Anion Gap at a glance

             At first we have to know which are the major anions and which are the major cations in our body.In respect of our clinical background Na⁺is the major cation and HCO₃^- & Cl^­- are the major anions .Besides these there are many unmeasured anions and cations present in our body.

We know the concentration of anions and the concentration of cations in plasma must be equal to maintain the normal electrical neutrality.

                [Measured cations  ]+ [Unmeasured cation] = [Measured anion] + [Unmeasured anion ]

So           [Measured cations  ] - [Measured anion]   =  [Unmeasured anion ] - [Unmeasured cation]

Now; if there any difference between the concentration of measured cations & anions or Unmeasured anions & cations this condition is known as Anion Gap.

So Anion gap may be defined as the difference between the concentration of measured cation and the concentration of measured anions. Or the concentration of Unmeasured anions & cations  .

Example: 

 we know [Na⁺] =144 mEq/L; [Cl^-] =108 mEq/L  ; [HCO₃^-] = 24 mEq/L 

Now, the plasma Anion Gap = [Na⁺] - [Cl^-]- [HCO₃^-]

                                                                144-108-24 = 10 mEq/L

• ·         Normal Anion Gap ranges are between 8-16 mEq/L.
• ·         Anion gap also increase if unmeasured Anion rise or unmeasured cation fall.
• ·         The most common unmeasured anions are albumin, phosphate, sulfate and other organic anions.
• ·         The most common Unmeasured cations are Ca⁺⁺ , K⁺, Mg⁺

Uses of Anion Gaps:

The main use of anion gap is to diagnosis different causes of Metabolic Acidosis. We know our body always tries to maintain equilibrium. In Metabolic Acidosis there is decrease of HCO₃^- concentration in the plasma. To compensate this, body starts to increase Cl^- concentration and maintain normal Anion Gap. So in Metabolic Acidosis if Anion gap is normal there must be hyperchloremia. But in some cases body can’t compensate this by increase Cl^- concentration or unmeasured anions or decrease unmeasured cations .As a result there is increase Anion Gap and body’s Cl^- concentration became normal.

             
Diseases related to Metabolic Acidosis & Anion gaps :

Normal Anion Gap (Hyperchloremia )	Increased anion Gap ( Normochloremia )
C.A.R.D Carbonic anhydrase inhibitors Addison’s diseases Renal tubular acidosis Diarrhea	Ketoacidosis Lactic acidosis Starvation Methanol poisoning Aspirin poisoning Ethylene glycol poisoning Chronic renal Failure

Collected and concise only for postgraduate students