The anion gap

Blood tests measure most cations (positively charged molecules) but only a few anions (negatively charged molecules). Anions and cations are equal in the human body, but if we add all the measured cations and anions together there will be a gap - this reflects the concentration of those anions not measured, for example plasma proteins, phosphates, and organic acids. This is called the anion gap and is calculated by:

The normal range is 10-16 mmol litre-1. In an increased anion gap metabolic acidosis, the body has gained an acid load. In a normal anion gap metabolic acidosis, the body has usually lost bicarbonate. Causes of an increased anion gap metabolic acidosis include (Box 3.1):

• excess acid through ingestion

• excess acid through the body's own production

• excess acid through an inability to excrete.

In a normal anion gap metabolic acidosis, bicarbonate is lost via the gastrointestinal tract (diarrhoea or ileostomy) or the kidneys (tubular damage or acetazolamide therapy). Occasionally there is reduced renal H+ excretion.

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