Anion Gap = the Concentration of All the Unmeasured Anions in the Plasma

Anion Gap

PY

= (Na+ + K+) – (Cl- + HCO3-)

- anion gap = the concentration of all the unmeasured anions in the plasma

- the normal anion gap depends on serum phosphate and serum albumin

- an elevated anion gap strongly suggests a metabolic acidosis

- normal 12 to 16

- > 30 then metabolic acidosis invariably present

- 20-29 then 1/3 will not have a metabolic acidosis

The effect of albumin & phosphate

- the normal anion gap depends on serum phosphate and serum albumin

- the normal AG = 0.2 x [albumin] (g/L) + 1.5 x [phosphate] (mmol/L)

- albumin is the major unmeasured anion and contributes almost the whole of the value of the anion gap.

- every one gram decrease in albumin will decrease anion gap by 2.5 to 3 mmoles.

- a normally high anion gap acidosis in a patient with hypoalbuminaemia may appear as a normal anion gap acidosis.

- this is particularly relevant in ICU patients where lower albumin levels are common.

Anion Gap Metabolic Acidosis – accumulation of organic acids or impaired H+ excretion

Lactate

Toxins

Ketones

Renal

- this metabolic acidosis is normally managed by albumin -> if albumin decreased by 1g then decrease anion gap by 2-3 points.

- lab tests to order = lactate, glucose, creatinine and urea, urinary ketones, serum levels of methanol, ethanol, paracetamol, salicylates and ethylene glycol.

Non-anion Gap Metabolic Acidosis – loss of HCO3- from ECF

Chloride

Acetazolamide/Addisons

GI causes – diarrhea/vomiting, fistulae (pancreatic, ureters, billary, small bowel, ileostomy)

Extra – RTA (1)

- calculating urinary anion gap helps to differentiate between a GI and renal cause of a normal anion gap acidosis (urinary anion gap = Na+ + K+ - Cl-) -> the remaining significant unmeasured ions are NH4+ and HCO3-

- renal causes increased urinary HCO3- excretion thus increased urinary AG

- GI causes increased NH4+ excretion thus decreased urinary AG

Causes of a Low Anion Gap

Decrease in unmeasured anions (albumin, dilution)

Increase in unmeasured cations (multimyeloma, hypercalcaemia, hypermagnesaemia, lithium OD, bromide OD, polymixin B)

Non random analytical errors (increased Na+, increased viscosity, iodide ingestion, increased lipids)

Jeremy Fernando (2010)