Anion gap

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Anion gap

Anion gap
= Concentration of all the unmeasured anions in the plasma

• Negatively charged proteins accounts for about 10% of all plasma anions, and make up the majority of unmeasured anions

Anion gap in metabolic acidosis

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

Calculating anion gap

Anion gap
= [Na+] - [Cl-] - [HCO3-]

• Reference range: 8-16mmol/L
• Due to the summation of a few measured values, errors involved is quite high

Anion gap in renal patients

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

Interpreting anion gap value

Anion gap greater than 30
--> Metabolic acidosis present

Anion gap between 20-29
--> ~1/3 of patients can be normal

• Increased when calcium, magnesium is decreased, when plasma albumin increased or when organic anion such as lactate accumulate
• Decreased when cations are increased or when plasma albumin is decreased.

 

Lactic acidosis

• Lactate levels of 5-10mmol/L are associated with high mortality if associated with sepsis

Effect of low albumin level

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

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

Common cations in urine

Na+, K+, NH4+, Mg2+, Ca2+

Common anions in urine

HCO3, Cl-, sulphates, phosphate

Application of urinary anion gap

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,

• Positive urinary anion gap suggest renal cause of hyperchloraemic acidosis
• Negative urinary anion gap suggest GIT cause of hyperchloraemic acidosis

NB:

• Used ONLY if clinically not obvious

 

Other notes

Delta ratio

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