Anion Gap: Made Easy

 Let's talk about Anion Gap!

First,what is ion?

An ion is a charged particle.

When it is positively charged then we call it "Cation" & when it is negativity charged then we call it "Anion"

   

In our body to maintain the electrical neutrality the total amount of Cation and the total amount of Anion remain same.

Example of Cation in blood are  Na+,K+,Ca²+,Mg²+ & Example of Anion in blood are  Cl-, HCO3-,SO²-,PO4²-,Organic anion such as Lactate,betahydroxybutyrate

Now when we advice to measure Serum Electrolytes ,then usually laboratory measures some Cation and those are  Na+, ও K+  & anions and those are  Cl- ,HCO3-

Sodium and potassium responsible fire 90% of measured Cation and chloride and bicarbonate responsible for 80% of Anion which we usually measure.So there is a gap of 10% of Anion which remain unmeasured and we call it Anion Gap.Normally it is 12±4 mEq/L.

Now,let discuss the above mentioned explanation in equation

‌As total amount of Cation and total ount of Anion remains same in amount in blood & among them some we can measure and some can not so we can write,

  MA+UA=MC+UC

  Or, UA-UC=MC-MA

        

(Where, MC=Measured cation,UC=Unmeasured cation,UA=Unmesured anion,MA=Measured Anion)

     

So

Anion Gap=UA-UC=MC-MA                (1)

 

So according to the equation we can write Anion Gap= (Na + K)–(Cl+HCO3)        (2)

So if we look at the equation (1) then we can see that if amount of unmeasured anion    increased or amount of unmeasured cation decreased then Anion Gap will be increased and in the same way if amount of measured anion decreased and measured cation increased then Anion Gap will be increased.                              

Let see some example

  Let assume a patient is suffering from lactic acidosis In lactic acidosis there is production of H+ ion(cation) and Lactate(anion).To neutralise the H+ ion the bicarbonate is used.

Now if we look at the right side of the equation (1) the we can see that the measured anion (bicarbonate) will be less and the Anion Gap will be increased.

But how balance occur? Here lactate which is a Anion fill the gap of bicarbonate and as we do not  measure this so, if we look at the left side of the equation (1) the we can see that the UA is more so Anion Gap will be increased.

The same also occur in Alcoholic acidosis,Ketoavidosis,Methanol poisoning,ethelene glycol poisoning,Salicylate poisoning etc where there is  High Anion Gap Metabolic Acidosis

Now when Normal Anion Gap Acidosis occur?

   ‌If bicarbonate is decreased such as Diarrhoea,Renal tubular acidosis(proximal type),Ureterosigmoidestomy etc then the amount of bicarbonate lost, the same amount of  chloride will be preserved in the blood by kidney.So if we look at the equation no (2) ,them we can see that there is no change in anions gap.

If chloride containing acids such as HCl,onium chloride etc is introduced in blood then there will be hydrogen ion as well as chloride ion,but to neutralize hydrogen ion bicarbonate will decreased but there will be no change of Anions gap cause same amount of chloride will fill-up the scarcity of bicarbonate ion.So there will be Normal anions gap acidosis

‌But if there is in organic avid containingSulphate,Phospahate  etc but except chloride then there will be high anion gap cause we don't consider sulphate,phosphate etc which are anion to calculate anions gap.So the remain unmeasured and as the are anions so there will be increased Anion Gap.

That's all.Thanks for reading.

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