Anion Gap

What is Anion Gap?

We need an equal amount of anions and cations to keep the body electrically neutral. So anion gap is not a real thing, it is just a diagnostic concept. Remember, in cations, we mainly measure Na, the other ‘unmeasured’ but important cations are K, Mg, Ca. Unmeasured means not routinely measured. In anions, we routinely measure Cl- and HCO3-, others important but ‘unmeasured’ anions are PO4, Sulfate and  Albumin.

The formula: Anion gap=Na-Cl-HCO3

(normally 8-16meq) is calculated mostly in metabolic acidosis as either normal anion gap or increased anion gap.

Now the most important thing to always remember is that, if due to anyreason, HCO3 or Chloride (measured anions) decreases, the relative concentration of unmeasured anions will increase, as if the unmeasured anions are trying to compensate the loss of their fellow measured anions by increasing their own concentration 

             In Metabolic Acidosis, HCO3 is low, assume that Na is unchanged for now,  so due to low HCO3, other anions like Cl-( measured) and other unmeasured anion will try to compensate by increasing themselves. If Cl- can increase to replace to loss of HCO3’s negative charge, the anion gap will remain within normal limits, if Cl- is too low and can't compensate, then, by the formula AG=Na(normal)- Cl(low)-HCO3(low), The Anion Gap will increase.

- Vaibhav Jain