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 non-bicarbonate 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 sulphate (SO4), 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. 
- GI loss of bicarbonate
- Renal loss of bicarbonate - renal tubular acidosis type 1 and 2
- Renal tubular acidosis type 4
- Diabetic ketoacidosis
- Alcohol ketoacidosis
- Ingestion of toxic substances
- Paracetamol ingestion (5-oxoproline toxicity)
- Salicylate intoxication
- Lactic acidosis
- Acute renal failure
- Chronic renal failure