Evaluation of metabolic acidosis

The presence of metabolic acidosis is a clue to the possible existence of several underlying medical conditions. Arterial pH <7.35 defines acidosis. Metabolic acidosis is indicated by a decrease in the plasma bicarbonate level and/or a marked increase in the serum anion gap (AG).

Metabolic acidosis may occur due to the following reasons:

1. Addition of strong acid that is buffered by and consumes bicarbonate ion

2. Loss of bicarbonate ion from the body fluids, usually through the GI tract or kidneys

3. Rapid addition to the extracellular fluid of a nonbicarbonate solution.

Differentiating between the causes of metabolic acidosis begins with calculation of serum AG. Serum AG is calculated by subtracting the sum of major measured anions, chloride (Cl-) and bicarbonate (HCO₃-), from the major measured cation, sodium (Na+).

• AG = Na+ - (Cl- + HCO₃-)

Normal serum AG is due to the difference between unmeasured anions such as sulfate (SO₄2-), phosphate (PO₄-), albumin, and organic anions, and unmeasured cations such as potassium (K+), magnesium (Mg+), and calcium (Ca2+). Plasma proteins also play a role in maintaining normal serum AG.[1]Dubin A, Menises MM, Masevicius FD, et al. Comparison of three different methods of evaluation of metabolic acid-base disorders. Crit Care Med. 2007 May;35(5):1264-70. http://www.ncbi.nlm.nih.gov/pubmed/17334252?tool=bestpractice.com