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:
- Addition of strong acid that is buffered by and consumes bicarbonate ion
- Loss of bicarbonate ion from the body fluids, usually through the GI tract or kidneys
- 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]
Acute metabolic acidosis, lasting minutes to several days, is relatively common among critically ill patients with reported incidence varying from 6% for more severe acidosis (plasma pH <7.20) to up to 64%.[1] [2] [3] [4] Chronic metabolic acidosis, lasting weeks to years, is much less common but its frequency is likely to increase with a rise in chronic kidney disease among older people.[5]
