Anion Gap and Acid-Base Balance

Measuring the anion gap in the blood helps evaluate acid-base balance and metabolic acidosis. The anion gap is the difference between the sum of positively charged (cationic) sodium and potassium and the sum of negatively charged (anionic) chloride and bicarbonate. 

The difference between these two values represents the presence of additional negatively charged compounds including many proteins, lactate, sulfate, keto acids, and other anions. Some labs may not include potassium in the equation in which case results are shifted downward.

Normally, if acids such as lactic acid or keto acids build up in the blood, bicarbonate should be able to neutralize them, keeping the anion gap within normal limits. However,  an excess of acid or depletion of bicarbonate will increase the anion gap, decrease pH, and likely indicate metabolic acidosis. 

An increasing anion gap has also been associated with inflammation, insulin resistance, increased fasting glucose, increased progression to diabetes, hypertension, decreased cardiorespiratory fitness, thiamine deficiency, hyperphosphatemia, and ingestion of both sugar-sweetened and diet soft drinks. 

A low anion gap is not common but can be associated with chronic vomiting, diarrhea, multiple myeloma, or intoxication with iodide, lithium, or bromide. Elevated blood lipids may falsely decrease AG.

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