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Dr. Ajith Kumar J MD
Dept. of Emergency Medicne
Travancore Medicity, Kollam
India
editor@emmedonline.com

Metabolic Acidosis & Anion Gap

In metabolic acidosis the anion gap may be either high or normal. That is it may either wide anion gap acidosis or a normal anion gap acidosis.
Normal value : 12 ± 4 mEq/L.

Anion Gap

The principle electrical neutrality requires that plasma have no net charge.
The predominant cation is Na+. The predominant anion is HCO ₃^–& Cl ^-_.
But the sum of HCO ₃^–& Cl ^- doesn’t equal that of sodium that means there are other anions in serum which also plays a major role.
Other anions are like serum protein, phosphate, sulfate, organic anions such as lactate and the conjugated base of ketoacids .

Where, MA: Measured anion , MC: Measured cations, UC : Unmeasured cations, UA: Unmeasured anions

Wide Anion Gap Acidosis

CATEGORY	ETIOLOGY
Renal Failure	Uremia
Lactic Acidosis	Sepsis, Shock , Liver disease, Isoniazid, Iron , Carbon monoxide, Methhemoglobin, Metformin , Cyanide
Ketoacidosis	Diabetic ketoacidosis, Alcholic ketoacidosis, Starvation ketoacidosis
Poisoning	Methanol , ethylene glycol, Salicylates, CO, Cyanide

High Anion Or Negative Anion Gap

Sometimes There wont be any metabolic acidosis but anion gap will be abnormal . If anion gap is present without Acid Base disturbance then it can be

ANION GAP	CAUSE	EXAMPLES
High	Decreased unmeasured cations	Hypomagnesemia, Hypokalemia , Hypocalcemia
Low or Negative	Increased unmeasured cation	High lithium, unmeasured positively charged proteins ( ie myeloma, polyclonal gammopathies)
	Decreased unmeasured anion	Hypoalbumin , Hypogammaglobulin
	Measured confounder	Bromide , high triglycerides.

Non Anion Gap Acidosis/Normal Anion Gap Acidosis

WITH TENDENCY TO HYPERKALEMIA	WITH TENDENCY TO HYPOKALEMIA
Subsidising diabetic ketoacidosis	Renal tubular acidosis , Type I
Early Uremic acidosis	Renal tubular acidosis , Type II
Early obstructive uropathy	Acetazolamide
Renal tubular acidosis IV	Acute diarrhea with losses of HCO₃^-& K+
Hypoaldosteronism	Uretrosigmoidostomy with increased resorption of [H+] & [Cl-] and losses of HCO₃^-& K+.
Infusion or ingestion of HCl	Obstruction of artificial ileal bladder
Potassium sparing diuretics	Dilutional acidosis

Nonanion gap acidosis occurs in the setting of bicarbonate loss, but without the presence of additional pathological anion. In a non anion gap acidosis [Cl^-] is increased to maintain electroneutrality and the calculated anion gap remains normal.

Check the urine anion gap (UNa + UK – UCl)
- It is used in a case of normal anion gap acidosis to known whether the loss via GI source or renal loss.
- If urine anion gap is positive, it is a renal cause (e.g. RTA; really only validated for types I, IV)
- If urine anion gap is negative, the cause is extra-renal (Appropriate renal response is to increase ammonium excretion as NH₄Cl, which causes the UAG to become negative, usually ranging from -20 to -50 mEq/L.

Gap – Gap acidosis / Mixed acid base disorder(Third Acid Base Disorder)

The third acid base disorder is diagnosed by comparing the excess AG to HCO3 deficit.
Gap- gap ratio = (Measured AG – 12) / (24 – Measured HCO3)
Gap- gap ratio = (∆AG)/ (∆HCO₃^-)
Ratio <1 is suggestive of concomitant normal anion gap acidosis
Ratio >1 is suggestive of concomitant metabolic alkalosis.

Add the delta gap to the patient’s bicarbonate if it is < 22, there is a concomitant non-gap metabolic acidosis; if it is > 26, there is a concomitant metabolic alkalosis.

Influence Of Albumin In Anion Gap

Albumin is a major source of unmeasured anion.
It is found that a 50% reduction in albumin concentration will result in 75 % reduction in the anion gap.
A reduction in plasma albumin concentration lower the baseline AG by 2.5 meq for every 1 gm/dl fall in albumin concentration. Hence anion gap has to be adjusted.
Adjusted AG = Observed AG + 2.5 (4.5 – measured albumin)
Eg: If a persons Calculated anion gap is 10 & albumin is 2gm/gdl ; then adjusted anion gap is 10+2.5(4.5 -2 ) = 16; this means the person who seem to be in NAGMA actually has WAGA.

Alternatively expected anion gap can also be calculated from the equation
Expected AG = (2 *albumin mg/dl) + 0.5 * PO4 (mg/dl)
If the calculated AG (Na⁺ - H^-+HCO^-3) is more than the Expected AG ;then it represents WAGA .

Updated on : 21/7/2015

Reference

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emmedonline
Dr. Ajith Kumar J MD
Dept. of Emergency Medicne
Travancore Medicity, Kollam
India
editor@emmedonline.com