chloride excess
Diuretics (Acetazolamide)


There is an interesting article published in Lancet, a new anion gap mnemonic for the 21st century: GOLD MARK, which points out the limitation of MUDPILES. MUDPILES is a cliche, isn't it?


Why you should consider Albumin while interpreting anion gap


Anions gap in a healthy individual depends heavily on serum phosphate and serum albumin which represent a larger proportion of unmeasured negative charges especially albumin which contributes almost the whole value of normal anion gap


Hence Hypoalbuminemia is a leading cause of reduced anion gap and may falsely lead to diagnosis non anion gap metabolic acidosis while in actuality the patient has anion gap metabolic acidosis. This is especially important in the PICU setting.


If your are interested in maths

the normal AG = 0.2 x [albumin] (g/L) + 1.5 x [phosphate] (mmol/L)

To normalise the effects of the hypoalbuminaemia on the AG, the corrected AG is calculated as AG + (0.25 X (40-albumin) expressed in g/L


How do you treat anion gap?

You don't need to !. You rather need to correct the disorder, unfortunately, the Henderson Hasselbalch puts HCO3 in the center of management of the acid-base disorder, while in reality it is just a variable constantly changing as a response to maintain electro-neutrality. If you treat the reason, HCO3 will normalize and so is your anion gap. 


Additional things to consider while calculating anion gap

  1. First and foremost, any laboratory error in measurement of any of the three or four parameters will lead to a wrong anion gap value misleading your diagnosis in practice, be careful
  2. An elevated anion gap strongly suggests the presence of a high anion gap metabolic acidosis but doesn't rule out the presence of coexisting normal anion gap metabolic acidosis ( further reading delta anion Gap and Gap Gap acidosis)
  3. Unmeasured proteins (e.g. immunoglobulins in patients with monoclonal gammopathies or myeloma) may lead to a paradoxically normal anion gap. this may be asked in OSCE station or clinical VIVA
  4. any delay in processing the ABG sample may lead to a mild reduction in anion gap, as cellular metabolism of leukocytes may result in increased HCO3 levels.


Recommended reading!

  1. GambleGram 
  2. Stewart principle of acid-base analysis 
  3. Urinary anion gap

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