Anion gap
= Concentration of all the unmeasured anions in the plasma
Metabolic acidosis
--> acid anion (e.g. lactate, acetoacetate, sulphate) produced
--> H+ produced consumes HCO3
--> HCO3 decrease and acid anions increase
--> Unmeasured anions increase
--> Anion gap increases
Anion gap
= [Na+] - [Cl-] - [HCO3-]
In renal patients, [K+] is often included because it tends to vary widely
Anion gap
= {[Na+] + [K+]} - {[Cl-] + [HCO3-]}
* Reference range: higher than 8-16mmol/L
Anion gap greater than 30
--> Metabolic acidosis present
Anion gap between 20-29
--> ~1/3 of patients can be normal
Hypoalbuminaemia causes a low anion gap
--> 1 gram drop decreases anion gap by 2.5-3 mmoles
Thus,
Lactic acidosis associated with low albumin may have normal anion gap
Urinary anion gap
= [Na+] + [K+] - [Cl-]
= unmeasured anion + unmeasured cations
Urinary anion gap can be used to differentiate GIT and renal causes of hyperchloraemic metabolic acidosis
Na+, K+, NH4+, Mg2+, Ca2+
HCO3, Cl-, sulphates, phosphate
In differentiating metabolic acidosis
Step 1: high anion gap or normal anion gap (hyperchloraemic)
Step 2: Hyperchloraemic acidosis can be caused by
* Loss of base via kidney (e.g. renal tubular acidosis)
* Loss of base via GIT (e.g. diarrhoea)
* Gain of mineral acid (e.g. HCl)
If acidosis is due to loss of base via GIT
--> Kidney is able to compensate by increasing NH4+ excretion
--> Decreased urinary anion gap
If acidosis is due to loss of base via kidney
--> Kidney not able to increase NH4+ excretion
--> Urinary anion gap unchanged
Thus,
NB:
Delta ratio
= Increase in anion gap/decrease in bicarbonate
When metabolic acid (HA) is produced,
--> H+ consumed a bicarbonate
--> A- is added to unmeasured anions (anion gap)
Thus,
If the acid completely dissociate, and if buffering was entirely by bicarbonate, then the delta ratio should be 1
But in reality,
Buffering is not entirely by bicarbonate
57% of the H+ is buffered intracellularly
Also,
A- tend to stay in ECF due to negative charge
Thus,
Bicarbonate doesn't fall as much as predicted, but anion gap tend to increase as much as predicted
--> Delta ratio > 1 in high anion gap acidosis